Sodium-potassium pump inhibitor in the mechanism of one-kidney, one wrap hypertension in dogs.

Using ouabain sensitive 86Rb uptake by the vessel wall, we previously showed that sodium-potassium pump activity is decreased in the arteries and veins, and that the sodium-potassium pump inhibitor (SPI) is increased in the plasma of dogs with one-kidney, one wrap (1-K, 1W) hypertension, a low renin model of hypertension. We also showed in rats with a similar type of hypertension that the membrane potential of vascular smooth muscle cells in arteries is decreased, and that this decrease can be reproduced in arterial cells in arteries from normal rats by applying plasma from the hypertensive animals. One endogenous SPI in human plasma has been reported to be ouabain or its isomer. In this study, we used a newly available Dupont ouabain enzyme immunoassay kit to examine plasma and kidneys for SPI in dogs with 1-K, 1W hypertension. We also examined 1) the inhibiting activity of plasma of Na+, K(+)-ATPase obtained from normal kidneys, and 2) the Na+, K(+)-ATPase activity of the kidneys from these hypertensive animals. 1-K, 1W hypertension was produced in dogs by wrapping the left kidney in a silk bag and removing the right kidney. The removed kidney was kept at -70 degrees C till assayed. After 4 weeks of hypertension, the remaining kidney was removed and stored at -70 degrees C till assayed. Blood samples were drawn before and at weeks 3 and 4 of hypertension. Plasma levels of "ouabain" and Na+, K(+)-ATPase inhibitory activity were increased at weeks 3 and 4 of hypertension, compared to pre-hypertension levels. Renal tissue "ouabain" levels were also increased at week 4 of hypertension. However, renal Na+, K(+)-ATPase activity was unchanged. These findings, using two different assays, confirm our 1980 conclusion that SPI is elevated in the plasma of dogs with 1-K, 1W hypertension. The absence of renal Na+, K(+)-ATPase inhibition, despite increased plasma and renal SPI in these animals, may have important implications for the development of this type of hypertension.

Effects of 6-iodo-amiloride, a sodium channel blocker, on cardiovascular parameters in spontaneously hypertensive and Wistar-Kyoto rats.

6-Iodo-amiloride, an analogue of the sodium channel blocker amiloride, was infused intravenously for 10 min in anaesthetized Okamoto spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats in doses ranging from 0.08 to 0.38 mg/100 g body weight. Systemic arterial blood pressure and urine flow were measured for 120 min. In SHR, 6-iodo-amiloride produced a prompt, sustained, dose-dependent decrease in pressure. The lower doses were associated with increased urine flow and sodium excretion, while higher doses were not. Paradoxically, in WKY all doses produced a small dose-independent sustained increase in pressure and were associated with diuresis and natriuresis. 6-Iodo-amiloride had no effect on cardiac output, dP/dt or heart rate in isolated working hearts from SHR or WKY. However, addition of 6-iodo-amiloride to physiological salt solution bathing an isolated Wistar rat tail artery produced hyperpolarization of impaled vascular smooth muscle cells. These studies show that 6-iodo-amiloride is a vasodilatory antihypertensive agent in SHR, and that this can be associated with natriuresis and diuresis.

Sodium channel blockers are vasodilator as well as natriuretic and diuretic agents.

Amiloride (100-400 micrograms) injected intra-arterially into the dog forelimb perfused at constant flow produced a prompt but transient dose-dependent decrease in perfusion pressure. Intravenous injection lowered systemic arterial pressure, but effects were small and transient except in doses exceeding 10 mg. We tested 11 analogues of amiloride, 3 other diuretics, and a hypotensive imidazopyrazine for vasodilator activity in the dog forelimb and found one analogue, 6-iodo-amiloride, with twice the activity of amiloride. Intravenous injection of 3 mg of 6-iodo-amiloride promptly decreased systemic arterial pressure and forelimb perfusion pressure 65 and 47 mm Hg respectively. The decreases with 3 mg of amiloride were only 5 and 23 mm Hg respectively. Intravenous infusion of 17 to 77 mg of 6-iodo-amiloride produced diuresis, natriuresis, and antikaliuresis and, with the higher doses, hypotension. The latter occurred promptly on starting the infusion and was sustained for the duration of the infusion. Wistar rats responded to an intravenous infusion of 0.38 mg/100 g in 11 minutes in the same manner. In the spontaneously hypertensive rat, this same dose produced a large, sustained antihypertensive effect with little change in the urinary parameters. These studies indicate that 6-iodo-amiloride is a vasodilator and a vasodepressor as well as natriuretic and diuretic in the normal dog and rat and that it produces a sustained, large fall in blood pressure, independently of urinary effects, in the spontaneously hypertensive rat. These results suggest that 6-iodo-amiloride and other sodium channel blockers might be useful as vasodilatory antihypertensive agents, particularly in those types of hypertension characterized by increased vascular smooth muscle cell permeability to sodium.

Response of the forelimb vasculature to vasoactive agents: effects of ouabain.

The effect of the local intra-arterial infusion of ouabain (11.8 micrograms/min.) on the response of the forelimb to vasoactive agents was examined. In seven dogs, bolus injections of CaCl2, MgSO4, KCl, norepinephrine, adenosine, acetylcholine, PGE1 and saline were made into the forelimb perfused at constant flow before and three times during ouabain infusion. Ouabain blocked potassium vasodilation and changed the response to CaCl2 from vasoconstriction to vasodilation. The response of the forelimb to the other vasoactive agents was initially unaffected by ouabain but with time the forelimb vasculature became less sensitive to all agents studied. These changes were not seen in a series of 5 saline infused control animals. In a third series of animals steady-state dose responses to CaCl2, Ca-gluconate and KCl were explored by infusing solutions intrabrachially at three dosages. Before ouabain, forelimb resistance increased as a function of Ca++ and decreased as a function of K+. Ouabain completely blocked potassium vasodilation and on the average blocked Ca++ vasoconstriction although a number of animals evidenced vasodilation to Ca++ during ouabain infusion. These data indicate that K+ vasodilation is Na+, K+-ATPase dependent and that Na+, K+-ATPase inhibition unmasks a vasodilatory action of locally applied Ca++.

Cardiovascular effects of vanadate in the dog.

We have investigated the cardiovascular actions of vanadate, a naturally occurring Na+-K+-ATPase inhibitor, in six series of pentobarbitalized dogs. In three of the series, isomotic sodium vanadate was infused intravenously at progressively faster rates while arterial pressure and other parameters were measured. In two other series, the solution was infused directly into the coronary artery with coronary flow held constant during measurement of perfusion pressure, left ventricular contractile force (LVCF), and dP/dt. In one series, the agent was infused into the brachial artery with brachial artery flow held constant, and small and large vessel resistances in skin and muscle were calculated. Intravenous infusion increased arterial pressure and reduced cardiac output, the latter resulting from both decreased heart rate and stroke volume. LVCF fell. Total peripheral, pulmonary, coronary, and renal resistances rose. Coronary and renal flows fell, and the latter was associated with reduced urine flow. Intracoronary infusion raised coronary resistance, but had little effect on heart rate, LVCF, and dP/dt. Intrabrachial infusion raisedthe resistance to flow through all components of the forelimb vascular bed. Thus, in the dog, vanadate activates vascular smooth muscle, but has little effect on cardiac muscle. In the latter respect, its action differs from that of ouabain.

Evidence from bioassay studies indicating a role for adenosine in cardiac ischemic and hypoxic dilation in the dog.

Previous studies using bioassay demonstrated the presence of a vasoactive substance or substances in coronary sinus blood during myocardial reactive hyperemia which had, on bioassay, characteristics of adenosine and/or AMP. In the present studies, specific blockers were applied to an improved bioassay system to define more precisely the nature of the substance or substances and to determine whether it also appears during local cardiac hypoxia. In the anesthetized dog, coronary sinus blood was bioassayed in an isolated autologous kidney during reactive and hypoxic dilation. During reactive dilation, the bioassay kidney responded with a large resistance increase which was blocked by theophylline and adenosine deaminase and converted to a decrease by adenosine autoblockade. The same was true for hypoxic dilation, except in this instance, only adenosine deaminase reduced the response (40%). Theophylline and adenosine autoblockade eliminated responses of the bioassay organ to both exogenous adenosine and exogenous AMP, but adenosine deaminase was specific for adenosine. These results confirm that one or more vasoactive substances appear in sinus blood in vasoactive quantities during cardiac reactive dilation, and that the peak concentration correlates roughly with the peak flow. In addition, they show that this also is the case for hypoxic dilation. More importantly, they demonstrate that the substance almost certainly is adenosine in the case of reactive dilation, and that increased adenosine levels also are present during hypoxic dilation, but in addition, suggest that AMP sometimes appears in coronary sinus blood during severe cardiac hypoxia in the dog heart.