The cardiovascular system in microgravity: symposium summary.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
R E Purdy
Publications and source records attributed to R E Purdy.
Effects of maturation and acute hypoxia on receptor-IP(3) coupling in ovine common carotid arteries.
Whereas previous studies have established that many mechanisms mediating pharmacomechanical coupling are subject to regulation, evidence of physiological regulation of the coupling efficiency between receptor activation and second-messenger production is scarce. The present studies address the hypothesis that acute hypoxia and maturation can influence the mass of second-messenger production for each activated agonist-bound receptor ("receptor gain"). For this assessment, receptor density and agonist affinity values were used to calculate 5-hydroxytryptamine (5-HT) concentrations that would produce standardized numbers of bound receptors (8.5 fmol/mg protein) in each experimental group and thus minimize effects of age or hypoxia on receptor density or agonist affinity. After 3 min of exposure to these 5-HT concentrations, normoxic magnitudes of contraction were similar (as %potassium maxima) in fetal (50 +/- 14%) and adult (40 +/- 9%) arteries, but hypoxia (PO(2) approximately 9--12 Torr for 30 min) depressed contractile tensions with a significantly different time course and magnitude in fetal (30 +/- 10%) and adult (17 +/- 11%) arteries (P < 0.05). Basal inositol 1,4,5-trisphosphate (IP(3)) values (in pmol/mg protein) were significantly greater in fetal (94 +/- 16) than in adult (44 +/- 6) arteries, and integrated areas above baseline for the IP(3) time courses (in nmol-s/mg protein) were significantly greater in fetal than in adult arteries both in normoxic (14.3 +/- 1.8 vs. 9.1 +/- 1.6) and hypoxic (15.0 +/- 2.1 vs. 8.6 +/- 1.2) conditions (P < 0.05). Hypoxia altered the IP(3) time courses both in the fetus and the adult but had no significant effect on IP(3 )mobilization or receptor gain. These data demonstrate that for the 5-HT(2a) receptor predominant in this preparation, receptor gain can be experimentally determined, is not influenced by acute hypoxia, but is greater in fetal than in adult ovine carotid arteries.
Animals↗
Simulated microgravity upregulates an endothelial vasoconstrictor prostaglandin.
Endothelial nitric oxide contributes to the vascular hyporesponsiveness to norepinephrine (NE) observed in carotid arteries from rats exposed to simulated microgravity. The goal of the present study was to determine whether a cyclooxygenase product of arachidonic acid also influences vascular responsiveness in this setting. Microgravity was simulated in rats by hindlimb unweighting (HU). After 20 days of HU, carotid arteries were isolated from control and HU-treated rats, and vascular rings were mounted in tissue baths for the measurement of isometric contraction. Two cyclooxygenase inhibitors, indomethacin and ibuprofen, and the selective thromboxane A(2) prostanoid-receptor antagonist, SQ-29548, had no effect on the contraction to NE in control vessels but markedly reduced contraction to NE in HU vessels. When the endothelium was removed, indomethacin no longer had any effect on the NE-induced contraction in HU vessels. In endothelium-intact vessels in the presence of indomethacin, the addition of the nitric oxide synthase inhibitor, N(G)-L-nitro-arginine methyl ester, to the medium bathing HU vessels increased the contraction to NE to the level of that of the control vessels. These results indicate that HU treatment induced two endothelial changes in carotid artery that opposed each other. Nitric oxide activity was increased and was responsible for the vascular hyporesponsiveness to NE. The activity of a vasoconstrictor prostaglandin was also increased, and attenuated the vasodilating effect of nitric oxide.
Analysis of Variance↗
Phenylephrine precontraction increases the sensitivity of rabbit femoral artery to serotonin by enabling 5-HT1-like receptors.
We used selective receptor antagonists to identify the receptors mediating the isometric contractile response to serotonin in control and phenylephrine (PHE)-precontracted rabbit femoral artery rings. Serotonin, in the absence of PHE, elicited monophasic concentration-response curves (CRCs) early, but biphasic CRCs late in the course of the study. In the monophasic curves, the threshold and maximal concentrations were 10 and 1,000 microM, respectively. In biphasic CRCs, the threshold and maximal concentrations of the first phase were 0.03 and 3 microM, respectively. The respective values for the second phase were 10 and 1,000 microM. Prazosin, 0.1 microM, a selective alpha1-adrenoceptor antagonist, inhibited the monophasic curves, but only the second phase of the biphasic curves. Ritanserin, 0.01 microM, a selective 5-HT2A-receptor antagonist, shifted the first phase of the biphasic serotonin CRCs to the right but had little effect on the second phase. PHE increased the sensitivity of rabbit femoral artery response to serotonin. This amplified response to serotonin was antagonized by 0.01 microM GR 127935T, a selective 5-HT1B-receptor antagonist. The selective 5-HT1 agonist, sumatriptan, had no effect in control femoral arteries, but caused a concentration-dependent contraction after PHE precontraction. These results suggest that 5-HT1-like receptors are normally inactive or "silent" in the absence of PHE. However, in the presence of PHE, these receptors become enabled and mediate the amplified response to serotonin. The evidence also suggests that, in the absence of PHE, alpha1-adrenoceptors mediated the contractile response to serotonin in the monophasic CRCs. In the biphasic curves observed late in the study, the first phase was mediated by 5-HT2A receptors, and the second, by the alpha1-adrenergic receptors.
Animals↗
Vascular hyporesponsiveness in simulated microgravity: role of nitric oxide-dependent mechanisms.
Simulated microgravity depresses the ability of arteries to constrict to norepinephrine (NE). In the present study the role of nitric oxide-dependent mechanisms on the vascular hyporesponsiveness to NE was investigated in peripheral arteries of the rat after 20 days of hindlimb unweighting (HU). Blood vessels from control rats and rats subjected to HU (HU rats) were cut into 3-mm rings and mounted in tissue baths for the measurement of isometric contraction. Mechanical removal of the endothelium from carotid artery rings, but not from aorta or femoral artery rings, of HU rats restored the contractile response to NE toward control. A 10-fold increase in sensitivity to ACh was observed in phenylephrine-precontracted carotid artery rings from HU rats. In the presence of the nitric oxide synthase (NOS) substrate L-arginine, the inducible NOS inhibitor aminoguanidine (AG) restored the contractile responses to NE to control levels in the femoral, but not carotid, artery rings from HU rats. In vivo blood pressure measurements revealed that the peak blood pressure increase to NE was significantly greater in the control than in the HU rats, but that to AG was less than one-half in control compared with HU rats. These results indicate that the endothelial vasodilator mechanisms may be upregulated in the carotid artery, whereas the inducible NOS expression/activity may be increased in the femoral artery from HU rats. These HU-mediated changes could produce a sustained elevation of vascular nitric oxide levels that, in turn, could contribute to the vascular hyporesponsiveness to NE.
Acetylcholine↗
Upregulation of NOS by simulated microgravity, potential cause of orthostatic intolerance.
Prolonged exposure to microgravity during spaceflight or extended bed rest results in cardiovascular deconditioning, marked by orthostatic intolerance and hyporesponsiveness to vasopressors. Earlier studies primarily explored fluid and electrolyte balance and baroreceptor and vasopressor systems in search of a possible mechanism. Given the potent vasodilatory and natriuretic actions of nitric oxide (NO), we hypothesized that cardiovascular adaptation to microgravity may involve upregulation of the NO system. Male Wistar rats were randomly assigned to a control group or a group subjected to simulated microgravity by hindlimb unloading (HU) for 20 days. Tissues were harvested after death for determination of total nitrate and nitrite (NOx) as well as endothelial (e), inducible (i), and neuronal (n) NO synthase (NOS) proteins by Western blot. Separate subgroups were used to test blood pressure response to norepinephrine and the iNOS inhibitor aminoguanidine. Compared with controls, the HU group showed a significant increase in tissue NOx content and an upregulation of iNOS protein abundance in thoracic aorta, heart, and kidney and of nNOS protein expression in the brain and kidney but no discernible change in eNOS expression. This was associated with marked attenuation of hypertensive response to norepinephrine and a significant increase in hypertensive response to aminoguanidine, suggesting enhanced iNOS-derived NO generation in the HU group. Upregulation of these NOS isotypes can contribute to cardiovascular adaptation to microgravity by promoting vasodilatory tone and natriuresis and depressing central sympathetic outflow. If true in humans, short-term administration of an iNOS inhibitor may ameliorate orthostatic intolerance in returning astronauts and patients after extended bed rest.
Animals↗
Evidence that simulated microgravity may alter the vascular nonreceptor tyrosine kinase second messenger pathway.
Simulated microgravity (hind limb unweighting; HU) reduces maximal contractile capacity to norepinephrine (NE) but not 5-hydroxytryptamine (5-HT) in the rat abdominal aorta of male Wistar rats. Our earlier study showed that voltage-operated calcium channels, the MAPK pathway [1], and vasoconstrictive prostaglandins contribute to the NE-induced contraction of control (C) but not HU, aorta rings. Genistein, a general tyrosine kinase inhibitor, caused a significant reduction in vascular contractility in C but not HU arteries. The present study explored the role of protein kinase C (PKC) and extracellular receptor-activated kinase 1 and 2 (ERK1/2) in the HU-induced vascular hyporesponsiveness to NE. Microgravity was simulated in Wistar rats by 20 day HU. The abdominal aorta was removed from control and HU rats, cut into 3 mm rings, and mounted in tissue baths to measure isometric contraction. Protein levels were determined using Western blot analysis. PD98059, a selective MAPKK inhibitor, caused a marked inhibition of NE-induced contraction in both C and HU arteries. Calphostin C, a PKC inhibitor, completely abolished the contractile response to NE in both C and HU tissues. Phosphorylated (activated) ERK1/2 protein mass was greater in C, compared to HU, aortas, and was reduced by genistein only in C tissues. MAPK total protein levels in the rat aorta were increased in the HU-treated, compared to C, animals. These results indicate that PKC represents an early transduction step in the contractile response to NE in the rat abdominal aorta. That inhibition of the step immediately before activation of MAPK reduced contraction in both C and HU tissues, while general tyrosine kinase inhibition with genistein blocked only the control responses, suggests that a nonreceptor tyrosine kinase may be involved in HU-induced vascular hyporesponsiveness to NE.
Animals↗
Precontraction with elevated concentrations of extracellular potassium enables both 5-HT1B and 5-HT2A "silent" receptors in rabbit ear artery.
The present study was conducted to determine the effect of a small (<10%) K+-induced precontraction on the response to vasoconstrictors in the rabbit aorta and ear artery rings. In both tissues, 15 mM K+ shifted the methoxamine concentration response curve (CRC) approximately 2.4-fold to the left. There was no change in the sensitivity of the control and amplified CRCs to the alpha1 adrenoceptor antagonist prazosin (100 nM). In the aorta, the CRC for serotonin was shifted 4.5-fold to the left in the presence of 15 mM K+, and both the control and amplified CRCs were antagonized equally by the 5-HT2A antagonist ketanserin (10 nM). In contrast, 16 and 20 mM K+ caused up to an approximately 60-fold leftward shift of the serotonin CRC in the rabbit ear artery. This effect of 16 mM K+ was not altered by mechanical removal of the endothelium or by in vitro denervation using 6-hydroxydopamine. The K+-amplified CRC was insensitive to 100 nM prazosin at serotonin concentrations below 3 microM, but was significantly antagonized by 10 nM ketanserin, suggesting that 5-HT2A receptors are involved in the K+-amplified response. The 5-HT1B-selective antagonist, GR 127935, did not affect control responses to serotonin, but significantly blocked the K+-amplified response. Furthermore, the combination of ketanserin and GR 127935 produced a significantly greater blockade of the amplified response than either antagonist alone, supporting the conclusion that both 5-HT2A and 5-HT1B receptors mediate the K+-amplified response to serotonin in the rabbit ear artery.
Animals↗
Na-K-ATPase inhibitor dissociated from hypertension-associated plasma protein.
It has been demonstrated that human plasma contains a low molecular weight sodium-potassium-stimulated adenosine triphosphatase (Na-K-ATPase) inhibitor, which can be dissociated from a circulating protein with a molecular weight of approximately 12,000 daltons. The dissociated factor was found to have a molecular weight <500 daltons, and shared many characteristics with ouabain. Similar to ouabain, this factor was found to be a potent inhibitor of both the Na-K-ATPase and potassium-stimulated para-nitrophenyl phosphatase (K-pNPPase) enzyme systems, and to bind to both high- and low-affinity binding sites on Na-K-ATPase, but unlike ouabain did not cross-react with digoxin antibody. The factor was further separated by HPLC and electrochemical detection into two active compounds (p-NKAI-1 and p-NKAI-2). P-NKAI-1 was demonstrated on mass spectroscopy to have a molecular weight of 408 daltons. In a vasoconstrictor assay employing rabbit femoral artery segments, this compound was a direct vasoconstrictor and potentiated the vasoconstriction produced by norepinephrine. It behaved similarly to ouabain in counteracting the relaxing effect on rabbit femoral artery of increasing potassium concentrations in the tissue bath.
4-Nitrophenylphosphatase↗
Simulated microgravity impairs vascular contractility: role of nitric oxide-dependent vasodilator mechanisms.
Explore the source record for details and available documents.
Animals↗
Simulated microgravity-induced vascular hyporesponsiveness: role of signal transduction.
Explore the source record for details and available documents.
Animals↗
Pharmacological characterization of the "silent" 5-HT1B-like receptors of rabbit ear artery.
Explore the source record for details and available documents.
Animals↗
Characterization of the "silent" 5-HT1B receptors of rabbit iliac artery.
Explore the source record for details and available documents.
Animals↗
Serotonin and vasoconstrictor synergism.
Contractile synergism between serotonin (5-hydroxytryptamine, 5-HT) and other vasoconstrictor substances has been observed in a number of peripheral and cerebrovascular blood vessels. This phenomenon may play an important role in certain pathological states such as hypertension, peripheral vascular disease, and coronary spasm. In the present review, we summarize studies on the synergism between serotonin and other vasoconstrictor agents and focus on a recently described type of vasoconstrictor synergism in which precontraction with a non-5-HT receptor agonist yields an enhanced contractile response to serotonin which is mediated by previously inactive or "silent" 5-HT receptor subtypes.
Animals↗
Effect of simulated microgravity on vascular contractility.
Microgravity was simulated in Sprague-Dawley (SD) and Wistar (W) rats by using a tail harness to elevate the hindquarters, producing hindlimb unweighting (HU). After 20 days of HU treatment, blood vessels from both HU and control rats were cut into 3-mm rings and mounted in tissue baths for the measurement of isometric contraction. HU treatment decreased the contractile response to 68 mM K+ in abdominal aorta from W rats. HU treatment also decreased the contraction to 68 mM K+ in carotid arteries from both rat strains and in femoral arteries from W but not SD rats. HU treatment reduced the maximal response to norepinephrine in all arteries except the femoral from SD rats. HU treatment reduced the maximal response of jugular vein from W rats to 68 mM K+ but had no effect on that response in femoral vein from either rat strain. HU treatment also had no significant effect on the maximal response to norepinephrine in veins. These results demonstrate that HU treatment caused a nearly universal reduction of contractility in arteries, but generally had no effect in veins.
Animals↗
Simulated microgravity increases myogenic tone in rat cerebral arteries.
Adaptation of the cerebral circulation to microgravity was investigated in rat middle cerebral arteries after 20 days of hindlimb unweighting (HU). Myogenic responses were measured in isolated, pressurized arteries from HU and control animals. Maximal passive lumen diameters, obtained in the absence of extracellular Ca2+ plus EDTA, were not significantly different between groups (249 vs. 258 micrometer). In physiological salt solution, arteries from both HU and control animals maintained a constant lumen diameter when subjected to incremental increases in transmural pressure (20-80 mmHg). However, the diameter of arteries from HU animals was significantly smaller than that of arteries from control animals at all pressures; this difference could be eliminated by exposure to the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester. After HU treatment, transient distensibility of the artery wall in response to pressure was also significantly decreased, whereas the frequency and amplitude of vasomotion were increased. The latter changes were not affected by NG-nitro-L-arginine methyl ester. Thus simulated microgravity increases cerebral artery myogenic tone through both nitric oxide synthase-dependent and -independent mechanisms.
Animals↗
Lead-induced hypertension is not associated with altered vascular reactivity in vitro.
In confirmation of a previous study (Am J Hypertens 1993;6:723), mean arterial blood pressure (MBP), as determined by tail cuff plethysmography, was found to be significantly elevated in Sprague-Dawley rats after 3 months of feeding 0.48 mmol/L (100 ppm) lead acetate/day (144 +/- 3.3 [SEM], in lead-treated [L] v 107 +/- 3.3 mm Hg in controls [C], P < .001). Thoracic aorta was excised from L and C animals (n = 6). Segments were suspended in tissue baths with Krebs' bicarbonate solution, then tested sequentially for vasoreactivity to 68 mmol/L K+, followed by graded concentrations of phenylephrine (PE), 0.01 to 0.3 micromol/L, acetylcholine (Ach), 0.001 to 3 micromol/L, nitroprusside (SNP), 0.0001 to 0.1 micromol/L, norepinephrine (NE), 0.001 to 300 micromol/L. There were no differences between L and C animals with respect to either vasoconstrictors (PE and NE) or vasodilators (Ach and SNP). The tissue levels of cGMP measured with and without phosphodiesterase inhibition, and in the absence and presence of either Ach or SNP, were comparable in the two groups. We conclude that the intrinsic vascular responsiveness is unchanged in lead-treated animals. The elevation of MBP is due to the presence of circulating factor(s) and hemodynamic changes.
Acetylcholine↗
Pharmacological characterization of the "silent" 5-hydroxytryptamine1B-like receptors of rabbit ear artery.
Serotonin acts on 5-hydroxytryptamine (5-HT)1B-like receptors in isolated rabbit ear artery precontracted with phenylephrine (PHE). These receptors are inactive, or "silent," in untreated vessels. Ear artery rings were mounted in tissue baths for the measurement of isometric contraction to further characterize these 5-HT1B-like receptors. The 5-HT1-selective receptor agonist sumatriptan failed to contract the untreated ear artery rings but caused a powerful, concentration-dependent contraction in PHE-precontracted vessels. The 5-HT1A/rat 1B receptor antagonist propranolol (1 microM) had no effect, whereas the 5-HT1B receptor antagonists rauwolscine (0.1 microM) and GR127935 (1-100 nM) markedly inhibited the contraction to sumatriptan. In vessels precontracted with phenylephrine, nifedipine reduced and calcium-free medium abolished the contractile response to serotonin. Relaxation to the adenylate cyclase activator forskolin was studied in contracted ear artery rings. Low concentrations (0.1-0.3 microM) of forskolin rapidly and completely relaxed ear artery rings contracted with PHE. In contrast, when PHE-precontracted vessels were contracted with either serotonin or sumatriptan, forskolin caused little or no relaxation at low concentrations and only partial relaxation at 10- to 30-fold higher concentrations. The resistance of these vessels to relaxation by forskolin was markedly reduced in the presence of GR127935 or in ear artery rings from pertussis toxin-treated rabbits. However, pertussis toxin treatment had no effect on the contractile response of PHE-precontracted ear artery rings to serotonin. It is concluded that the silent 5-HT1-like receptor of rabbit ear artery closely resembles the 5-HT1B receptor subtype. This receptor is inversely coupled to adenylate cyclase through a pertussis toxin-sensitive G protein; however, this coupling is unlikely to contribute to the serotonin-induced contraction of PHE-precontracted ear artery rings. Instead, this contraction is mediated at the second-messenger level by pertussis toxin-insensitive influx of calcium.
Adenylate Cyclase Toxin↗