Does long-term experimental antiorthostasis lead to cardiovascular deconditioning in the rat?

Microgravity or simulated microgravity induces acute and chronic cardiovascular responses, whose mechanism is pivotal for understanding of physiological adaptation and pathophysiological consequences. We investigated hemodynamic responses of conscious Wistar rats to 45? head-down tilt (HDT) for 7 days. Arterial blood pressure (BP) was recorded by telemetry. Heart rate (HR), spectral properties and the spontaneous baroreflex sensitivity (sBRS) were calculated. Head-up tilt (HUT) was applied for 2 h before and after HDT to assess the degree of any possible cardiovascular deconditioning. Horizontal control BP and HR were 112.5+/-2.8 mmHg and 344.7+/-10 bpm, respectively. HDT elicited an elevation in BP and HR by 8.3 % and 8.8 %, respectively, in less than 1 h. These elevations in BP and HR were maintained for 2 and 3 days, respectively, and then normalized. Heart rate variability was unchanged, while sBRS was permanently reduced from the beginning of HDT (1.01+/-0.08 vs. 0.74+/-0.05 ms/mmHg). HUT tests before and after HDT resulted in BP elevations (6.9 vs. 11.6 %) and sBRS reduction (0.44 vs. 0.37 ms/mmHg), respectively. The pressor response during the post-HDT HUT test was accompanied by tachycardia (13.7 %). In conclusion, chronic HDT does not lead to symptoms of cardiovascular deconditioning. However the depressed sBRS and tachycardic response seen during the post-HDT HUT test may indicate disturbances in cardiovascular control.

Nitric oxide modulates the interaction of pressure-induced wall mechanics and myogenic response of rat intramural coronary arterioles.

Interactions between the biomechanical characteristics and pressure-induced active response of coronary microvessels are still not well known. We tested the hypothesis that pressure-dependent biomechanical characteristics of the coronary vascular wall are modulated by the active myogenic response and local vasodilators. We have utilized data obtained previously in isolated rat intramural coronary arterioles (approximately 100 microm in diameter), in which the diameter was investigated as a function of intraluminal pressure (Szekeres et al.: J. Cardiovasc. Pharmacol., 43, 242-249, 2004). To characterize the magnitude of myogenic response, diameter was expressed as percent of passive diameter as a function of pressure (normalized diameter; ND). In addition, circumferential wall stress (WS) and incremental distensibility (ID) were calculated. In control conditions, after an initial increase between 0-30 mm Hg, ND decreased substantially as pressure increased from 30 to 150 mm Hg. Correspondingly, WS gradually increased as a function of pressure (from 0.3 +/- 0.03 to 34.7 +/- 4.4 kPa) exhibiting a plateau phase between 40-80 mm Hg. In contrast, ID decreased and reached negative values (min: -104.9 +/- 21.9 10(-6) m2/N at 50 mm Hg). Inhibition of nitric oxide (NO) synthase by L-NNA decreased basal diameter (approximately 35% at 2 mm Hg), eliminated pressure-induced changes in ND, reduced the slope of pressure-WS curve, and decreased ID at lower pressures. Simultaneous administration of L-NNA and adenosine (which restored initial diameter, i.e. length of smooth muscle) restored--in part--the pressure-induced reduction in ND, reversed the pressure-induced behavior of WS to control, but not that of ID. These results not only confirm that in coronary arterioles wall stress is regulated by the myogenic response, but also suggest that there is interplay between the mechanical behavior of the wall and the myogenic response. Furthermore, the presence of NO seems to be necessary for maintaining a higher distensibility of intramural coronary arterioles allowing increases in diameter to lower pressures, which then activate the myogenic mechanism resulting in constrictions and full development of myogenic tone, as indicated by the presence of negative slope of pressure-diameter curve in the presence of NO.

Pharmacologic inhomogeneity between the reactivity of intramural coronary arteries and arterioles.

We hypothesized that because of their size, anatomic location, and hemodynamic function, coronary arteries and arterioles would respond differently to vasoactive substances. Intramural arteries (281.7 +/- 23.1 microm) and arterioles (77.3 +/- 6.6 microm) of the left anterior descending coronary of rats were isolated and cannulated. Spontaneous tone was lower in arteries than in arterioles (81.1 +/- 5.7 vs. 53.0 +/- 3.9% of passive diameter, p < 0.05 at 60 mm Hg intraluminal pressure). Arterial tone was adjusted by the thromboxane receptor agonist U46619 (5 x 10(-8) M ) to reach an active tone close to that of arterioles. Bradykinin elicited dilations in both types of vessels. Acetylcholine (10(-6) - 10(-5) M ) dilated arteries (by 42.6 +/- 11.5 microm) but constricted arterioles (by 16.4 +/- 9.3 microm). Sodium nitroprusside and adenosine elicited significantly greater dilations in arterioles than in arteries (by 7.9 and 11.9%, respectively, p < 0.05), whereas dilations to norepinephrine were similar. Inhibition of nitric oxide synthesis caused a significantly smaller constriction in arteries (10.2 +/- 3.31%) than in arterioles (31.6 +/- 6.9%) and completely blocked bradykinin-and acetylcholine-induced dilations, whereas it did not affect dilations to sodium nitroprusside, adenosine, and norepinephrine. Compared with arteries, arterioles have a greater spontaneous tone and enhanced nitric oxide modulation of basal tone and exhibit greater responsiveness to nitric oxide and adenosine. In addition, nitric oxide synthase is activated differently by pharmacologic stimuli in these segments. The qualitative and quantitative differences among vasoactive responses of coronary arteries and arterioles demonstrated in this study suggest segment-specific roles for endothelial and metabolic factors in regulation of coronary vascular resistance.

Concomitant accumulation of intracellular free calcium and arachidonic acid in the ischemic-reperfused rat heart.

This study was designed to elucidate the relationship between enhanced cytoplasmic calcium levels (Ca2+i) and membrane phospholipid degradation, a key step in the loss of cellular integrity during cardiac ischemia/reperfusion-induced damage. Isolated rat hearts were subjected to 15 min ischemia followed by 30 min reperfusion. Ca2+i was estimated by the Indo-1 fluorescence ratio technique. Degradation of membrane phospholipids as indicated by the increase of tissue arachidonic acid content was assessed in tissue samples taken from the myocardium at various points of the ischemia/reperfusion period. The hemodynamic parameters showed almost complete recovery during reperfusion. Fluorescence ratio increased significantly during ischemia, but showed a considerable heart-to-heart variation during reperfusion. Based upon the type of change of fluorescence ratio during reperfusion, the hearts were allotted to two separate subgroups. Normalization of fluorescence ratio was associated with low post-ischemic arachidonic acid levels. In contrast, elevated fluorescence ratio coincided with enhanced arachidonic acid levels. This observation is suggestive for a relationship between the Ca2+-related fluorescence ratio and arachidonic acid accumulation probably due to a calcium-mediated stimulation of phospholipase A2.

Evidence for the expression of cyclooxygenase-2 enzyme in periodontitis.

We investigated the role of the inducible isoform of cyclooxygenase (COX-2) in a rat model of periodontitis using a selective COX-2 inhibitor NS-398. Periodontitis was produced by a silk ligature placed around the lower left 1st molar. Animals were treated with NS-398 (3 mg kg(-1) i.p., 2 times per day for 7 days) or vehicle. At Day 8, the gingivomucosal tissues encircling the mandibular 1st molars were removed on both sides for COX-2 immunohistochemistry, measurement of plasma extravasation by the Evans blue technique, and alveolar bone loss by videomicroscopy. Immunohistochemical analysis revealed numerous strongly COX-2-positive cells in the subepithelial tissues in the ligated side and only a few COX-2-reactive cells in the contralateral (control) side. Ligation significantly increased Evans blue extravasation in the gingivomucosal tissue and alveolar bone destruction compared to the control side. NS-398 treatment significantly reduced the plasma extravasation and alveolar bone resorption of the ligated side compared to vehicle administration. The present results suggest that COX-2 is induced by periodontitis, and plays an important role in gingival inflammation and alveolar bone destruction. In a previous study (Br J Pharmacol 1998;123:353-60) we found the expression of the inducible isoform of nitric oxide synthase in this model. Therefore, based on our own data and the literature, we propose that selective inhibition of these inducible enzymes might be a basis for adjunctive therapy, or new therapeutic approaches in periodontitis.

Segmental differences in geometric, elastic and contractile characteristics of small intramural coronary arteries of the rat.

The depedence of elastic and contractile properties on the caliber of small intramural coronary arteries was investigated in the rat in vitro. Different segments of the left anterior descending coronary artery branching system were prepared for microarteriography. The segments were cannulated at both ends, immersed in oxygenated normal Krebs Ringer (nKR) solution. Intraluminal pressure was changed at a rate of about 0.5 mm Hg/s between 0 and 150 mm Hg in repeated cycles. The outer diameter was continuously measured with microangiometry. Pressure-diameter curves were recorded after preconditioning pressure cycles in nKR, with PGF2alpha in the bath (7.5 x 10(-6) M), and in maximal relaxation with papaverine (2.8 x 10(-4) M). Biomechanical parameters were computed for vessels grouped according to their calibers (inner diameters: 50-150, 150-250, 250-350, >350 microm). Distensibility and contractility decreased with increasing caliber of the vessels, while the elastic modulus increased. Spontaneous tone was (at 100 mm Hg in mechanically preconditioned vessels) 18.8 +/- 4.5, 8.4 +/- 4.4, 9.7 +/- 3.7 and 8.3 +/- 3.8% in the four groups, respectively. PGF2alpha contraction was maximal around the 300- microm caliber. Our study is the first direct demonstration that intramural small coronary arteries exhibit characteristic variability in their elastic and contractile properties as a function of their caliber. Such differences may be important in segmentally specific control processes of the coronary microcirculation.

Effect of a new nitric oxide donor on the biomechanical performance of the isolated ischaemic rat heart.

The effect of a new nitric oxide (NO) donor, a meso-ionic 3-aryl substituted oxatriazole-5-imine derivative, GEA 3162 was studied on constant flow-perfused ischaemic Langendorff rat heart. The perfusion was kept constant at a rate of 16 mL min-1. Ischaemia was induced by a low flow rate of 0.8 mL min-1 for 30 min, and was followed by a 40-minute reperfusion. In the first set of experiments the effects of GEA 3162-infusion were examined on perfusion pressure, left ventricular pressure, heart rate and left ventricular dP/dt. GEA 3162 infusion did not affect the pre-ischaemic maximum of left ventricular pressure. During reperfusion, maximal left ventricular pressure, maximal and minimal dP/dt values in the GEA 3162-treated group significantly exceeded those of the untreated controls (by 19.3, 36.0 and 18.0%, respectively). During reperfusion, perfusion pressure increased continuously in the control group indicating an increasing coronary resistance, but it was kept at a continuous low level with GEA 3162 treatment. In a second set of experiments bradykinin was infused in order to test the endothelial function before ischaemia and during late reperfusion. Bradykinin elicited significant vasodilation in the control group during reperfusion, meanwhile it did not cause further change in coronary resistance in the GEA 3162-infused group. We suggest, that GEA 3162 may have a protective effect on isolated rat heart in ischaemia and reperfusion, that results in an improved cardiac performance compared with untreated hearts.

Somatostatin induces vasodilatation in the cat mesenteric artery via endothelium-derived nitric oxide and prostaglandins.

Direct vascular effects of somatostatin (ST) were investigated in cat superior mesenteric artery (SMA) segments in vitro. Changes of outer diameter were measured at constant axial length and perfusion pressure. SMA segments in the resting state were not affected by ST, regardless of endothelial integrity. Noradrenaline-preconstricted SMA segments were dilated concentration-dependently by ST (EC50 10(-13) mol/l). At maximal dilatation (by 10(-11) mol/l ST) the preconstriction was diminished to 45 +/- 9% (P<0.001). Perfusion with Triton X-100, or NG-nitro-L-arginine nearly abolished the ST-induced dilatation, while indomethacin treatment partially suppressed it. We conclude that ST dilates preconstricted cat SMA segments mainly via the endothelial release of nitric oxide, and additionally via prostaglandins.

Nitric oxide-dependent opposite effects of somatostatin on arterial and venous caliber in situ.

The vascular effects of somatostatin (ST) and its mechanism of action are not well understood. In the present study, we investigated the direct effects of ST on the vascular tone of rat saphenous artery and vein using videomicroangiometry in situ. ST was administered either in superfusion or in infusion. We found opposite effects in arteries and veins: ST (10(-12)-10(-7) M) dilated the artery (outer diameter increased from 533 +/- 28 to 600 +/- 29 microns, administered in superfusion) and contracted the vein (from 709 +/- 26 to 640 +/- 26 microns and from 775 +/- 30 to 708 +/- 60 microns in superfusion and infusion, respectively). These effects of ST were completely abolished after deendothelization (air bolus maintained for 6 min in vessel lumen) and after local infusion of NG-nitro-L-arginine (L-NNA; 10(-4) M), a nitric oxide (NO) synthesis inhibitor. An NO-dependent basal vasodilator tone in the rat saphenous vein responsible for 10.9 +/- 0.3% of the total vessel diameter was found. After ST administration the venous diameter reduction was similar to that measured after deendothelization or L-NNA. We conclude that ST in situ induces NO release from endothelial cells of rat saphenous artery causing vasodilation, whereas, in contrast, it inhibits the basal NO-dependent vasodilator tone of the saphenous vein inducing vasoconstriction.

Regional differences in nitric oxide-dependent vascular responses to somatostatin.

The mechanisms of the vascular effects of somatostatin (ST) are not well known. This study compares the direct effect of ST in different vascular regions and species. Isolated perfused segments of the cat superior mesenteric artery in vitro did not exhibit a vascular response in the resting state, however, ST-induced vasodilatation was observed with norepinephrine preconstriction. In contrast, ST only slightly dilated superior mesenteric vein segments. In the artery, NG-nitro-L-arginine inhibited both ST and endothelium-dependent nitric oxide (NO) mediated response. No regular dose-response curves were found when ST was applied on the large mesenteric artery in the cat, but rings of small mesenteric artery from both cats and dogs exhibited dose-dependent relaxations. These effects were also NO-dependent. Local application of ST on the rat saphenous artery in situ elicited NO-mediated dose-dependent vasodilatation. However, ST constricted rat saphenous veins in the case of either adventitial or intraluminal application. It is concluded that ST exerts different actions on the arterial and the venous vessel wall. The major response in arteries is endothelium-mediated vasodilatation seen in various species and vascular beds. Large and small arteries respond differently to ST but these differences require further elucidation.

[The role of vascular nitric oxide (NO) in the pathomechanism of pre-eclampsia]. / A vascularis nitrogén-oxid (NO) szerepe a praeeclampsia patomechanizmusában.

The significance of nitric-oxide as a biological mediator is increasingly elucidated in various physiological processes. In addition to the endothelium dependent vasodilator effect it also plays an important role as a neurotransmitter. Possible synthesis of endothelial nitric-oxide is an essential factor in the regulation of blood pressure and utero-placental circulation. Recently the relationship of nitric-oxide and preeclampsia has extensively been studied. The results of human studies and animal experiments suggest that insufficient production of endothelial nitric-oxide probably has an important role in the pathomechanism of preeclampsia.

Prolonged effects of MK-801 in the cat during focal cerebral ischemia and recovery: survival, EEG activity and histopathology.

Previously we reported an improvement in histological outcome in cats treated with MK-801 shortly after the induction of temporary middle cerebral artery occlusion, and examined after 2 h of ischemia followed by 4 h of reperfusion. This study investigates the prolonged effects of the same drug treatment. Focal cerebral ischemia was produced in 34 cats by temporary occlusion of the left middle cerebral artery for 2 h. Stroke severity was determined using the ratio of the EEG amplitude from the ipsilateral to that of the contralateral hemisphere. Thirty minutes after the onset of ischemia, cats were treated i.v. with either 1 mg/kg MK-801 or saline. Electrocortical activity of the animals who survived were followed for 6 days postocclusion at which point they were sacrificed for histopathological analysis. Twelve of the animals died during recovery, of which 4 were MK-801 treated, and 8 were saline controls. The EEG ratios in the non-surviving animals were more depressed than in the animals that survived, whereas the depression in the EEG amplitude in both the treated and the control surviving animals was equal. Among the survivors no reduction in infarct size with MK-801 treatment was observed. Thus treatment with MK-801 in the middle cerebral artery occlusion model in the cat leads to a significant increase in the rate of survival (P < 0.05), but no prolonged improvement in late histopathology, in contrast with acute histological findings using this model. MK-801 treatment may be shifting the stroke model towards the survival of animals with larger infarcts. Histological recovery during prolonged reperfusion may eliminate the early neuroprotective effects seen with MK-801 treatment.

Acute improvement in histological outcome by MK-801 following focal cerebral ischemia and reperfusion in the cat independent of blood flow changes.

The present study reports on the acute effects of MK-801 on the histopathological outcome and blood flow changes during focal cerebral ischemia and reperfusion. In addition, acute changes in the EEG and blood pressure are also reported. In 16 halothane-anesthetized cats, the left middle cerebral artery (MCA) was occluded for 2 h followed by 4 h of reperfusion. Thirty minutes after the onset of ischemia, eight animals were treated with 1 mg/kg of MK-801, while eight animals received saline. Blood flow from the peripheral MCA territory was measured with H2 clearance. There was a comparable reduction in blood flow (down to 20% of control) in the ischemic gyri of the two groups followed by a partial recovery after recirculation. There was a similar decrease in the EEG amplitude over the ischemic central MCA territory in the treated and the untreated group. Treatment with MK-801 induced a burst suppression in the EEG and a transient drop (11.4 +/- 6.5 mm Hg) in the mean arterial pressure. The volume of early ischemic damage decreased by one-third in the MK-801-treated group compared to the untreated one, both in the total hemisphere (from 29 +/- 10 to 20 +/- 5%) and in the hemispheric cortex (range 36 +/- 8 to 24 +/- 13%). A major fraction of this improvement was localized to the middle and posterior parietal (mainly perifocal) regions of the MCA territory. These results show that in our model, MK-801 improves histopathological outcome despite the lack of apparent effect on the cortical blood flow, and an adverse effect on the systemic blood pressure. This is the first report that describes data on a reproducible model of reperfusion after temporary occlusion of the MCA in a cat, extending the findings of the Glasgow group, who observed similar neuroprotection in models of permanent MCA occlusion.

Regulation of femoral vascular resistance by adenine nucleotides via endothelial and smooth muscle receptors.

It is well established that adenosine (ADO) and adenine nucleotides are potent vasodilators, but their role in local blood flow control is still under debate. Recent findings on contribution of vascular endothelium to the vasomotor regulation pointed out this problem. In the present study the effects of adenine nucleotides were investigated in vivo on the femoral arterial flow (FAF) and femoral vascular resistance (FVR). Selective suppression of the endothelium mediated dilation was achieved by gossypol (35 mumol/l). On intact hindlimbs ADO (4 mmol/l) and ATP (0.5 mmol/l) elicited 3.5-fold increase of FAF, in average. Resistance decreased by 6.24 +/- 0.58 and 7.23 +/- 1.12 peripheral resistance units (PRU100), respectively. After gossypol, ATP-induced dilation was either significantly suppressed (resistance-decrease was 3.70 +/- 0.58 PRU100; p less than 0.02 vs control) or turned to strong constriction (FAF decreased by 50%). ADO-induced dilation remained unchanged. These results, in agreement with in vitro data, suggest that adenosine directly relaxes the vascular smooth muscle of resistance vessels via P1-purinoceptors, while ATP-induced vasomotion is composed of a dilator effect mediated by endothelial P2y-receptors, and a direct constrictor effect on the vascular smooth muscle via P2x-purinoceptors.

The effect of calcium entry blocker S-emopamil on cerebrocortical metabolism and blood flow changes evoked by graded hypotension.

Our studies demonstrate that in the explanation of the protective effect of the calcium antagonist (S)-emopamil the possibility of beneficial metabolic causes (lower O2 consumption) must also be considered beside of blood flow increasing effects. It is suggested that (S)-emopamil may be a useful drug for the treatment of cerebro-ischemic disorders.

Effect of intraarterial somatostatin infusion on SMA blood flow.

Experiments were carried out on anaesthetized cats to study the effect of somatostatin on the mesenteric circulation. Intraarterial infusions of somatostatin were applied into the superior mesenteric artery. The effect of atropine, propranolol and phentolamine were investigated. Catecholamine release or uptake of the mesenteric vascular bed during somatostatin infusions were also measured. We found dose dependent vasodilatory effect of somatostatin on the mesenteric vasculature that was not influenced by atropine and by the adrenoreceptors or by denervation. A direct effect of somatostatin on the vascular smooth muscle membrane is assumed.

Selective inhibition of endothelium-dependent dilation in resistance-sized vessels in vivo.

In vivo experiments were performed in autoperfused hindlimbs of rabbits to investigate the role of endothelium-mediated vasomotion in resistance-sized vessels. The flow responses to the vasodilators acetylcholine (ACh), ATP, and substance P (SP), all of which have been shown to act in an endothelium-dependent manner in large conduit arteries, were studied before and after exposure of the hindleg vasculature to gossypol (a potent inhibitor of endothelium-mediated vasodilation in vitro). The flow responses to adenosine (ADO), nitroglycerin (GTN), and prostaglandin E2 (PGE2), which induce relaxation by a direct effect on vascular smooth muscle, were tested in the same manner. All vasodilators induced dose-dependent increases in femoral flow up to two- to threefold when administered intra-arterially. After gossypol, the flow responses to the endothelium-dependent compounds (ACh, ATP, and SP) were severely reduced (by 88 +/- 3%, P less than 0.01) or sometimes were converted to constrictions (ATP). The flow increases induced by ADO, PGE2, and GTN remained largely unaffected. Sham treatment (gossypol solute only), exposure to indomethacin (10 microM), and ganglionic blockade had no differential effect on the flow responses. The selective action of gossypol in suppressing the flow responses to the endothelium-dependent compounds ACh, ATP, and SP is consistent with a vasomotor role for endothelial cells in resistance-sized vessels in vivo.