Nitroprusside-induced vascular relaxation in DOCA hypertensive rats

Na+/Ca2+ exchanger overexpression in smooth muscle augments cytosolic Ca2+ in femoral arteries of living mice

Plasma membrane Na+/Ca2+ exchanger-1 (NCX1) helps regulate the cytosolic Ca2+ concentration ([Ca2+]CYT) in arterial myocytes. NCX1 mediates both Ca2+ entry and exit and tends to promote net Ca2+ entry in partially constricted arteries. Mean blood pressure (telemetry) is elevated by ≈10 mmHg in transgenic (TG) mice that overexpress NCX1 specifically in smooth muscle. We tested the hypothesis that NCX1 overexpression mediates Ca2+ gain and elevated [Ca2+]CYT in exposed femoral arteries that also express the Ca2+ biosensor exogenous myosin light chain kinase. [Ca2+]CYT and the NCX1-dependent (SEA0400-sensitive) component, ≈15% of total basal constriction in controls, were increased in TG arteries, but constrictions to phenylephrine and ANG II were comparable in TG and control arteries. Normalized phenylephrine dose-response curves and constriction to 30 and 300 ng/kg iv ANG II were virtually identical in control and TG arteries. ANG II-evoked constrictions, superimposed on elevated basal tone, accounted for the larger blood pressure responses to ANG II in TG arteries. TG and control mouse arteries fit the same pCa-constriction relationship over a wide range of pCa (≈125-500 nM). Vasodilation to acetylcholine, normalized to passive diameter, was also comparable in TG and control arteries, implying normal endothelial function. TG artery Na+ nitroprusside (nitric oxide donor)-induced dilations were, however, shifted to lower Na+ nitroprusside concentrations, indicating that TG myocyte vasodilator mechanisms were augmented. Maximum arterial dilation was comparable in TG and control mice, although passive diameter was ≈6-7% smaller in TG mice. The changes in TG arteries were apparently largely functional rather than structural, despite the congenital hypertension. NEW & NOTEWORTHY Smooth muscle Na+/Ca2+ exchanger-1 transgene overexpression (TG mice) increases femoral artery basal cytosolic Ca2+ concentration ([Ca2+]CYT) and tone in vivo and raises blood pressure. Arterial constriction to phenylephrine and angiotensin II are normal but superimposed on the augmented basal [Ca2+]CYT and tone (constriction) in TG mouse arteries. Similar effects in resistance arteries would explain the elevated blood pressure. Acetylcholine-induced vasodilation is unimpaired, implying a normal endothelium, but TG arteries are hypersensitive to sodium nitroprusside.

Altered coronary dilation in deoxycorticosterone acetate-salt hypertension

OBJECTIVE

To compare coronary dilation in uninephrectomized hypertensive deoxycorticosterone acetate (DOCA)-salt rats (HTRs), treated for 2 or 4 weeks, with age-matched uninephrectomized normotensive rats (NTRs). DESIGN AND METHODS Coronary perfusion pressure was recorded in isolated hearts perfused at a constant flow rate to evaluate coronary resistance.

RESULT

A decreased vasoconstriction due to NG-nitro-Larginine (NNLA, 30 pmol/I) in hearts from HTRs suggested a reduced basal nitric oxide (NO) release. In contrast, coronary vasodilation due to the NO donor, sodium nitroprusside (3 pmol/I), remained unaffected in 2-week HTRs, and was enhanced in 4-week HTRs. Cumulative dose-response curves to bradykinin induced an important vasodilation in NTRs, with a maximal response that remained unaffected in the presence of either NNLA (30 pmol/I), indomethacin (10 pmol/l) or the two combined. In contrast, hearts from HTRs showed a diminished maximal relaxation to bradykinin, suggesting an altered endothelium-dependent relaxation. The presence of NNLA or indomethacin had no effect on the weak relaxation observed in HTRs. However, NNLA and indomethacin combined unmasked an important relaxation due to bradykinin in HTRs. The addition of clotrimazole (1 pmol/I) to NNLA and indomethacin blunted the relaxation due to bradykinin in both NTRs and HTRs. Perfusion with superoxide dismutase (120 IU/ml) restored most of the coronary relaxation due to bradykinin in hearts from HTRs. Bradykinin-induced prostaglandin 12 (PGI2) and E2 (PGE2) production was unaffected by hypertension. No increase in thromboxane A2 (TXA2) due to bradykinin was detected. Finally, reduced reactivity to papaverine and forskolin was observed in hearts from HTRs.

CONCLUSION

DOCA-salt hypertension is associated with alterations in coronary reactivity. Basal NO formation appears to be reduced in HTRs, but the intact relaxation to exogenous NO suggests a preserved guanylate cyclase pathway. In addition, alteration in adenylate cyclase activity, and not in prostaglandin production, may explain the blunted cAMP-mediated responses in HTRs. The combined nitric-oxide synthase (NOS) and cyclo-oxygenase (COX) inhibition unmasked an endothelium-derived hyperpolarizing factor (EDHF) involvement in the coronary dilation due to bradykinin in hearts from HTRs, suggesting that endothelial NO and PGI2, although unable to induce coronary smooth-muscle relaxation, can inhibit EDHF production in HTRs. Impairment in the adenylate cyclase pathway and the suppression of NO by free radicals may explain the blunted vasodilation in DOCA-salt hypertension.

Smooth muscle hypertrophy and arterial remodelling in deoxycorticosterone acetate-salt hypertension

Morphometric analyses were made of medial-intimal cross-sectional area and lumen diameter in transverse sections of large arteries and small arterioles from normal and deoxycorticosterone acetate-salt hypertensive animals whose vasculatures were perfusion fixed at their in vivo mean arterial pressures. During the borderline and established phases of hypertension, changes in medial cross sectional area and lumen diameter were not detected in any vessel. During chronic mineralocorticoid hypertension, significant (p < 0.05) medial hypertrophy and an increased lumen diameter were found in the abdominal aorta whereas these parameters were normal in other large conduits, small arteries, and arterioles examined. These results indicate that medial hypertrophy is a late consequence of chronic hypertension in this model and is confined primarily to the abdominal aorta. Further, remodelling without medial hypertrophy may explain the normal lumen diameters of small arteries and arterioles in this model of hypertension.

Enhanced release of endothelium-derived relaxing factor in mineralocorticoid hypertension

Ring segments of superior mesenteric arteries studied in vitro were used to determine the role of the vascular endothelium in regulating vascular contractile and relaxant sensitivity in deoxycorticosterone acetate (DOCA)-salt hypertension. Wistar rats were given DOCA (20 mg/kg s.c. twice per week) and 1% NaCl drinking water for 5 weeks. In ring segments containing endothelium, there was a decrease in contractile sensitivity to arginine vasopressin, no change in contractile sensitivity to norepinephrine and KCl, and no change in relaxant sensitivity to acetylcholine or isoproterenol in arteries from hypertensive rats compared with normotensive controls. Removal of the vascular endothelium by rubbing had no effect on the contractile response to arginine vasopressin and KCl or the relaxant response to isoproterenol in control arteries. In arteries without endothelium, DOCA-salt hypertension caused a threefold increase in contractile sensitivity for arginine vasopressin, norepinephrine, and KCl; a 50% reduction in maximal relaxation to isoproterenol; and a threefold decrease in relaxant sensitivity to sodium nitroprusside. Indomethacin (10 microM) had no effect on contraction or relaxation. However, N-monomethyl L-arginine unmasked altered contractile sensitivity to norepinephrine in arteries from DOCA-salt hypertensive rats. These data show that the endothelium compensates for increased contractile and reduced relaxant responses of vascular muscle in DOCA-salt hypertension by increasing the release of endothelium-derived relaxing factor. These data suggest that altered vascular responsiveness is masked by the endothelium, thus preventing these alterations from contributing to increased peripheral resistance during the development of DOCA-salt hypertension.

Vascular responsiveness in rats resistant to aldosterone-salt hypertension

Wistar-Furth rats have been shown to be resistant to mineralocorticoid-salt hypertension, but the mechanism for this resistance is unknown. In the current experiments, adult male Wistar and Wistar-Furth rats were given a subcutaneous aldosterone infusion (0.15 microgram/hr) for 4 weeks, and changes in blood pressure and vascular reactivity were studied. Rats received a 1% NaCl, 0.2% KCl solution to drink. After 4 weeks of aldosterone infusion, systolic blood pressure measured using a tail-cuff technique had increased by 60 mm Hg in Wistar rats but was unchanged in Wistar-Furth rats. Hypokalemia occurred in both strains in response to the aldosterone infusion. Isolated, helically cut strips of common carotid artery and aorta were prepared for isometric force recording. Cumulative concentration-response curves to norepinephrine, serotonin, KCl, calcium, nitroprusside, and acetylcholine were performed in carotid artery strips, and concentration-response curves to ouabain were performed in aortic strips. Increased vascular contractile sensitivity to KCl, ouabain, norepinephrine, and serotonin was observed in vessels from Wistar rats treated with aldosterone and salt. The same treatment in Wistar-Furth rats produced only increased vascular sensitivity to ouabain and serotonin, and these changes were of smaller magnitude than those seen in Wistar rats. Aldosterone-salt treatment produced decreased vascular sensitivity to acetylcholine and nitroprusside in both Wistar and Wistar-Furth rats. These results support the hypothesis that resistance of Wistar-Furth rats to aldosterone-salt hypertension is due to resistance to the effects of aldosterone-salt treatment that normally result in increased vasoconstrictor sensitivity.

Endothelium-dependent and endothelium-independent vasodilation in resistance arteries from hypertensive rats

The endothelium-dependent and presumed endothelium-independent vasodilators acetylcholine and sodium nitroprusside, respectively, were used to characterize relaxation responses of mesenteric resistance arteries from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto rats (WKY). Vessels were preconstricted using concentrations of norepinephrine or 5-hydroxytryptamine, which reduced their diameters by 50 to 60%. Relaxation responses to acetylcholine (10(-8) - 10(-7) M) were significantly smaller (p less than 0.05) in vessel segments from SHRSP, but the maximal relaxations at higher concentrations were the same in both strains. However, SHRSP vessels relaxed to a greater extent than did those of the WKY at all concentrations of sodium nitroprusside. Endothelium removal significantly enhanced sodium nitroprusside-induced dilations in both rat strains, and the dilations were significantly greater in segments from SHRSP in the concentration range of 3 X 10(-8) to 10(-6) M. The decreased relaxation to acetylcholine in resistance arteries from adult hypertensive rats compared with those from the normotensive strain suggests that functional alterations in the endothelium may play a role in hypertensive disease.

Systemic and coronary hemodynamic effects of pinacidil in awake normotensive and hypertensive dogs

We studied the systemic and coronary hemodynamic effects of a new antihypertensive agent, pinacidil, in nine morphine-sedated chronically instrumented dogs with one-kidney renal hypertension and eight similarly treated sham-operated normotensive dogs. The renal hypertensive dogs exhibited higher mean aortic blood pressure, total peripheral vascular resistance, and plasma renin activity before pinacidil administration than the sham-operated animals. The renal hypertensive dogs also had a lower left ventricular norepinephrine content, but the two groups did not differ significantly in plasma norepinephrine levels, cardiac output, or heart rate. Pinacidil decreased mean aortic pressure and total peripheral vascular resistance and increased cardiac output and heart rate in both groups. The changes in aortic pressure, total peripheral vascular resistance, and cardiac output were similar between the two groups, but the increase in heart rate was attenuated in renal hypertension. The peak rate of rise of left ventricular pressure (dP/dt), the ratio of left ventricular dP/dt and the developed pressure during isovolumic contraction (dP/dt/P), myocardial oxygen consumption, and plasma norepinephrine levels increased after pinacidil administration in the sham-operated dogs, but did not change in the renal hypertension group. The two groups did not differ in their responses of left ventricular dP/dt to intravenous isoproterenol. Pinacidil also caused coronary vasodilation in both groups, as evidenced by an increase in coronary blood flow and decreases in coronary vascular resistance and myocardial oxygen extraction. The decrease in myocardial oxygen extraction was similar in the two groups, but the increase in coronary blood flow was significantly less (p less than 0.05), probably because of the absence of an increase in myocardial oxygen consumption in the renal hypertensive dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane mechanisms in arterial hypertension

The purpose of this review is to focus on alterations in vascular muscle membrane potentials (Em), ionic permeabilities, and ionic transport systems which may either contribute to or be a consequence of the hypertensive state. Three models of hypertension are discussed: 1) deoxycorticosterone-salt (DOCA-salt)-induced hypertension; 2) low-renin (presumably volume expanded) renal hypertension (LRRH); and 3) the spontaneously hypertensive rat (SHR) of the Okamoto-Aoki Kyoto-Wistar strain and its normotensive genetic control (WKY). The importance of studying all possible mechanisms of increased contraction in vascular smooth muscle is stressed.