Responses of fetal ovine systemic and umbilical arteries to angiotensin II.

Angiotensin II (ANG II) contracts umbilical arteries and has been hypothesized to regulate fetal blood pressure primarily by altering umbilical vascular resistance. To determine whether systemic arteries in term fetal sheep are sensitive to ANG II, isometric contraction of endothelium-intact isolated fetal renal, mesenteric, and umbilical arteries in response to ANG II was studied. ANG II (10(-7) M) elicited contractile responses in all three vessels (43 +/- 8%, 99 +/- 21%, and 105 +/- 5% of the maximal response seen with 90 mM KCl for renal, mesenteric, and umbilical arteries, respectively). The time course of the contractile responses differed among the vessels: renal and mesenteric arteries exhibited rapid transient contraction followed by relaxation, whereas umbilical artery displayed a more slowly developing but sustained contraction (1 +/- 0%, 3 +/- 1%,and 93 +/- 4% of maximal contractile response at 5 min, for renal, mesenteric, and umbilical arteries, respectively). The AT1 receptor antagonist, losartan (10(-6) M), abolished contractile responses in renal and mesenteric arteries but only slowed the contraction in umbilical artery in response to ANG II and had no effect on maximal tension. AT2 receptor blockade (PD 123319, 10(-7) M) had no significant effect on the response to ANG II in any vessel. Indomethacin (10(-6) M) significantly potentiated contraction to ANG II in renal and mesenteric but not umbilical arteries. Northern and Western blot analyses demonstrated the presence of AT1 mRNA and protein in all three vessels. Immunostaining for the AT1 receptor was present in endothelium and the tunica media. These findings demonstrate the AT1 receptor is present and functionally active in fetal systemic arteries and are consistent with previous findings that the umbilical circulation displays a greater responsiveness to ANG II than the systemic vasculature.

Ontogeny of CLCN3 chloride channel gene expression in human pulmonary epithelium.

Human fetal bronchopulmonary epithelia secrete liquid, and this chloride (Cl)-dependent process is important for normal lung growth. At the time of birth there is a maturational transition from a secretory to an absorptive phenotype. The pathways for Cl exit from the apical membrane which are required for fetal lung liquid secretion are unknown but are thought to be independent of the cystic fibrosis transmembrane conductance regulator. We determined the ontogeny of expression of the CLCN family of voltage-dependent Cl channel genes (CLCN2 through 6, K(a) and K(b)) in the human lung to identify potential pathways for pulmonary liquid secretion. Only CLCN3 and CLCN6 messenger RNA were detected by Northern analysis of fetal whole lung tissue. Ribonuclease protection assays confirmed the expression of CLCN3 and also revealed expression of CLCN2. The ontogeny of expression of these two channels was similar, peaking in midgestation and declining postnatally. In situ hybridization localized the CLCN2 and CLCN3 messages to airway and distal pulmonary epithelia and to pulmonary blood vessels. We conclude that CLCN3 is expressed in human airway epithelia and expression is developmentally regulated. The contribution of these channels to pulmonary epithelial liquid transport and lung development remains to be determined.

Role of Cl(-) channels in alpha-adrenoceptor-mediated vasoconstriction in the anesthetized rat.

In vitro studies have provided evidence that Cl(-) ion currents are important for activation of vascular smooth muscle contraction. The stilbene, 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), disrupts Cl(-) metabolism by blocking Cl(-) channels and by inhibiting Cl(-) bicarbonate exchange. The aims of this study were to: (i) characterize the hemodynamic responses produced by DIDS in pentobarbital anesthetized rats, and (ii) examine vasoconstrictor responses to norepinephrine before and after administration of DIDS. DIDS (2.5-50 micromol/kg, 92.5 micromol/kg total dose, i.v.) produced dose-dependent but transient reductions in mean arterial blood pressure and in hindquarter, renal and mesenteric vascular resistances. Prior to the administration of DIDS, norepinephrine (1. 0-5.0 microgram/kg, i.v.) produced dose-dependent increases in mean arterial pressure, renal resistance and mesenteric resistance, but decreases in hindquarter resistance that were inversely related to dose. After administration of DIDS, the peak pressor responses produced by norepinephrine were either slightly diminished (1.0, 2.5 microgram/kg) or unchanged (5.0 microgram/kg). Peak norepinephrine-induced changes in hindquarter and renal vascular resistance were unaffected by DIDS, while increases in mesenteric resistance were augmented. The total norepinephrine-induced increases in mean arterial pressure (mm Hgxs) were markedly reduced by DIDS. These effects of DIDS on norepinephrine-induced responses were similar, but not identical to those of the voltage-sensitive Ca(2+) channel blocker, nifedipine (500 nmol/kg, i.v.). These findings suggest that DIDS may interfere with norepinephrine-induced depolarization of resistance arteries, thereby preventing activation of voltage-sensitive Ca(2+) channels.

Complex RNA processing of TDRKH, a novel gene encoding the putative RNA-binding tudor and KH domains.

The sequence from a human EST (IMAGE:259322) with homology to the nucleotide-sensitive chloride conductance regulator (ICln) was used to screen a human aortic cDNA library. The probe sequence was from a region of the EST lacking homology to ICln, and the goal was to isolate an ICln-like gene. A 2843bp cDNA clone with an open reading frame coding for a 561 amino acid protein was isolated. This clone had no homology to ICln. PROSITE analysis of the putative protein sequence reveals one tudor and two K homology (KH) domains. The gene has therefore been named TDRKH. Both KH and tudor motifs are involved in binding to RNA or single-strand DNA. PCR analysis demonstrated that TDRKH is alternatively spliced in several ways and alternatively polyadenylated at multiple sites. Northern analysis confirmed the presence of messages of multiple lengths with predominant bands at 2.8 and 4.0 kb and also demonstrated that TDRKH is widely expressed in human tissues. Within an intron of TDRKH, there is a region with 90% homology to ICln. This sequence, which is incorporated into the alternatively spliced message represented by IMAGE:259322, contains a 2 bp deletion that disrupts the ICln reading frame and therefore represents an ICln pseudogene. The TDRKH gene was mapped to the Epidermal Differentiation Complex (EDC) at chromosome 1q21 by radiation hybrid mapping and STS content of genomic clones from that region. The EDC contains a large cluster of related genes involved in terminal differentiation of the epidermis. It remains to be determined whether TDRKH has a specific role in epithelial function.

Endothelium modulates anion channel-dependent aortic contractions to iodide.

Anion currents contribute to vascular smooth muscle (VSM) membrane potential. The substitution of extracellular chloride (Cl) with iodide (I) or bromide (Br) initially inhibited and then potentiated isometric contractile responses of rat aortic rings to norepinephrine. Anion substitution alone produced a small relaxation, which occurred despite a lack of active tone and minimal subsequent contraction of endothelium-intact rings (4.2 +/- 1.2% of the response to 90 mM KCl). Endothelium-denuded rings underwent a similar initial relaxation but then contracted vigorously (I > Br). Responses to 130 mM I (93.7 +/- 1.9% of 90 mM KCl) were inhibited by nifedipine (10(-6) M), niflumic acid (10(-5) M), tamoxifen (10(-5) M), DIDS (10(-4) M), and HCO(-)(3)-free buffer (HEPES 10 mM) but not by bumetanide (10(-5) M). Intact rings treated with N(omega)-nitro-L-arginine (10(-4) M) responded weakly to I (15.5 +/- 2.1% of 90 mM KCl), whereas hemoglobin (10(-5) M), indomethacin (10(-6) M), 17-octadecynoic acid (10(-5) M), and 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one (10(-6) M) all failed to augment the response of intact rings to I. We hypothesize that VSM takes up I primarily via an anion exchanger. Subsequent I efflux through anion channels having a selectivity of I > Br > Cl produces depolarization. In endothelium-denuded or agonist-stimulated vessels, this current is sufficient to activate voltage-dependent calcium channels and cause contraction. Neither nitric oxide nor prostaglandins are the primary endothelial modulator of these anion channels. If they are regulated by an endothelium-dependent hyperpolarizing factor it is not a cytochrome P-450 metabolite.

Expression of CLCN voltage-gated chloride channel genes in human blood vessels.

Chloride (Cl) ion channels play a critical role in the response of both vascular smooth muscle (VSM) and endothelial (ENDO) cells to agonist stimulation. In VSM, agonist-induced Cl currents produce membrane depolarization, resulting in calcium influx through voltage-sensitive channels. ENDO cells also activate Cl currents after either agonist application or perturbation of cell volume. Although some of these currents have been characterized biophysically, the genes involved have not been identified. The CLCN family of voltage-dependent Cl channel genes comprises nine members (CLCN1-7, Ka and Kb) which demonstrate quite diverse functional characteristics while sharing significant sequence homology. We used Northern-blot analysis to study the expression of these Cl channel genes in cultured human aortic and coronary VSM cells and in aortic ENDO cells. CLCN3 is by far the most abundant CLC channel mRNA in both VSM and ENDO cells. Lower levels of expression are seen for CLCN2, CLCN4, CLCN5 and CLCN6. Expression levels were similar in VSM and ENDO cells except for CLCN4 which was more highly expressed in ENDO cells. In situ hybridization was used to confirm the expression of CLCN3 in intact human fetal lung. CLCN3 message was seen in VSM and ENDO cells of both large and small pulmonary vessels, indicating that their detection by Northern blotting was not an artifact of cell culture. CLCN3 is also expressed in pulmonary epithelial and bronchial smooth muscle cells but not in chondrocytes or pulmonary interstitial cells. Recent studies suggest that CLCN3 may encode the swelling-induced Cl conductance. We used whole cell patch clamp recording to demonstrate swelling-induced Cl currents in these cultured VSM cells. This suggests that the CLCN3 protein is expressed; however, the functional role of this current in VSM remains to be determined.

Chloride ion currents contribute functionally to norepinephrine-induced vascular contraction.

Norepinephrine (NE) increases Cl- efflux from vascular smooth muscle (VSM) cells. An increase in Cl- conductance produces membrane depolarization. We hypothesized that if Cl- currents are important for agonist-induced depolarization, then interfering with cellular Cl- handling should alter contractility. Isometric contraction of rat aortic rings was studied in a bicarbonate buffer. Substitution of extracellular Cl- with 130 mM methanesulfonate (MS; 8 mM Cl-) did not cause contraction. NE- and serotonin-induced contractions were potentiated in this low-Cl- buffer, whereas responses to K+, BAY K 8644, or NE in the absence of Ca2+ were unaltered. Substitution of Cl- with I- or Br- suppressed responses to NE. Inhibition of Cl- transport with bumetanide (10(-5) M) or bicarbonate-free conditions (10 mM HEPES) inhibited NE- but not KCl-induced contraction. The Cl--channel blockers DIDS (10(-3) M), anthracene-9-carboxylic acid (10(-3) M), and niflumic acid (10(-5) M) all inhibited NE-induced contraction, whereas tamoxifen (10(-5) M) did not. Finally, disruption of sarcoplasmic reticular function with cyclopiazonic acid (10(-7) M) or ryanodine (10(-5) M) prevented the increase in the peak response to NE produced by low-Cl- buffer. We conclude that a Cl- current with a permeability sequence of I- > Br- > Cl- > MS is critical to agonist-induced contraction of VSM.

The endothelium modulates the contribution of chloride currents to norepinephrine-induced vascular contraction.

Activation of a Cl- current is critical to agonist-induced activation of rat aortic smooth muscle contraction. Substituting extracellular Cl- with 130 mM methanesulfonate (8 mM Cl-) increases the contractile response to norepinephrine (NE) but not to KCl. We hypothesized that endothelial factors modulate this effect. Removing the endothelium (rubbing) or treatment with N-nitro L-arginine (L-NNA) markedly increased the potentiation of NE-induced contraction by low-Cl- buffer. Indomethacin had no effect. The previously demonstrated ability of Cl--channel blockers (DIDS, anthracene-9-carboxylic acid, niflumic acid) or Cl- transport inhibitors (bumetanide, bicarbonate-free buffer) to inhibit responses to NE was not altered by L-NNA. Low-Cl- buffer alone did not contract intact rings but produced nifedipine-sensitive contractile responses after rubbing or L-NNA treatment. These data suggest that the Cl- conductance of smooth muscle in intact blood vessels is low but increases with withdrawal of reduced nitric oxide (NO') or agonist stimulation. Rubbing or L-NNA increased the sensitivity of rings to KCl but not to NE. Nifedipine reduced both sensitivity and maximum response to NE in intact vessels. L-NNA increased the maximum response to NE in nifedipine-treated rings without changing sensitivity. We conclude that although NO' affects both the voltage-dependent and voltage-independent components of contraction, sensitivity to NE is determined by the voltage-dependent portion. The voltage change required for a full response to NE is dependent on activation of a Cl- current that may be under the tonic regulatory influence of NO'.

Calcium-activated chloride current in rabbit coronary artery myocytes.

Whole-cell patch-clamp techniques were used to study enzymatically dispersed epicardial coronary artery smooth muscle cells. Depolarizing voltage pulses of 500-millisecond duration from -60 mV (118 mmol/L CsCl, 22 mmol/L tetraethylammonium chloride, and 5 mmol/L EGTA pipette solution) elicited inward L-type calcium currents (ICa). When EGTA was omitted from the pipette solution, an outward current was superimposed on the calcium current, and repolarizing voltage steps produced an inward tail current (IT). The amplitude of these inward currents was proportional to the ICa amplitude from -30 to +50 mV. The time course of decay of the current was well fit by a single exponential equation. The time constant (tau) of this equation did not change with the size of IT but was clearly voltage dependent (shorter at more negative potentials). Changing the chloride reversal potential from -1.3 to -39.7 mV by anion substitution using methanesulfonate as the chloride replacement in the pipette solution shifted the zero current level of IT from 0.9 +/- 0.56 to -33.1 +/- 0.85 mV. The tail current was blocked by nifedipine (10(-6) mol/L) and by isosmolar calcium substitution with barium in the bath solution and was enhanced by the dihydropyridine agonist Bay K 8644 (10(-6) mol/L). IT was also blocked by the chloride channel blockers DIDS (10(-4) mol/L) and niflumic acid (10(-5) mol/L). Caffeine (10(-2) mol/L), which releases intracellular calcium stores, caused an inward current at holding potentials (-60 mV), which was inhibited by DIDS. Caffeine also inhibited subsequent attempts to elicit IT by depolarizing pulses (88% reduction in IT).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of ramipril on alpha-adrenoceptor-mediated oscillatory contractions in tail artery of hypertensive rats.

Recent studies indicate that norepinephrine-induced contractile oscillations in the tail artery from stroke-prone spontaneously hypertensive rats (SHRSP) may be a vascular phenomenon independent of blood pressure level. The objectives of this study were: (1) to characterize pharmacologically the alpha-adrenoceptor mediating norepinephrine-induced oscillations in tail artery; and (2) to investigate the relationship between blood pressure level, altered by treatments with hydralazine/hydrochlorothiazide or the angiotensin converting enzyme inhibitor ramipril, and the observation of norepinephrine-induced oscillations in tail artery. The alpha 2-adrenoceptor agonists clonidine and guanabenz potently stimulated oscillatory contractions in the tail artery while the alpha 1-adrenoceptor agonists phenylephrine and methoxamine were considerably less potent. Yohimbine, an alpha 2-adrenoceptor antagonist, but not the alpha 1-adrenoceptor antagonist prazosin demonstrated high affinity for the receptor mediating norepinephrine-induced oscillatory contractions. These results support the hypothesis that norepinephrine-induced oscillatory contractions in the tail artery from SHRSP occur primarily through stimulation of alpha 2-adrenoceptors. Ramipril lowered blood pressure in SHRSP after 4 weeks of treatment during 6-10 weeks of life but did not alter the ability of the alpha 2-adrenoceptor agonist clonidine (10(-5) M) to induce contractile oscillations in tail arteries from SHRSP, indicating these oscillations are not a secondary effect of high blood pressure. These studies suggest that norepinephrine-induced oscillations in tail artery from SHRSP may be a vascular trait separate and distinct from blood pressure level and angiotensin II expression early in life.

Potassium conductance and oscillatory contractions in tail arteries from genetically hypertensive rats.

Tail arteries isolated from the stroke-prone substrain of the spontaneously hypertensive rat (SHR-SP) exhibit oscillatory contractile responses to norepinephrine. Simultaneous recording of force generation and membrane potential (Em) has previously demonstrated that the contractile phase of these oscillations is associated with bursts of calcium-dependent action potentials. The smooth muscle cells are electrically quiescent during the relaxation phase of the oscillations. The present studies were designed to test the hypothesis that this quiescent period results from the stimulation of a calcium-activated potassium conductance (gKCa) in the cells responsible for triggering the bursting activity. Isolated tail artery strips from SHR-SP and Wistar-Kyoto rats (WKY) were prepared for measurement of isometric force generation or for simultaneous recording of force and Em. The channel-specific toxins apamin (4 x 10(-7) mol/l) and charybdotoxin (4.7 x 10(-8) did not alter the oscillatory pattern of contraction in response to norepinephrine. Oscillations were converted to sustained contraction by barium (10(-4) mmol), quinidine (5.8 x 10(-5) mmol) and elevation of extracellular potassium (20 mmol/l). Em recordings show that both potassium and barium convert bursting activity into tonic firing. Only 20 mmol/k+ caused significant depolarization in addition to that produced by norepinephrine. In contrast, quinidine appears to alter oscillatory behavior by interfering with calcium-spike generation. Norepinephrine-induced electrical activity is diminished in the presence of quinidine. These results suggest that potassium conductance plays an important role in controlling Em, electrical spiking and therefore oscillatory contractile activity in response to norepinephrine in the tail arteries of SHR-SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Regenerative electrical activity and arterial contraction in hypertensive rats.

Isolate tail arteries from spontaneously hypertensive rats-stroke prone strain (SHRSP) display oscillatory contractile responses to norepinephrine. These oscillations are not observed in tail arteries from normotensive Wistar-Kyoto rats (WKY). The mechanism underlying these oscillatory contractions was investigated by simultaneous measurement of isometric force development and membrane potential (Em) from tail artery strips in vitro. After equilibration in physiological salt solution containing 1.6 mM calcium (37 degrees C), resting Em was not different between WKY (-52 +/- 1.1 mV) and SHRSP (-52 +/- 0.4 mV). Norepinephrine (3 x 10(-7) M) produced a similar degree of depolarization in tissues from the two strains (WKY = (-42.5 +/- 0.9, SHRSP = -41 +/- 0.8). However, while Em recordings from WKY arteries were quiescent, those from SHRSP displayed bursts of electrical spiking activity that were temporally associated with the rising phase of oscillations in contractile force. The frequency and duration of these bursts of action potentials increased with the concentration of norepinephrine. Action potentials were not observed in calcium-free solution or in presence of nifedipine (3 x 10(-7) M). Releasing the passive stretch on the tissues caused a decrease in the rate of spiking. These studies demonstrate catecholamine-induced regenerative electrical activity in tail arteries from SHRSP that is dependent on extracellular calcium. This activity is unique to tail arteries from this strain.

Calcium and contractile responses to ouabain and potassium-free solution in aortae from spontaneously hypertensive rats.

This study examines the role of calcium in contractions induced by Na+, K+ ATPase inhibition in blood vessels from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Helical strips of aortae from SHR contract more rapidly in response to ouabain or potassium-free conditions than those from WKY rats. Dose-response curves to calcium in SHR aortae treated with 10(-3) mol/l ouabain were shifted to the left of those in WKY. Treatment with 10(-3) mol/l EGTA shifted dose-response curves to calcium in ouabain-treated strips to the left in both strains. The magnitude of the leftward shift induced by EGTA was greater in WKY aortic strips than in those from SHR. Similarly, treatment with EGTA increased the rate of contractile responses to potassium-free solution in WKY aortae to a greater extent than in SHR aortae. Verapamil (10(-6) mol/l) depressed contractions induced by ouabain and potassium-free solution and abolished the differences between SHR and WKY aortae in terms of contraction rate. Calcium-free conditions completely blocked contractions caused by sodium pump inhibition. These results suggest that the difference in responsiveness to sodium pump inhibition in SHR and WKY rats results from an alteration in calcium entry through verapamil-sensitive calcium channels.

Neonatal resuscitation.

Three decades of ongoing research and obstetric and pediatric education have seen neonatal resuscitation develop into a well-organized delivery room procedure. Because neonatal resuscitation does not occur frequently in the Emergency Department, few are well prepared. A designated site in the Emergency Department, trained personnel, appropriate equipment and well defined procedures are necessary. These recommendations for the organization of the resuscitation site, procedures, therapeutic drugs, and required equipment must be individualized to each Emergency Department.

Cyclosporine augments reactivity of isolated blood vessels.

Administration of cyclosporine (CS) as an immunosuppressive agent in clinical transplantation is associated with multiple side effects including nephrotoxicity and hypertension. These two effects could be related in that the renal changes may be secondary to alterations in organ blood flow. The present studies investigate the ability of CS to augment contractile responsiveness in blood vessels from normotensive rats. Isometric force generation was measured in isolated tail arteries and portal veins. CS (8.3 X 10(-6)M) potentiated tail artery contractile responses to sympathetic nerve stimulation, exogenous norepinephrine, and increases in extracellular potassium concentration. Portal veins undergo spontaneous contractions which are related to the firing of calcium-driven action potentials in the smooth muscle cells. CS significantly increased the frequency of these spontaneous contractile events. These results suggest that components of CS toxicity may involve a direct action on vascular smooth muscle and/or on vascular adrenergic neurotransmission.

Free radical-mediated endothelial damage in blood vessels after electrical stimulation.

The endothelium plays an important role in mediating vasodilator effects of several agents (acetylcholine, thrombin, A23187, etc.). The goal of this study was to determine the ability of oxygen free radicals generated by electrical field stimulation to alter endothelial function in isolated tissue systems. Tail artery strips and the mesenteric microvasculature isolated from Sprague-Dawley rats were used. Following smooth muscle contraction induced by norepinephrine, these preparations relaxed in response to acetylcholine chloride or ionophore A23187. All vessels were then subjected to electrical stimulation (9 V, 1-2 ms, 4 Hz) of the physiological buffer in which they were bathed or perfused. In some of these preparations, an antioxidant, (10(-4) M sodium ascorbate, 3.6 X 10(-5) M glutathione, 1.3 X 10(-2) M dimethyl sulfoxide) was included in the buffer. Relaxation responses persisted in vessels where an antioxidant had been included in the electrically stimulated buffer. Tissues stimulated without this protection did not relax on subsequent exposures to endothelium-dependent vasodilators. Scanning-electron microscopy of the tissues revealed significant endothelial damage (cell membrane pitting) in tissues exposed to electrical stimulation without antioxidant protection. These results suggest that electrical stimulation causes endothelial damage in isolated vascular preparations. This seemingly adverse effect proves to be a useful tool for removing the endothelium in studies of isolated vascular tissues.

Contractile responses to ouabain and potassium-free solution in vascular tissue from renal hypertensive rats.

This study characterizes contractions to ouabain and potassium-free solution in isolated vascular segments from two-kidney, one clip (2-K, 1C) hypertensive rats. Aorta, mesenteric artery, and vena cava from hypertensive rats were more sensitive (lower threshold) to ouabain than those from normotensive rats. Contractions of hypertensive vascular segments to potassium-free solution and to ouabain (10(-3) mol/l) were faster than those in normotensive vessels. Monensin potentiated contractions to ouabain and increased the rate of force development to potassium-free solution to a greater extent in normotensive aortae than in hypertensive aortae. Amiloride, low sodium solution, verapamil and calcium-free solution depressed contractions to ouabain and potassium-free solution in both hypertensive and normotensive aortae. These observations demonstrate augmented responsiveness to ouabain and potassium-free solution in hypertensive blood vessels. Interventions which influence transmembrane sodium and calcium movements altered contractions to ouabain and potassium-free solution. The results are consistent with the hypothesis that vascular cells of hypertensive rats have enhanced sodium pump activity.

Calcium influx and vascular reactivity in systemic hypertension.

Numerous studies have focused on functional vascular changes that characterize the hypertensive state. Recent evidence that suggests that increased vascular reactivity in hypertension is due to changes in the delivery of activator Ca++ through channels in the cell membrane will be reviewed. The primary evidence supporting this hypothesis comes from studies that characterize the effects of Ca++-free solution and calcium channel blockers on contractile properties of isolated vascular smooth muscle. In the present study, experiments were performed to investigate the role of Ca++ influx in vascular contractions produced by interventions that cause membrane depolarization. Isometric tension development in helical strips of carotid arteries from stroke-prone spontaneously hypertensive rats in response to elevated K+ and tetraethylammonium chloride was greater than that in carotid arteries from Wistar-Kyoto normotensive rats. The rate of tension development to K+-free solution in carotid arteries from stroke-prone spontaneously hypertensive rats was faster than in Wistar-Kyoto normotensive rat arteries. Contractile responses to all 3 depolarizing interventions were reduced in arterial strips incubated in Ca++-free solution containing the chelator ethylene glycol bis-(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid and in arterial strips treated with the Ca++ channel blocker verapamil. These results are consistent with the hypothesis that constrictor stimuli that produce membrane depolarization cause an opening of Ca++ channels in the plasma membrane that are sensitive to the organic channel blockers. Further, a change in Ca++ permeability or membrane depolarizing mechanisms contributes to increased contractile responsiveness in carotid arteries of stroke-prone spontaneously hypertensive rats.

Calcium and vascular smooth muscle membrane in hypertension.

A major focus of past and recent research in hypertension has been on the characterization of the nature of the vasculature changes which lead to the observed increase in total peripheral resistance responsible for the elevation of arterial pressure. Here we survey recent evidence which suggests that altered handling of calcium is a primary membrane defect in hypertension. Evidence for the primacy of this defect is provided by studies demonstrating a reduced ability of the membrane to bind calcium in diverse tissues from hypertensive animals. The reduced calcium binding ability appears to be responsible for a greater membrane permeability to monovalent and divalent cations. This greater permeability contributes to the increased sensitivity to vasoconstrictor stimuli of vascular smooth muscle in hypertension.