Biomedical subjects

R C Webb

Publications and source records attributed to R C Webb.

At least 73 records · Page 4Linked to original sources

PubMed1995

Lead acetate-induced contraction in rabbit mesenteric artery: interaction with calcium and protein kinase C.

Animal studies have demonstrated that low-level lead exposure produces hypertension and that lead can cause contraction of vascular smooth muscle directly. The physiological effects of lead have been associated with both stimulation and inhibition of protein kinase C (PKC). Given that vascular smooth muscle contractility is generally enhanced when protein kinase C is activated, we have tested the hypothesis that lead contracts vascular smooth muscle via stimulation of PKC. Helically-cut strips of rabbit mesenteric artery were mounted in muscle baths for measurement of isometric force development. Cumulative addition of lead acetate (10(-10)-10(-3) M) to the muscle bath produced contractions (concentration necessary to produce half-maximal response -log EC50 = 5.16 +/- 0.07). Maximal contraction to lead acetate in arteries denuded of endothelium did not differ from those in intact vessels, supporting the hypothesis that lead-induced contraction is an endothelium-independent event. Contractions to lead acetate were potentiated by the PKC activators, phorbol 12-myristate 13-acetate (TPA; 3 x 10(-7) M) and mezerein (3 x 10(-7) M), as indicated by leftward shifts in the concentration-response curve and increase in the potency of lead (-log EC50 with TPA: 6.94 +/- 0.07; -log EC50 with mezerein: 6.07 +/- 0.04). H-7 (6 x 10(-6) M), an inhibitor of PKC, decreased maximal contraction to lead approximately 65% and slightly, but insignificantly, decreased the potency of lead (-log EC50 = 4.82 +/- 0.1). The inactive phorbol ester, phorbol 12-myristate 13-acetate 4-0-methyl ether (1 x 10(-6) M), did not alter contractile responses to lead (-log EC50 = 4.92 +/- 0.09). Vascular contraction to lead partially depends on extracellular calcium as the L-type voltage-gated calcium channel antagonist, verapamil (3 x 10(-6) M), decreased lead-induced contractions by 50%. These data indicate that lead interacts with PKC in an endothelium-independent, calcium-dependent manner to cause vascular smooth muscle contraction and suggest that lead-induced increases in vascular contractility may play a role in lead-induced hypertension.

Analysis of Variance↗

PubMed1995

Long-term effects of brief antihypertensive treatment on systolic blood pressure and vascular reactivity in young genetically hypertensive rats.

Recent studies have shown that angiotensin converting enzyme (ACE) inhibitor treatment in young spontaneously hypertensive rats (SHR) reduces blood pressure into adulthood. This study explored changes in vascular reactivity in adult normotensive (WKY) rats and stroke-prone SHR (SHRSP) receiving the following treatments at 6-10 weeks of age: (a) ACE inhibitor (ramipril); (b) hydralazine/hydrochlorothiazide (hydral/HCTZ); or (c) no treatment. The hypothesis tested was that vascular changes and blood pressure would be reduced in adult SHRSP treated with ramipril during development. At 17 weeks of age, rats were anesthetized and vascular tissue was excised. Isolated experiments in the aorta included characterization of initial phasic and tonic contractions to 0.1 microM angiotensin II (AII). A phenylephrine (PE) concentration-response curve was performed on carotid arteries, and threshold values were determined. All WKY groups showed lower systolic blood pressure (131 +/- 4 mmHg) and reduced phasic AII induced contraction (7.4 +/- 4.7%) compared with SHRSP (217 +/- 4 mmHg; 37.2 +/- 4%). Antihypertensive treatment reduced blood pressure (ramipril: 168 +/- 2; hydral/HCTZ: 198 +/- 6 mmHg) but not phasic AII responses in adult SHRSP; adult WKY rats were unaffected by treatment. Threshold values for PE in carotid arteries were lower in SHRSP than in WKY, indicating increased sensitivity. However, SHRSP treated with ramipril did not demonstrate increased sensitivity to PE. These data support the hypothesis that blood pressure and sensitivity to PE but not contractile responsiveness to AII in adult SHRSP are determined by an AII-sensitive mechanism during development.

Aging↗

PubMed1995

Nitric oxide regulation of ADP-ribosylation of G proteins in hypertension.

Nitric oxide stimulates endogenous ADP-ribosylation of cytosolic and membrane-bound proteins. Endogenous ADP-ribosyltransferases modify several intracellular proteins including the heterotrimeric GTP-binding proteins (G proteins). ADP-ribosylation of G proteins in vascular smooth muscle leads to increased activation of adenylate cyclase and decreased activation of phospholipase C leading to vasodilation. We hypothesize that in hypertension, chronically depressed endothelium-derived nitric oxide levels lead to decreased ADP-ribosylation of G proteins. This reduced ADP-ribosylation leads to vasoconstriction since activation of the G proteins by agonists is unopposed. Thus, disinhibition of G proteins, mediated by nitric oxide deficit, is responsible for the observed increased sensitivity to vasoconstrictor agonists in hypertension. This novel role for nitric oxide in hypertension will provide a new area of research for antihypertensive therapeutic intervention.

Adenosine Diphosphate Ribose↗

PubMed1995

Nitric oxide inhibition of endothelial cell mitogenesis and proliferation.

BACKGROUND: Endothelial cell (EC) proliferation is essential in vascular repair after injury to the vessel wall. Impaired EC proliferation may be an important factor contributing to vessel wall disease. Nitric oxide (NO) inhibits proliferation of many cells, including smooth muscle cells (SMC). We tested the hypothesis that NO inhibits EC proliferation and DNA synthesis. METHODS: Cultured canine venous ECs were treated with NO donors: S-nitroso-N-acetylpenicillamine (SNAP), S-nitroso-glutathione (GSNO), or spermine NONOate (SP NO). Proliferation was determined by cell counts after 48 hours. Parallel proliferation studies were done with rat aortic SMC. ECs synchronized in S phase were treated with the NO donor diethylamine NONOate (DEA NO), and DNA synthesis was measured as the incorporation of tritiated thymidine. A NO antagonist, cPTIO, was used to reverse the effects of DEA NO: RESULTS: Concentration-dependent (1 to 100 mmol/L) inhibition of EC proliferation (11% to 71% inhibition; p < 0.05) was seen with SNAP. Similar inhibition of proliferation was noted with the NO donors GSNO and SP NO and in SMC treated with SNAP. DEA NO caused concentration-dependent (0.1 to 1 mmol/L) inhibition of EC DNA synthesis (39% to 85% inhibition; p < 0.05), which was reversed by cPTIO. CONCLUSIONS: NO inhibits proliferation and mitogenesis of cultured ECs. This may occur in certain pathologic states, where production of NO in plaques and diseased vessels impedes reendothelialization, thus contributing to adverse thrombotic and vasospastic events.

Animals↗

PubMed1995

The role of gap junctional communication in contractile oscillations in arteries from normotensive and hypertensive rats.

OBJECTIVES: To test the hypotheses that vascular supersensitivity correlates with the appearance of contractile oscillations; vascular oscillations are mediated by gap junctions; and gap junctional communication is altered in the vasculature in stroke-prone spontaneously hypertensive (SHRSP) compared with Wistar-Kyoto (WKY) rats. DESIGN AND METHODS: Helical strips of mesenteric and tail arteries from SHRSP and WKY rats were mounted in tissue baths for measurement of isometric force. Cultures of mesenteric arterial cells were used for measurement of Lucifer yellow dye transfer and abundance of connexin43 mRNA, a monomer of gap junctions. RESULTS: Mesenteric arteries from SHRSP that displayed spontaneous oscillations were more sensitive to the contractile agonist 5-hydroxytryptamine than those from SHRSP and WKY rats that displayed no oscillations. In addition, SHRSP tail arteries displayed norepinephrine-induced oscillations. The putative gap junction up-regulator tetraethylammonium (10(-3)-10(-1) mol/l) induced oscillations (1-5 cycle/min) in arteries from both rat strains. These oscillations were not altered by endothelium removal or phentolamine and were blocked by heptanol (10(-3) mol/l). Although tetraethylammonium and heptanol caused similar effects in both arteries, heptanol-sensitive agonist-induced oscillations persisted only in the tail artery of SHRSP. Tetraethylammonium increased dye transfer between contiguous cells approximately 35% above basal levels both for SHRSP and WKY cells. In both cell lines, heptanol reduced basal- and tetraethylammonium-stimulated dye transfer. Total RNA from WKY rat and SHRSP cultured cells hybridized strongly and to a similar magnitude with a complementary DNA probe for messenger RNA for connexin43. CONCLUSIONS: Gap junctional communication is important in vascular reactivity and might play a part in the development of oscillations. Altered gap junctional communication could not be demonstrated in cell cultures nor in some contractile experiments. It is possible that the culture conditions failed to mimic conditions in vivo that differentially regulate gap junctions in the hypertensive state, but it is also possible that gap junctional activity is not abnormal in hypertension.

Alcohols↗

PubMed1995

Sarcoplasmic reticulum Ca2+ uptake is not decreased in aorta from deoxycorticosterone acetate hypertensive rats: functional assessment with cyclopiazonic acid.

Ca2+ plays a major role in vascular contraction, and a defect in intracellular Ca2+ regulation has been associated with increased vascular reactivity in hypertension. To test the hypothesis that the sarcoplasmic reticulum does not adequately buffer Ca2+ in deoxycorticosterone acetate (DOCA) hypertension, contractile experiments were performed with a specific inhibitor of the sarcoplasmic reticulum Ca2+ ATPase, cyclopiazonic acid (CPA). Contractile force in aortic strips from DOCA and control rats was measured, using standard muscle bath procedures, to evaluate (i) Ca2+ handling, assessing caffeine and serotonin (5HT) induced contractions in Ca(2+)-free buffer and (ii) relaxation rate after 5HT washout. Contractile responses elicited with 5HT (3 x 10(-6) mol/L) and caffeine (20 mmol/L) were greater in DOCA than in control arteries. CPA (1 x 10(-7) to 3 x 10(-5) mol/L) reduced phasic contractions to 5HT and caffeine in DOCA and control aorta, and no differences in the IC50 values were observed. Aortae from DOCA rats contracted when placed in normal buffer, subsequent to treatment with Ca(2+)-free buffer, but control aortae did not. CPA potentiated these responses in DOCA aorta and only caused a modest contraction in control aorta. CPA-induced contraction did not occur in Ca(2+)-free buffer, and it was inhibited by nifedipine (IC50 = 4 x 10(-9) mol/L). The relaxation rate, after 5HT washout (3 x 10(-6) mol/L), was increased in DOCA aorta (2.6 +/- 0.3 min) compared with control (1.7 +/- 0.2 min), and CPA (10(-5) mol/L) increased the relaxation rate in both groups. The results support the hypothesis of defective Ca2+ handling in DOCA hypertension. However, an increased Ca2+ influx, and not a decreased buffering ability of the sarcoplasmic reticulum, contributes to the enhanced vascular reactivity observed in DOCA hypertension.

Animals↗

PubMed1995

Inhibition of vascular nitric oxide-cGMP pathway by plasma from ischemic hindlimb of rats.

The hypothesis was tested that plasma from ischemic hindlimbs facilitates hypertension. Ischemia-induced hypertension was generated in rats by infrarenal aortic cross clamping for 5 h after which plasma was obtained from femoral vein blood. In vitro contractile activity of naive aortic rings incubated for 2 h in plasma collected from ischemic rats demonstrated reduced relaxation to acetylcholine and nitroglycerin. Methylene blue (10(-5) M) induced greater contraction in rings incubated in control vs. ischemic plasma, suggesting that endogenous guanylate cyclase activity is decreased by ischemic plasma. However, 8-bromo-guanosine 3',5'-cyclic monophosphate (cGMP) relaxed equally strips incubated in ischemic or control plasma. Acetylcholine-induced nitrite release was significantly lower in ischemic vs. control plasma-incubated strips (8.6 +/- 2.7 vs. 28.2 +/- 2.3 ng/10 mg tissue wt, respectively). The impaired relaxation to acetylcholine in ischemic plasma-incubated rings was significantly increased by L-arginine but not by prior treatment of ischemic plasma with heating or superoxide dismutase and catalase. These findings suggest the impaired relaxation is mediated through inhibition of the nitric oxide-cGMP pathway. Prolonged blunting of vasodilation by ischemic plasma may therefore contribute to maintenance of a sustained vasoconstriction and ischemic hypertension.

Acetylcholine↗

PubMed1995

5-Hydroxytryptamine2B receptor mediates contraction in the mesenteric artery of mineralocorticoid hypertensive rats.

Vascular responsiveness to 5-hydroxytryptamine (5-HT) is dramatically increased in hypertension. The hypothesis that augmented vasoconstriction to 5-HT in hypertension is due to a change in receptor subtype on vascular myocytes was tested. Mesenteric arteries from deoxycorticosterone acetate (DOCA)-salt hypertensive (systolic blood pressure > 180 mm Hg) and sham normotensive (systolic blood pressure < 130 mm Hg) rats were mounted in isolated tissue baths for measurement of isometric contractile force. The receptor mediating contraction in isolated mesenteric arteries from sham and DOCA-salt hypertensive rats is a member of the 5-HT2 family based on rank order of agonist potency (5-HT = alpha-methyl-5-HT [5-HT2 receptor agonist]>tryptamine>5-hydroxykynuramine). 5-HT was approximately 10-fold more potent in contracting mesenteric arteries from DOCA-salt hypertensive rats compared with arteries from sham normotensive rats. The tryptophan metabolite kynuramine, which possesses significant contractile activity at the 5-HT2B receptor, contracted hypertensive arteries significantly (50% of 5-HT maximum) but not sham arteries. Ketanserin (5-HT2A antagonist) competitively inhibited contraction to 5-HT in arteries from normotensive rats (-log dissociation constant [mol/L]; pKB = 8.54) but not from hypertensive rats (pKB > 6.5). Moreover, contraction to kynuramine was not blocked by ketanserin. Thus, under normal conditions, 5-HT2A receptors mediate contraction to 5-HT. However, in DOCA-salt hypertension, ketanserin-insensitive 5-HT2 receptors, possibly 5-HT2B receptors, mediate mesenteric arterial contraction to 5-HT.

Algorithms↗

PubMed1994

Ramipril prevents hypersensitivity to phenylephrine in aorta from streptozotocin-induced diabetic rats.

This study investigated the protective effect of the angiotensin converting enzyme inhibitor, ramipril, on endothelium-dependent responses in arteries from control (CON) and streptozotocin-induced (STZ) diabetic rats. Three hypotheses were tested: 1) there is an endothelium-dependent component to the increased alpha-adrenergic responsiveness characteristic of diabetes; 2) endothelium-dependent, acetylcholine-induced relaxation is attenuated in aorta from diabetic rats; and 3) ramipril (3 mg/kg daily in the food, 12-15 weeks) will prevent functional vascular changes in diabetic rats. Vascular function was assessed in aortic rings using standard muscle bath procedures for measurement of isometric force. Sensitivity to phenylephrine was increased in aortic rings from diabetic compared to control values [pD2 values (-log ED50): CON = 6.22 +/- 0.12, STZ = 7.54 +/- 0.11), and removal of the endothelium (-Endo) increased phenylephrine sensitivity (CON-Endo = 7.40 +/- 0.11, STZ-Endo = 8.32 +/- 0.18). The magnitude of the shift in responsiveness following endothelium removal was greatest in control rats. Ramipril treatment (Ram) partially normalized phenylephrine responsiveness in intact (STZ + Ram = 6.55 +/- 0.11) and denuded (STZ-Endo + Ram = 7.75 +/- 0.10) vessels. Vasodilatation to acetylcholine and nitroglycerin was not altered in diabetic rats nor was it affected by ramipril treatment. Diabetes increases contractile sensitivity to phenylephrine but not to vasodilators and chronic ramipril treatment prevents this increase in contractile sensitivity. Ramipril treatment did not alter the hyperglycaemic condition induced by streptozotocin. The changes in phenylephrine sensitivity appear to involve an endothelial and a smooth muscle component.

Acetylcholine↗

PubMed1994

A photoactivable source of relaxing factor in genetic hypertension.

Deendothelialized rings of rabbit aorta relax after exposure to UV light because of release of a relaxing factor that is similar if not identical to nitric oxide. We tested the hypothesis that production of the photo-induced relaxing factor is impaired in a rat model of genetic hypertension. Thoracic aortas were removed from adult Wistar-Kyoto rats and stroke-prone spontaneously hypertensive rats. The vessels were cut into rings, denuded of endothelium, and placed in a muscle bath for isometric force measurement. Rings were contracted with phenylephrine, and relaxation was measured after exposure to UV light. Aortic rings from stroke-prone spontaneously hypertensive rats relaxed to a greater extent after exposure to UV light than did rings from Wistar-Kyoto rats. An inhibitor of nitric oxide synthase (N omega-nitro-L-arginine) greatly potentiated the relaxation responses to light in both strains, and these enhanced relaxations were attenuated by tetraethylammonium chloride, potassium chloride, ouabain, or inhibitors of guanylate cyclase. These results suggest that UV irradiation induces relaxation in aortic smooth muscle that is greater in hypertensive than normotensive rats and is greatly enhanced after addition of inhibitors of nitric oxide production. Thus, the unidentified photo-induced relaxing factor is not solely nitric oxide but may also represent either a hyperpolarizing factor, because depolarization blocks the responses entirely, or possibly smooth muscle guanylate cyclase that might itself be photoactivable.

Animals↗

PubMed1994

Enhanced vascular reactivity to mastoparan, a G protein activator, in genetically hypertensive rats.

Vascular smooth muscle from stroke-prone spontaneously hypertensive rats has an increased responsiveness to the vasoconstrictors angiotensin II and serotonin. This abnormality is postulated to contribute to the hypertension characteristic of this strain of rats. We hypothesized that a portion of the increased responsiveness may be due to altered function of G proteins. This hypothesis was tested using mastoparan, a peptide that mimics ligand-bound receptors to stimulate G proteins directly. In addition, we investigated the mechanism of mastoparan-induced contraction of vascular smooth muscle. Changes in isometric tension were recorded in denuded carotid artery strips from hypertensive and normotensive (Wistar-Kyoto) rats. Vascular strips from the hypertensive rats had a significantly greater response to mastoparan at all concentrations between 10(-8) and 10(-5) mol/L. A G protein inhibitor, N-ethylmaleimide (10(-3) mol/L), attenuated the response to mastoparan (10(-7) mol/L) (67 +/- 4% of control response), whereas pertussis toxin treatment did not. Inhibition of phospholipase C also significantly decreased the mastoparan-induced response (23 +/- 12% of control), and nifedipine (10(-3) mol/L), a calcium channel blocker, completely blocked the mastoparan-induced contraction. Indomethacin treatment did not affect the mastoparan contraction even though mastoparan has been shown to stimulate phospholipase A2 in other cell types. In conclusion, we observed an increased response in carotid arteries from genetically hypertensive rats to a pharmacological intervention that appears to act via G protein-linked phospholipase C stimulation and L-type calcium channel activation, suggesting that the increased vascular reactivity in stroke-prone spontaneously hypertensive rats is due in part to altered function of G proteins.

Animals↗

PubMed1994

Endothelium dependency of contractile activity differs in infant and adult vertebral arteries.

Contractions to serotonin (5-HT) and endothelin-1 (ET-1) in infant (0-2 yr) and adult (38-71 yr) vertebral arteries were examined in the presence of either the cyclooxygenase inhibitor indomethacin or NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide production. In addition, endothelium-dependent relaxations to acetylcholine were characterized in arteries contracted with agonist. The results showed that: (a) Contractions of infant arteries to 5-HT or ET-1 decreased to 44 +/- 8% and 27 +/- 13%, respectively, within 10 min. Indomethacin or removal of endothelium abolished this decreased response, whereas L-NMMA had no effect. (b) Adult arteries produced sustained contractions to 5-HT or ET-1 that were unaffected by indomethacin, endothelium denudation, or L-NMMA. (c) Endothelium-dependent relaxations to acetylcholine were greater in infant than adult arteries and were abolished by indomethacin (but not L-NMMA) in infants and L-NMMA (but not indomethacin) in adults. Thus, endothelium-dependent responses in infant arteries are attenuated because of increased prostaglandin activity not observed in adult tissues. Additionally, there is an age-dependent change in the primary mechanism responsible for acetylcholine-induced vasodilation. Apparently, endothelium dependency of acetylcholine-induced relaxation is highly dependent on cyclooxygenase activity in the infant vertebral artery, but in the adult artery, nitric oxide is linked to the vasodilator response.

Adult↗

PubMed1994

Enhanced vascular responsiveness to Bay K 8644 in mineralocorticoid- and N-nitro arginine-induced hypertension.

The present study evaluates the response to the L-type voltage gated calcium channel agonist Bay K 8644 in two forms of experimental hypertension (mineralocorticoid- and hypertension induced by the nitric oxide synthase inhibitor N omega-nitro-L-arginine (N-Nitro arginine)) and under conditions of acute stretch. These studies test the hypothesis that increased L-type calcium channel activity in vasculature is a hallmark or general characteristic of hypertension. Male Sprague-Dawley rats were made hypertensive by subcutaneous implantation of deoxycorticosterone acetate (200 mg/kg DOCA) and given normal or high salt water (1% NaCl + 0.2% KCl); other rats were made hypertensive by ingestion of N-Nitro arginine (2% in water). Systolic blood pressures (SBP) were taken by the standard tail cuff method. Following development of hypertension, rats were anesthetized, and aortae or mesenteric arteries were isolated for measurement of isometric contractile force. Cumulative concentration response curves to Bay K 8644 (10(-10) to 10(-6) M), KCl (6 to 100 mM), or phenylephrine (10(-10)-3 x 10(-7) M) were evaluated. Isolated mesenteric arteries from rats given both DOCA and salt were most sensitive to Bay K 8644 (SBP = 191 +/- 6 mmHg, -log EC50 = 7.78 +/- 0.13), followed by rats receiving high salt alone (SBP = 118 +/- 6 mmHg, -log EC50 = 7.30 +/- 0.17), DOCA alone (SBP = 152 +/- 2 mmHg, -log EC50 = 7.25 +/- 0.15), and finally normal sham rats (SBP = 111 +/- 5 mm Hg, -log EC50 > or = 6.80 +/- 0.10). These data indicate that both DOCA and salt intake can independently influence responsiveness to Bay K 8644.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1994

Effects of ramipril on contractile oscillations in arteries from genetically hypertensive rats.

We have tested the hypothesis that altered vascular reactivity, specifically the appearance of spontaneous and BayK 8644 (L-type voltage gated calcium channel agonist)-induced oscillations in the carotid artery and the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA)-induced oscillations in the aorta from stroke-prone spontaneously hypertensive rats (SHRS), are dependent upon angiotensin II production early in life. SHRSP and normotensive Wistar-Kyoto (WKY) rats were treated from 6-10 weeks of age with vehicle, hydralazine/hydrochlorothiazide (used as a control for lowered blood pressure) or the angiotensin converting enzyme inhibitor ramipril (3 mg/kg/day). Systolic blood pressures were measured weekly in rats from 6 to 17 weeks of age. In SHRSP (at 17 weeks of age), ramipril-treatment but not hydralazine/hydrochlorothiazide attenuated the long term expression of elevated systolic blood pressure in adult SHRSP while blood pressures of all adult WKY rats were unaffected by any treatment. At 17 weeks, rats were killed and arteries removed for in vitro measurement of isometric contractile activity. Only the incidence of spontaneous oscillations (carotid artery) was affected by ramipril treatment; ramipril did not change the frequency of BayK 8644-induced oscillations in the artery or the frequency of CPA-induced oscillations in aorta from either SHRSP or WKY. These data indicate that while spontaneous oscillations in the carotid artery may be dependent on an angiotensin II-sensitive mechanism during development, agonist-induced oscillations (CPA and BayK 8644) appear not to be angiotensin II-dependent. Thus, not all of the contractile oscillations which appear in vascular smooth muscle from SHRSP are angiotensin II-dependent, suggesting that some of these vascular abnormalities may develop at a time separate from that in which increased blood pressure is firmly established and may not be associated with the for maintenance of elevated blood pressure.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1994

Recycling and buffering of intracellular calcium in vascular smooth muscle from genetically hypertensive rats.

OBJECTIVE: To test the hypothesis that impaired Ca2+ recycling by the sarcoplasmic reticulum Ca-ATPase contributes to augmented force development in arteries from stroke-prone spontaneously hypertensive rats (SHRSP). METHODS: Force development to caffeine (0.3-30 mmol/l) in the absence of extracellular Ca2+ was compared in aortic strips from SHRSP and Wistar-Kyoto (WKY) rats. In another protocol the strips were rinsed at the peak of contraction to caffeine (20 mmol/l) and subsequently restimulated with the alkaloid. The second response, dependent on recycled Ca2+, was used as a measure of sarcoplasmic reticulum function. A third protocol evaluated caffeine-induced contractions after Ca2+ depletion and reloading. In these latter experiments the effects of thapsigargin, an inhibitor of the sarcoplasmic reticulum Ca-ATPase, and ryanodine, an activator of sarcoplasmic reticulum Ca2+ release channels, were used to evaluate Ca2+ buffering. Finally, unidirectional 45Ca2+ influx was measured. RESULTS: Contractions to caffeine (0.3-30 mmol/l) were larger in SHRSP aortic strips than in WKY rat strips. After a rinse at the peak of the initial response to caffeine, SHRSP segments contracted more when challenged a second time. Thapsigargin (0.3-10 mumol/l) caused a concentration-dependent contraction during Ca2+ loading that was greater in SHRSP than in WKY rat strips, and a concentration-dependent inhibition of caffeine-induced contraction with similar median inhibitory concentrations in the two groups. Ryanodine did not cause contraction during Ca2+ loading, but caffeine-induced contractions were reduced after ryanodine treatment in both groups. 45Ca2+ influx was increased in SHRSP aortic segments. CONCLUSIONS: The greater force development to caffeine in SHRSP aortic strips probably reflects a greater storage of activator Ca2+ in the sarcoplasmic reticulum. On the basis of the pharmacological properties of thapsigargin and ryanodine, it appears that the larger store is caused by enhanced Ca2+ influx across the sarcolemma rather than by recycling of Ca2+ by sarcoplasmic reticulum Ca-ATPase. Experiments evaluating the secondary response to caffeine also support the interpretation that recycling of activator Ca2+ into the sarcoplasmic reticulum does not explain the augmented force development in SHRSP aortic segments.

Animals↗

PubMed1994

Mechanism of ergonovine-induced contraction in the mesenteric artery from deoxycorticosterone acetate-salt hypertensive rat.

Contractile responsiveness of the rat mesenteric artery to the ergot alkaloid ergonovine is enhanced in deoxycorticosterone acetate (DOCA) hypertension. This study determines if this abnormality is mediated through serotonergic or alpha adrenergic receptors and investigates the cellular mechanism of the contraction. Mesenteric arteries were dissected from DOCA hypertensive and normotensive rats for use in isolated tissue experiments. Contractions to 5-hydroxytryptamine, phenylephrine, norepinephrine, dopamine and histamine were of a similar maximum in the hypertensive and sham artery with phenylephrine, dopamine and serotonin more potent in hypertension. Ergonovine contracted hypertensive arteries (maximum = 245 +/- 27 mg) but only minimally in sham arteries (maximum = 83 +/- 19 mg). Endothelium removal did not enhance contraction to ergonovine. The alpha-1 antagonist prazosin (10(-6) M) and the serotonergic antagonist 1-naphthylpiperazine (10(-6) M) shifted the ergonovine concentration response of the hypertensive artery rightward. The alpha-2 receptor antagonist idazoxan (10(-6) M) and dopamine antagonist haloperidol did not affect contraction to ergonovine. Contraction to ergonovine was not altered by indomethacin or 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate and was minimally affected by genistein, indicating that ergonovine does not activate pathways which involve cyclooxygenase, phospholipase C or tyrosine kinases, respectively. The protein kinase C inhibitor chelerythrine (10(-5) M), nifedipine (10(-6) M) and calcium-free medium blocked ergonovine-induced contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

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.

Animals↗

PubMed1993

Vascular myocyte-derived nitric oxide is an autocrine that limits vasoconstriction.

Vascular myocytes and endothelial cells possess the enzymatic machinery to generate nitric oxide from L-arginine. This study tests the hypothesis that myocyte-derived nitric oxide has an autocrine function to inhibit contraction. Rat aortic rings were placed in muscle baths for isometric force measurement. Denuded and intact rings contracted to N omega nitro-L-arginine; L-arginine reversed these contractions. Compared to intact rings, contractile sensitivity to phenylephrine was increased in denuded rings; N omega nitro-L-arginine caused a further enhancement of phenylephrine sensitivity. Acetylcholine contracted denuded rings but not intact rings; these contractions were also potentiated by N omega nitro-L-arginine. In intact rings contracted with phenylephrine, acetylcholine caused relaxation that was inhibited by N omega nitro-L-arginine. Denuded rings did not relax to acetylcholine. In summary, contractile responses of rat aortae to interventions that alter nitric oxide production are the composite of enzymatic activity in both the endothelial cells and myocytes. Thus, myocyte-derived nitric oxide modulates vascular tone.

Acetylcholine↗