Ca2+ Sensitization Mechanism in Stretch-induced Myogenic Tone

It has been suggested that Ca2+ sensitization mechanisms might contribute to myogenic tone, however, specific mechanisms have not yet been fully identified. Therefore, we investigated the role of protein kinase C (PKC)- or RhoA-induced Ca2+ sensitization in myogenic tone of the rabbit basilar vessel. Myogenic tone was developed by stretch of rabbit basilar artery. Fura-2 Ca2+ signals, contractile responses, PKC immunoblots, translocation of PKC and RhoA, and phosphorylation of myosin light chains were measured. Stretch of the resting vessel evoked a myogenic contraction and an increase in the intracellular Ca2+ concentration ([Ca2+]i) only in the presence of extracellular Ca2+. Stretch evoked greater contraction than high K+ at a given [Ca2+]i. The stretch-induced increase in [Ca2+]i and contractile force were inhibited by treatment of the tissue with nifedipine, a blocker of voltage-dependent Ca2+ channel, but not with gadolinium, a blocker of stretch-activated cation channels. The PKC inhibitors, H-7 and calphostin C, and a RhoA-activated protein kinase (ROK) inhibitor, Y-27632, inhibited the stretch-induced myogenic tone without changing [Ca2+]i. Immunoblotting using isoform-specific antibodies showed the presence of PKCalpha and PKCepsilon in the rabbit basilar artery. PKCalpha, but not PKCepsilon, and RhoA were translocated from the cytosol to the cell membrane by stretch. Phosphorylation of the myosin light chains was increased by stretch and the increased phosphorylation was blocked by treatment of the tissue with H-7 and Y-27632, respectively. Our results are consistent with important roles for PKC and RhoA in the generation of myogenic tone. Furthermore, enhanced phosphorylation of the myosin light chains by activation of PKCalpha and/or RhoA may be key mechanisms for the Ca2+ sensitization associated with myogenic tone in basilar vessels.

Effect of lysophosphatidylcholine on whole cell K+ current in rabbit coronary smooth muscle cells

BACKGROUND: Impairment of relaxing response and augmentation of contractile response to vasoactive substances have been reported in atherosclerotic arteries. These alterations in vascular reactivity are considered as an underlying mechanism for the development of acute vasospasm in atherosclerotic coronary artery. Recently, it has been reported that lysophophatidylcholine (LPC), an oxidative metabolite of low density lipoprotein causes this functional abnormality. However, the precise mechanism of LPC induced change of vascular reactivity is still uncertain. METHOD: In this study, to elucidate the underlying mechanisms of abnormal vascular reactivity in atherosclerotic coronary artery, we examined the effect of LPC on whole cell K+current using patch clamping technique in rabbit coronary smooth muscle cells. RESULTS: Application of LPC(1microM) showed dual effect on whole cell outward current which depends on the magnitude of test potentials. At relatively high depolarizing test potentials (> 10 mV), LPC increased amplitude of outward current which was blocked by Gd3+ not by iberiotoxin (100 nM) and TEA (1 mM). Reversal potential of this Gd3+sensitive, LPC-induced current was -9.7 +/- 0.6 mV. At less depolarizing test potentials (< 10 mV), LPC decreased whole cell K+currents in a dose dependent manner (from 0.01 to 10 microM) in the range of -30 mV to +0 mV. Half maximal inhibition of K+current was 1.509 microM at 0 mV test potential (n =5). Depolarizing holding potential (0 mV) prevented this LPC-induced inhibition of K+current. Steady state activation and inactivation parameters of K+current were significantly shifted to the positive direction by application of LPC (p < 0.01, n =8). Pretreatment of staurosporine (100 nM), a blocker of protein kinase C partially blocked LPC-induced decrease of K+currents. CONCLUSION: LPC-induced inhibition of voltage dependent K+current may explain abnormal vascular reactivity in atherosclerotic coronary artery.

Membrane stretch increases the activity of Ca(2+)-activated K+ channels in rabbit coronary vascular smooth muscles

It has been proposed that Ca(2+)-activated K+ channels play an essential role in maintaining vascular tone during stretch of blood vessel. However, the underlying mechanism of stretch-induced change of Ca(2+)-activated K+ channel activities are still unknown. The present experiment was designed to investigate the effect of membrane stretch on these channels whose activity was measured from rabbit coronary smooth muscle cells using a patch clamp technique. Ca(2+)-activated K+ channel were identified by their Ca2+ and voltage dependencies and its large conductances as in other preparations. Perfusion of cells with a hypotonic solution, which mimics stretching the cell membrane by making a cell swelling, produced an increase in channel activity in cell-attached patch mode. The similar increase was observed when negative pressure was applied into the patch pipette for stretching the cell membrane within a patch area. In inside-out patch, stretch still increased channel activity even under the conditions which exclude the possible involvement of secondary messengers, or of transmembrane Ca2+ influx via stretch-activated cation channels. Pretreatment of arachidonic acid or albumin showed no effect on stretch-induced channel activation, excluding the possibility of fatty acids mediated channel activation during membrane stretch. These results indicate that the stretch may directly increase the activity of Ca(2+)-activated K+ channels in our experimental condition.

The effect of NO donor on contraction, cytosolic Ca2+ level and ionic currents in guinea-pig ileal smooth muscle

This study was designed to clarify the mechanism of the inhibitory action of a nitric oxide (NO) donor, 3-morpholino-sydnonimine (SIN-1), on contraction, cytosolic Ca2+ level ((Ca2+)i), and ionic currents in guinea-pig ileum. SIN-1 (0.01~100 micrometer) inhibited 25 mM KCl- or histamine (10 micrometer)-induced contraction in a concentration-dependent manner. SIN-1 reduced both the 25 mM KCl- and the histamine-stimulated increases in muscle tension in parallel with decreased (Ca2+)i. Using the patch clamp technique with a holding potential of -60 mV, SIN-1 (10 micrometer) decreased peak Ba currents (IBa) by 30.9+/-5.4% (n=6) when voltage was stepped from -60 mV to +10 mV and this effect was blocked by ODQ (1 micrometer), a soluble guanylyl cyclase inhibitor. Cu/Zn SOD (100 U/ml), the free radical scavenger, had little effect on basal IBa, and SIN-1 (10 micrometer) inhibited peak IBa by 32.4+/-5.8% (n=5) in the presence of Cu/Zn SOD. In a cell clamped at a holding-potential of -40 mV, application of 10 micrometer histamine induced an inward current. The histamine-induced inward current was markedly and reversibly inhibited by 10 micrometer SIN-1, and this effect was abolished by ODQ (1 micrometer). In addition, SIN-1 markedly increased the depolarization-activated outward K+ currents in the all potential ranges. We concluded that SIN-1 inhibits smooth muscle contraction mainly by decreasing (Ca2+)i resulted from the inhibition of L-type Ca2+ channels and the inhibition of nonselective cation currents and/or by the activation of K+ currents via a cGMP-dependent pathway.

Attenuation of endothelial relaxation in umbilical arteries from preeclampsia patients / 대한산부인과학회잡지

OBJECTIVES AND METHODS: To directly examine the function of the endothelial cell(EC) and smooth muscle cell in umbilical arteries acquired from preeclampsia patients between June 1998 to November 1999, using a conventional tension measurement and bioassay experiment. RESULTS: Relaxation responses to EC-dependent relaxing agents including bradykinin and A23187 in human umbilical artery rings were significantly decreased in preeclampsia(p<0.01). Relaxation responses to EC-independent agents(SNP and SNAP) were also inhibited in umbilical artery rings acquired from preeclampsia patients(p<0.01). To test the change of endothelial cell function in preeclampsia without involvement of smooth muscle dysfunction, we used human umbilical artery and rabbit femoral artery as a donor and detector, respectively, in bioassay experiment. Relaxation responses to EC-dependent agents(A23187 and bradykinin) showed similar results to conventional tension measurement (p<0.01). Relaxation responses to 8-bromo-cGMP in human umbilical artery rings were also significantly decreased in preeclampsia(p<0.01). CONCLUSIONS: It can be concluded that increased vascular resistance in preeclampsia is not only due to the disturbance of endothelial function, but also to smooth muscle dysfunction.

The involvement of K+ channels and the possible pathway of EDHF in the rabbit femoral artery

Experiments were designed to characterize the cellular mechanisms of action of endothelium-derived vasodilator substances in the rabbit femoral artery. Acetylcholine (ACh, 10(-8)-10(-5) M) induced a concentration-dependent relaxation of isolated endothelium-intact arterial rings precontracted with norepinephrine (NE, 10(-6) M). The ACh-induced response was abolished by the removal of endothelium. NG-nitro-L-arginine (L-NAME, 10(-4) M), an inhibitor of NO synthase, partially inhibited ACh-induced endothelium-dependent relaxation, whereas indomethacin (10(-5) M) showed no effect on ACh-induced relaxation. 25 mM KCl partially inhibited ACh-induced relaxation by shifting the concentration-response curve and abolished the response when combined with L-NAME and NE. In the presence of L-NAME, ACh-induced relaxation was unaffected by glibenclamide (10(-5) M) but significantly reduced by apamin (10(-6) M), and almost completely blocked by tetraethylammonium (TEA, 10(-3) M), iberiotoxin (10(-7) M) and 4-aminopyridine (4-AP, 5 x 10(-3) M). The cytochrome P450 inhibitors, 7-ethoxyresorufin (7-ER, 10(-5) M) and miconazole (10(-5) M) also significantly inhibited ACh-induced relaxation. Ouabain (10(-6) M), an inhibitor of Na+, K(+)-ATPase, or K(+)-free solution, also significantly inhibited ACh-induced relaxation. ACh-induced relaxation was not significantly inhibited by 18-alpha-glycyrrhetinic acid (18 alpha-GA, 10(-4) M). These results of this study indicate that ACh-induced endothelium-dependent relaxation of the rabbit femoral artery occurs via a mechanism that involves activation of Na+, K(+)-ATPase and/or activation of both the voltage-gated K+ channel (Kv) and the large-conductance, Ca(2+)-activated K+ channel (BKCa). The results further suggest that EDHF released by ACh may be a cytochrome P450 product.

Effect of PKC-dependent Change of K+ Current Activity on Histamine-induced Contraction of Rabbit Coronary Artery

BACKGROUND: Histamine, released from mast cells in atheromatous plaque, has been known to cause cardiac ischemia or sudden cardiac death in atherosclerosis patient. Previous reports have suggested that histamine induced coronary vasoconstriction was due to increase in IP(3) and DAG, which induce release of Ca2+ from SR and increase the Ca2+ sensitivity of contractile element via activation of PKC. Recently, it was reported that application of histamine cause depolarization of intestinal smooth muscle, which may contribute to histamine-induced contraction via augmenting Ca2+ influx through activation of Ca2+ channels. However, the underyling mechanism of histamine-induced depolarization and its contribution to the magnitude of coronary vasoconstriction are still uncertain. METHOD: To elucidate the underlying mechanism of Ca2+ influx change during histamine-induced vasoconstriction, we examined the effect of Ca2+ channel antagonist and PKC blocker on histamine-induced contractions, and then measured the effect of PKC antagonist on whole cell K+ current using patch clamping method in rabbit coronary smooth muscle cells. RESULTS: Application of histamine induced phasic and tonic constraction of coronary rings via activation of H(1) receptors. Pretreatment of Ca2+ channel antagonist (nifedipine, 1 microM) or PKC blockers (10 nM staurosporine and 10 microM Go6976) markedly inhibited histamine-induced tonic contraction, which suggest that the magnitude of tonic contraction depend on the Ca2+ influx. Application of 4-AP, a blocker of voltage-dependent K+ channels, increased resting tone of coronary rings, and combined treatment of nifedipine blocked this 4-AP induced increase of resting tone. Application of active analoge of DAG (1,2-DiC(8)) significantly inhibited the activity of voltage-dependent K+ current in single smooth muscle cell, meanwhile the inactive analogue of DAG (1,3-DiC(8)) has no apparent effect on the activity of voltage-dependent K+ current. Furthermore, pretreatment of calphostin C (1 microM), a blocker of PKC, diminished the 1,2-DiC(8)-induced inhibition of K+ current. CONCLUSION: PKC dependent inhibition of voltage-dependent K+ current may be responsible for the maintaining of histamine-induced tonic contraction in rabbit coronary artery.

Mechanisms of relaxation of coronary artery by hypoxia

This study was designed to clarify the dependency of hypoxic coronary vasodilation (HCD) on the endothelium and the role of the K+ channels on HCD in the rabbit coronary artery. HCD was investigated in an isolated left circumflex coronary artery precontracted with prostaglandin F2 alpha. Vascular rings were suspended for isometric tension recording in an organ chamber filled with Krebs-Henseleit (KH) solution. Hypoxia was induced by gassing the chamber with 95% N2 + 5% CO2 and was maintained for 15 approximately 25 min. Hypoxia elicited a vasodilation in the precontracted coronary artery with and without endothelium. There was no difference between the amplitude of the HCD induced by two consecutive hypoxic challenges and the effects of 20% O2 + 5% CO2 + 75% N2 and 95% O2 + 5% CO2 control K-H solution of subsequent responses to hypoxia. Inhibition of the cyclooxygenase pathway by treatment with indomethacin had no effect on HCD. Blockades of the tetraethylammonium chloride-sensitive K+ channel abolished HCD. Apamin, a blocker of the small conductance Ca(2+)-activated K+ (KCa) channel, and iberiotoxin, a blocker of the large conductance KCa channel had no effect on HCD, respectively. Glibenclamide, a blocker of the ATP-sensitive K+ (K+ATP) channel, reduced HCD. Cromakalim, an opener of the K+ATP channel, relaxed the coronary artery precontracted with prostaglandin F2 alpha. The degree of relaxation by cromakalim was similar to that by hypoxia while glibenclamide reduced both hypoxia- and cromakalim-induced vasodilatations. In conclusion, these results suggest that HCD is independent on endothelium and HCD is considered to be induced by activation of K+ATP channel.

Effects of hypoxia on pulmonary vascular contractility

Although hypoxic pulmonary vasoconstriction (HPV) has been recognized by many researchers, the precise mechanism remains unknown. As isolated pulmonary arteries will constrict in vitro in the response to hypoxia, the oxygen sensor/transduction mechanism must reside in the pulmonary arterial smooth muscle or in the endothelium, or in both. Unfortunately, much of the current evidence is conflicting, especially as to the dependency of HPV on the endothelium and the role of a K+ channel. Therefore, this experiment was attempted to clarify the dependency of HPV on the endothelium and the role of a K+ channel on HPV in rat pulmonary artery. The effects of hypoxia were investigated in isolated main pulmonary arteries precontracted with norepinephrine. Vascular rings were suspended for isometric tension recording in an organ chamber filled with a Krebs-Henseleit solution. Hypoxia was induced by gassing the chamber with 95% N2 + 5% CO2 and this was maintained for 20 min. Hypoxia elicited a vasoconstriction in arteries with endothelium. Mechanical disruption of the endothelium abolished HPV. There was no difference between the amplitude of the HPV induced by two consecutive hypoxic challenges and the effect of normoxic and hyperoxic control Krebs-Henseleit solution on a subsequent response to hypoxia. Inhibition of NO synthesis by treatment with N(omega)-nitro-L-arginine reduced HPV, but inhibition of a cyclooxygenase pathway by treatment with indomethacin had no effect on HPV. Blockades of a tetraetylammonium chloride-sensitive K+ channel abolished HPV. Verapamil, a Ca2+ entry blocker reduced HPV. In conclusion, these results suggest that HPV was dependent on the endothelium and that HPV can be considered to be induced by inhibition of the mechanisms of NO-dependent vasodilation such as the opening of a K+ channels.

Study on the Mechanism of Hypoxic Induced Vasodilatation and Vasoconstriction

BACKGROUND: Although hypoxic pulmonary vasoconstriction (HPC) and hypoxic coronary vasodilatation (HCD) have been recognized by many researchers, the precise mechanism remains unknown. As isolated arteries will constrict or relax in vitro in response to hypoxia, the oxygen sensor/transduction mechanism must reside in the arterial smooth muscle, the endothelium, or both. Unfortunately, much of the current evidence is conflicting, especially concerning to the dependency of HPC and HCD on the endothelium and the role of the K+ channel. Therefore, this experiment was attempted to clarify the dependency of HPC and HCD on the endothelium and the role of the K+ channel on HPC and HCD. METHODS: HPC was investigated in isolated main pulmonary arteries precontracted with norepinephrine (NE). HCD was investigated in isolated left circumflex coronary artery precontracted with prostaglandin F2 alpha. Vascular rings were suspended for isometric tension recording in an organ chamber filled with Krebs-Henseleit solution. Hypoxia was induced by gassing the chamber with 95% N2 +5% CO2, which was maintained for 15 - 25 min. RESULTS: 1)Hypoxia elicited a vasoconstriction in NE-precontracted pulmonary arteries with endothelium, but a vasodilatation in PGF 2 alpha-precontracted coronary arteries with and without endothelium. There was no difference between the amplitude of the HPC and HCD induced by two consecutive hypoxic challenges and the effect of normoxic and hyperoxic control Krebs-Henseleit solution on subsequent response to hypoxia. 2)Inhibition of NO synthesis by the treatment with Nw-nitro-L-arginine reduced HPC in pulmonary arteries, but inhibition of the cyclooxygenase pathway by treatment with indomethacin had no effect on HPC and HCD, respectively. 3)Blockades of the TEA-sensitive K+ channel abolished HPC and HCD. 4)Apamin, a small conductance Ca2+/-activated K+ (KCa) channel blocker, and iberiotoxin, a large conductance KCa channel blocker, had no effect on the HCD. 5)Glibenclamide, an ATP-sensitive K+ (KATP) channel blocker, reduced HCD. 6)Cromakalim, an K(ATP) channel opener, relaxed the coronary artery precontracted with prostaglandin F2 alpha. The degree of relaxation by cromakalim was similar to that by hypoxia and glibenclamide reduced both hypoxia- and cromakalim-induced vasodilations. 7)Verapamil, a Ca2+ entry blocker, caffeine, a Ca2+ emptying drug; and ryanodine, an inhibitor of Ca2+ release from SR, reduced HPC, respectively. CONCLUSION: HPC is dependent on the endothelium and is considered to be induced by inhibition of the mechanisms of NO-dependent vasodilation while HCD is independent of the endothelium and is considered to be induced by activation of the K(ATP) channel.

Modulation of outward potassium currents by nitric oxide in longitudinal smooth muscle cells of guinea-pig ileum

To investigate the possible involvement of outward potassium (K+) currents in nitric oxide-induced relaxation in intestinal smooth muscle, we used whole-cell patch clamp technique in freshly dispersed guinea-pig ileum longitudinal smooth muscle cells. When cells were held at -60 mV and depolarized from - 40 mV to + 50 mV in 10 mV increments, sustained outward K+ currents were evoked. The outward K+ currents were markedly increased by the addition of 10 muM sodium nitroprusside (SNP). 10 muM S-nitroso-N-acetylpenicillamine (SNAP) and 1 mM 8-Bromo-cyclic GMP (8-Br-cGMP) also showed a similar effect to that of SNP. 1 mM tetraethylammonium (TEA) significantly reduced depolarization-activated outward K+ currents. SNP-enhanced outward K+ currents were blocked by the application of TEA. High EGTA containing pipette solution (10 mM) reduced the control currents and also inhibited the SNP-enhanced outward K+ currents. 5 mM 4-aminopyridine (4-AP) significantly reduced the control currents but showed no effect on SNP-enhanced outward K+ currents. 0.3 muM apamin and 10 muM glibenclamide showed no effect on SNP-enhanced outward K+ currents. 1 muM 1H-(1,2,4)oxadiazolo (4,3-a)quinoxaline-1-one (ODQ), a specific inhibitor of soluble guanylate cyclase, significantly blocked SNP-enhanced K+ currents. We conclude that NO donors activate the Ca2+-activated K+ channels in guinea-pig ileal smooth muscle via activation of guanylate cyclase.

Action mechanisms of NANC neurotransmitters in smooth muscle of guinea pig ileum

The relaxation induced by stimulation of the inhibitory non-adrenergic, non-cholinergic (iNANC) nerve is mediated by the release of iNANC neurotransmitters such as nitric oxide (NO), vasoactive intestinal peptide (VIP) and adenosine triphosphate (ATP). The mechanisms of NO, VIP or ATP-induced relaxation have been partly determined in previous studies, but the detailed mechanism remains unknown. We tried to identify the nature of iNANC neurotransmitters in the smooth muscle of guinea pig ileum and to determine the mechanism of the inhibitory effect of nitric oxide. We measured the effect of NO-donors, VIP and ATP on the intracellular Ca2+ concentration((Ca2+)i), by means of a fluorescence dye (fura 2) and tension simultaneously in the isolated guinea pig ileal smooth muscle. Following are the results obtained. 1. Sodium nitropnisside (SNP: 10(-5) M) or S-nitro-N-acetyl-penicillamine (SNAP: 10(-5) M) decreased resting (Ca2+)i and tension of muscle. SNP or SNAP also inhibited rhythmic oscillation of (Ca2+)i and tension. In 40mM K+ solution or carbachol (CCh:10(-6) M)-induced precontracted muscle, SNP decreased muscle tension. VIP did not change (Ca2+)i and tension in the resting or precontracted muscle, but ATP increased resting (Ca2+)i and tension in the resting muscle. 2. 1H-(1,2,4)oxadiazol(4,3-a)quinoxalin-1-one (ODQ:1 muM), a specific inhibitor of soluble guanylate cyclase, limited the inhibitory effect of SNP. 3. Glibenclamide (10 muM), a blocker of KATP channel, and 4-aminopyridine (4-AP:5 mM), a blocker of delayed rectifier K channel, apamin (0.1 muM), a blocker of small conductance KCa. channel had no effect on the inhibitory effect of SNP. Iberiotoxin (0.1 muM), a blocker of large conductance KCa channel, significantly increased the resting (Ca2+)i, and tension, and limited the inhibitory effect of SNP. 4. Nifedipine (1 muM) or elimination of external Ca2+ decreased not only resting (Ca2+)i and tension but also oscillation of (Ca2+)i and tension. Ryanodine (5 muM) and cyclopiazonic acid (10 muM) decreased oscillation of (Ca2+)i and tension. 5. SNP decreased Ca2+ sensitivity of contractile protein. In conclusion, these results suggest that 1) NO is an inhibitory neurotransmitter in the guinea pig ileum, 2) the inhibitory effect of SNP on the (Ca2+)i and tension of the muscle is due to a decrease in (Ca2+)i by activation of the large conductance KCa channel and a decrease in the sensitivity of contractile elements to Ca2+ through activation of G-kinase.

The effect of sodium nitroprusside on muscle tension in guinea-pig ileum

Nitric oxide (NO) has been known as a mediator of nonadrenergic, noncholinergic inhibitory neurotransmitter in intestinal smooth muscles. It has been suggested that NO donor such as sodium nitroprusside (SNP) produces relaxation of smooth muscle via activation of guanylate cyclase and elevation of cGMP levels. We have therefore investigated the effects of NO, using SNP, on muscle tension in the longitudinal smooth muscle of guinea-pig ileum. The possible role of cGMP was also investigated as well as the involvement of K+ channel on SNP-induced inhibitory effect. The results are summarized as follows; high KCl-or CCh-activated contractions were inhibited by SNP in a concentration-dependent manner. 8-Br-cGMP also showed a similar effect in that of SNP. TEA (1 mM) significantly reduced the SNP-induced inhibitory effect. SNP-induced effect was further reduced by the presence of 10 mM TEA. On the other hand, 4-AP (0.1 mM), glibenclamide (10 muM) and apamin (0.1 muM) showed little effects on SNP-induced relaxation. Zaprinast significantly potentiated the SNP-induced inhibitory effect in all ranges. ODQ also significantly decreased the SNP-induced inhibitory effect. Pretreatment with CPA (10 muM) slightly reduced the SNP-induced inhibitory effect. From the above results, both effect mediated by NO and cGMP might be responsible for the activation of Ca2+/-activated K+ channel by SNP in guinea-pig ileum. And this K+ channel activation by SNP also contributes to the SNP-induced membrane hyperpolarization and relaxation.

Changes in Cytosolic Ca2+ Concentration of Single Rabbit Coronary Artery Smooth Muscle Cell during Ischemic Cardioplegic Period

BACKGROUND: No-reflow is a specific type of vascular damage occuring when removal of coronary occlusion dose not lead to restoration of coronary flow. There are three major explanations for the no-reflow phenomenon such as endothelial cell edema, microvascular plugging by platelets or thrombi and coronary occlusion by ischemic contracture of the myocardium. But detailed mechanisms of no-reflow phenomenon are not known. The objects of this study are to elucidate the possibility whether elevation of cytosolic Ca2+ concentration during ischemic cardioplegic period is mechanism of no-reflow phenomenon or not. METHODS: Changes in cytosolic Ca2+ concentration were measured under varying experimental condition. Free [Ca2+] in the cytosole [Ca2+]i of single rabbit coronary artery cells was measured with fluorescent Ca2+ indicator, Fura-2. RESULTS: Resting [Ca2+]i was 134.2+/-34 nM (n=43). When single cells were perfused with cardioplegic or ischemic cardioplegic solution, [Ca2+]i was significantly increased and degree of [Ca2+]i elevation was further augmented by ischemic cardioplegic solution. Pretreatment of sarcoplasmic reticulum emptying agent (20mM caffeine) had no effect on cardioplegia-induced [Ca2+]i change, but application of Ca2+ channel blocker (5x10-7M nifedipine) or an antagonist of Na+/Ca2+ exchange (5mM Ni2+ ) partially (nifedipine) or completely (nickel) inhibited the [Ca2+]i elevation. Pretreament of caffeine had no effect on ischemic cardioplegia-induced [Ca2+]i change, but application of nifedipine or nickel partially inhibited the [Ca2+]i elevation. Magnitude of ischemic cardioplegia-induced [Ca2+]i elevation was dependent on the Ca2+ concentration of perfusate from 0 to 2.5mM. When Ni2+ was added to reperfusion solution, recovery of ischemic cardioplegia-induced [Ca2+]i elevation was very rapid compared with control. CONCLUSIONS: From the above results, it may be speculated that ischemic cardioplegia-induced [Ca2+]i elevation may act as one of the mechanism of no-reflow phenomenon in rabbit coronary artery.

The Effect of Saponin on the Vascular Contractility of the Rabbit Aortic Ring

PURPOSE: There have been conflicting reports on vascular response to Panax ginseng. The conflicting reports may be due to difference of ingredient of Panax ginseng. The aim of the present study was to investigate the effect of saponin, the main ingredient of Panax ginseng, on the vascular contractility. METHODS: The rabbit aortic rings were cut and mounted on the force transducer to record an isometric tension on polygraph. To elucidate the mechanism of saponin effect on vascular smooth muscle, the contractility of the vascular smooth muscle were measured under varying experimental condition. RESULTS: 1) When the aortic rings were precontracted with norepinephrine, saponin caused biphasic(initial relaxation-sustained contraction) dose-response in the endothelium dependent manner. But saponin had no effect on the resting tension. 2) When EDRF inhibitors such as methylene blue(10(-5)M), hemoglobin(10(-5)M), N-omega-nitro-L-arginine(100microM) were added to precontracted ring with norepinephrine, the initial relaxation caused by 2mg% saponin was inhibited. 3) When Ca(2+)-channel blocker, nifedipine(5x10(-7)M), was added to precontracted rings with norepinephrine, the sustsined contraction by saponin was inhibited. 4) When hemoglobin(10(-5)M) was added to precontracted rings with norepinephrine, the contractility by norepinephrine was increased and this effect was further augmented by 2mg% saponin. CONCLUSIONS: From the above results, it may be concluded that saponin stimulated the release of both an endothelium-dependent relaxing factor and endothelium-dependent contracting factor.

Characteristics of Ca2+ release mechanisms from an intracellular Ca2+ store in rabbit coronary artery

To elucidate the Ca2+ release mechanisms in the rabbit coronary artery, arterial preparations were permeabilized with beta-escin and changes in tension were measured under varying experimental conditions. Additionally, we investigated properties and distribution of two kinds of Ca2+ release mechanisms, Ca2+-induced Ca2+ release (CICR) and IP3-induced Ca2+ release (IICR). The results obtained were summarized as follows; 1. When a rabbit coronary artery was incubated in a relaxing solution containing 30 microM beta-escin for 40 min. sensitivity to externally added Ca2+ was much higher in beta-escin permeabilized muscle than in intact preparations. The contractile effect of IP3 in beta-escin permeabilized muscle was also demonstrated; 2. Caffeine and IP3 contracted coronary arteries were permeabilized with beta-escin, but the amplitude of contraction was much larger in the presence of caffeine than of IP3. 3. Intracellular heparin completely inhibited the contractions induced by IP3, but not those by caffeine. On the other hand, procaine inhibited the responses to caffeine, but not those to IP3. Ryanodine inhibited both the caffeine- and IP3-induced contractions. 4. The amplitude of contractile responses was much larger to the maximal stimulation of CICR by applying caffeine than to the maximal stimulation of IICR by applying IP3. After the maximal CICR stimulation by caffeine, the activation of IICR by IP3 without the reloading of Ca2+ could no longer evoke contraction. On the other hand, after the maximal IICR activation, the activation of CICR could still evoke contraction although the amplitude of the contraction was smaller when compared with the case without the initial IICR stimulation. 5. Acetylcholine contracted coronary artery smooth muscles were permeabilized with beta-escin. However, in the absence of added guanosine triphosphate (GTP), the responses were very small. Acetylcholine-induced contraction was inhibited by heparin, but not by procaine. From the above results, it may be concluded that there are two kinds of mechanisms of Ca2+ release, CICR and IICR, in the rabbit coronary artery smooth muscle cell. Also, whereas the CICR mechanism distributes on the membrane of the whole smooth muscle Ca2+ store, the IICR mechanism distributes only on a part of it.

Changes in intracellular Ca2+ concentration of rabbit coronary artery smooth muscle cell during ischemic cardioplegic period

To elucidate the possibility whether an elevation of intracellular Ca2+ concentration ([Ca2+]i) in rabbit coronary artery myocytes during ischemic cardioplegic period may serve as one of the mechanisms of the "no-reflow' phenomenon or not, the changes in [Ca2+]i were measured under ischemic cardioplegia conditions using a fluorescent Ca2+ indicator, fura 2/AM. When single cells were perfused with cardioplegic or ischemic cardioplegic solutions, [Ca2+]i was significantly increased and the degree of [Ca2+] elevation was further augmented by the ischemic cardioplegic solution. Pretreatment of a sarcoplasmic reticulum emptying agent, 20 mM caffeine, had no effect on ischemic cardioplegia-induced [Ca2+]i changes, but application of a Ca2+ channel blocker, 5 x 10 (-1)M nifedipine, or an antagonist of Na+/Ca2+ exchange, 5 mM Ni2+, significantly inhibited the [Ca2+]i elevation, respectively. The magnitude of ischemic cardioplegia-induced [Ca2+]i elevation was dependent on the Ca2+ concentration of perfusate in the range of 0 and 25 mM. When Ni2+ was added to the reperfusion solution, recovery of ischemic cardioplegia-induced [Ca2+]i elevation was very rapid compared with the controls. It is concluded that ischemic cardioplegia-induced [Ca2+]i elevation may serve as one of the mechanisms of the "no-reflow' phenomenon in rabbit coronary artery smooth muscle cells. We propose that Na+/Ca2+ exchange may serve as a key function in ischemic cardioplegia-induced [Ca2+]i elevation.

Activation of Ca(2+)-activated K+ channels by beta agonist in rabbit coronary smooth muscle cells

Isoproterenol (ISO), a beta agonist, causes hyperpolarization of coronary smooth muscle cells via an increase in K+ conductance. This hyperpolarization may cause the coronary vasorelaxation by decreasing the cytoplasmic Ca2+ concentration. It is well known that the activation of beta adrenoreceptors stimulates the adenylate cyclase activity, and the resulting K+ channel phosphorylation by cAMP-dependent protein kinase may be responsible for ISO-induced increase in K+ channel activity. However, it is not clear whether the increase in K+ channel activity by ISO is exclusively due to the activation of adenylate cyclase or not. In this research, the effect of ISO on the isometric tension and the mechanism of ISO-induced K+ channel activation were investigated in various patch clamp conditions. The summarized results are as follows. ISO- and pinacidil induced vasorelaxation was significantly inhibited by the application of TEA or by increasing the external K+ concentration. In the whole cell clamp mode, application of ISO increased K+ outward current, and this effect was completely eliminated by propranolol. In the cell-attached patch, application of ISO or forskolin increased Ca(2+)-activated K+ channel activity. Application of ISO to the bath in the outside-out patches or GTP in the inside-out patches stimulated Ca(2+)-activated K+ channels. From the above results, both A-kinase dependent channel phosphorylation and direct GTP-binding protein mediated effect might be responsible for the the activation of Ca(2+)-activated K+ channel by ISO in rabbit coronary smooth muscle cells. And this K+ channel activation also contributes to the ISO-induced vasorelaxation.

The effect of lovastatin on proliferation of cultured rat mesangial and aortic smooth muscle cells

In order to investigate the anti-proliferative effect of 3-hydroxy-3-methylglutaryl coenzyme. A reductase inhibitor, we evaluated the effects of lovastatin on DNA replication and the proliferation of rat mesangial and aortic smooth muscle cells, both of which were mesenchymal origin cells. Proliferations were determined by measuring [3H]thymidine uptake, and counting the number of cells. Growth-arrested mesangial and aortic smooth muscle cells were exposed to platelet-derived growth factor (PDGF), endothelin (ET) and angiotensin II (Ang II) to stimulate mitogenesis. All agents exhibited dose-dependent stimulation of [3H] thymidine uptake. PDGF was more potent than the others. Ang II increased [3H] thymidine uptake without demonstrable mitogenic activity. Lovastatin inhibited PDGF (10 ng/ml in mesangial cell, 25 ng/ml in smooth muscle cell)-, ET (10(-7)M)- and Ang II (10(-7)M)-induced [3H] thymidine uptake significantly in a dose-dependent manner in both cells. The increase of cell number in response to PDGF and ET treatment were also inhibited at 10 microM of lovastatin. The inhibitory effect of lovastatin was largely overcome in the presence of exogenous mevalonate at 200 microM, with 75.5% restoration from lovastatin-induced inhibition on PDGF-induced [3H] thymidine uptake in mesangial cells (77.8% in aortic smooth muscle cells). However, the addition of cholesterol did not prevent inhibition by lovastatin. In conclusion, lovastatin had an inhibitory effect on mesangial and aortic smooth muscle cell proliferation, and mevalonate was essential for DNA replication in both types of cells. Lovastatin may reduce glomerular and atherosclerotic injury through an anti-proliferative effect on mesangial and vascular smooth muscle cells, in addition to lowering circulating lipids.

Effects of mastoparan on a vascular contractility in rabbit aorta

Mastoparan is an amphiphilic tetradecapeptide derived from wasp venom which activates G-proteins. Several secondary effects have been attributed to this peptide, including activation of phospholipase and phosphatidylinositol kinase. The aim of the present study was to investigate the effects of mastoparan on vascular contractility. Rabbit aortic rings were cut and mounted on a force transducer to record isometric tension on a polygraph. The effects of mastoparan were then investigated on the contractile responses in the isolated rabbit aorta with or without endothelium. The results were summarized as follows; 1. Mastoparan caused biphasic response, a transient relaxation followed by a further contraction, in norepinephrine (NE)-precontracted ring with endothelium. These effects were not observed in the aorta in the absence of endothelium. 2. Mastoparan-induced transient relaxation was significantly inhibited by treatment with a N-omega-nitro-L-arginine or methylene blue. 3. When an inhibitor of phospholipase C, neomycin was added to the precontracted aortic ring with NE, the transient relaxation induced by mastoparan was inhibited, but sustained contraction was not inhibited. 4. When an inhibitor of phospholipase A2, quinacrine and inhibitor of the cyclooxygenase pathway, indomethacin, were added to a precontracted ring with NE, the transient relaxation induced by mastoparan was not inhibited, but sustained contraction was inhibited. 5. Mastoparan induced a contraction of the aorta either with or without endothelium. Indomethacin and nifedipine inhibited mastoparan-induced contraction. From the above results, we concluded that mastoparan acts on the endothelium and modifies the release of endothelium-derived relaxing factors such as nitric oxide and also endothelium-derived contracting factors such as metabolites of arachidonic acid.