David L. Clough

Humoral factors and the sodium-potassium pump in volume expanded hypertension

Abstract Bioassay studies in the old and recent literature suggest the presence of an unknown slowly acting pressor agent in the blood of animals and man with volume expanded (low renin) hypertension. Recent studies in our laboratories suggest that the sodium-potassium pump activity of blood vessels is suppressed in animals with one-kidney, one wrapped, one-kidney, one clip, and one-kidney, DOCA, salt hypertension. Similar reduction of Na + , K + -ATPase activity has been observed in the left ventricle of animals with one-kidney, one clip and one-kidney, DOCA salt hypertension. The changes do not appear to result from increased pressure since they have also been observed in veins and right ventricle. Acute volume expansion of the normal rat with saline suppresses pump activity in the tail artery and plasma from these animals suppresses pump activity when applied to a tail artery from another rat. Data in the literature indicate that the adrenergic nerve terminals are depleted of norepinephrine. Suppression of pump activity, with ouabain for example, is known to activate cardiovascular muscle and reduce norepinephrine uptake by nerve terminals. These observations suggest a role for a slowly acting ouabain-like humoral agent, which acts directly on cardiovascular muscle to increase contractility and on nerve endings to reduce reflex compensation, in the genesis of volume expanded hypertension.

The Sodium-Potassium Pump in Volume Expanded Hypertension

Decreased arterial Na+-K+ pump and cardiac Na+, K-+ATPase activities have now been demonstrated in several types of experimental volume expanded hypertension. The changes are not secondary to elevated pressure since they also occur in veins and right ventricle where the pressure is not elevated. Decreased arterial Na+-K+ pump activity can be reproduced by acute volume expansion of the normal rat and plasma extracts from this rat suppress pump activity when applied to arteries from another rat. Suppression of Na+-K+ pump activity in arteries, veins and heart, with ouabain for example, leads to increased contractile activity. Thus the volume expansion, reduced pump activity, and hypertension appear to be causally related through an ouabain-like humoral agent. Certain other evidence suggests that the pump defect extends to the sympathetic nerve endings, thereby reducing the efficiency of neural compensatory mechanisms.

Effects of rat atrial extract on sodium transport and blood pressure in the rat.

Abstract Atrial cardiocytes contain specific atrial granules (SAGs) which are the storage site of atrial natriuretic factor (ANF). The purpose of the present study was to determine whether ANF produces natriuresis by inhibiting Na+-K+ pump activity and whether this factor is similar to the humoral sodium transport inhibiting factor (HSTIF) previously demonstrated in acutely volume expanded animals and humans as well as in experimental and human essential hypertension. Our results indicate that, in contrast to the HSTIF, ANF does not inhibit membrane Na+, K+-ATPase, vascular smooth muscle cell Na+-K+ pump activity, or sodium transport in the toad bladder. Intravenous infusion of ANF in the bilaterally nephrectomized, hexamethoniumtreated rat produces only a small transient pressor response, probably due to potentiation of endogenous norepinephrine. These findings strongly suggest that the ANF is not the same as the HSTIF detected on acute volume expansion and in some forms of hypertension. They also suggest that the diuretic and natriuretic effects of ANF are due to mechanism(s) other than blood pressure elevation and inhibition of Na+-K+ pump activity.

Protection by pyrdvate against inhibition of Na+,K+-ATPase by a free radical generating system containing T-butylhydroperoxide

Global tissue damage due to oxygen-derived free radicals has been implicated in several pathological processes including exposure to ionizing radiation, and postischemic reperfusion of the heart or kidney. Recently pyruvate, a hydroperoxide scavenger, has been shown to protect against functional damage during postischemic reperfusion of the heart and in acute renal failure. In the present study, pyruvate was found to protect against inactivation of partially purified guinea pig renal and rat cardiac Na+,K(+)-ATPase which occurred when microsomal membranes were assayed for 1 hr at 37 degrees C (pH 7.5) in the presence of a free radical generating system (FRGS) containing 0.3 mM t-butylhydroperoxide and horseradish peroxidase. The presence of the FRG system inhibited the guinea pig renal Na+,K(+)-ATPase activity by 48.2 +/- 4.8% (N = 10, P < .05) and the presence of 0.2 to 20 mM pyruvate partially protected the Na+,K(+)-ATPase. At 5 mM pyruvate Na+,K(+)-ATPase was inhibited by only 18.8 +/- 2.5% (N = 10, P < .05) but increasing the pyruvate concentration gave no further protection. Equimolar concentrations of glucose, mannitol or lactate were without effect. The protection appeared to require an alpha-keto acid since alpha- but not beta-ketoglutarate was also effective and the mechanism is most probably the scavenging of t-BHO2. The results of the present study therefore support the hypothesis that, if free radical damage to native Na+,K(+)-ATPase does contribute to global tissue injury in certain pathological processes, pyruvate, in addition to being a powerful metabolic effector of recovery, may also protect against oxidative damage.

Vascular Na+-K+ Pump Activity in Dahl S and R Rats

Abstract Ouabain-sensitive 86rubidium uptake was used to estimate sodium-potassium pump activity in the tail arteries of Dahl salt-sensitive and Dahl salt-resistant rats on normal and high oral intakes of salt. Uptake was increased in the salt-sensitive strain relative to the resistant strain at a given salt intake. It was also increased in a given strain when the salt intake was increased. In each case the increased ouabain-sensitive uptake was associated with increased ouabain-insensitive uptake which in part reflects the permeability of the cell membrane to rubidium. The results suggest that the increased pump activity is a secondary compensatory response to increased passive penetration of sodium. In this respect, the Dahl salt-sensitive rat is similar to SHR, another genetic model, but different from the other low-renin, presumably, volume-expanded models of hypertension we have studied.

Role of a humoral sodium-potassium pump inhibitor in experimental low renin hypertension

Recent evidence suggests that the vascular sodium-potassium pump suppression previously observed in animals with various models of low renin hypertension results from a circulating heat stable ouabain-like agent. It appears to come from or be influenced by the anteroventral third ventricle area of the brain and its action on blood vessels results in depolarization of the smooth muscle cell. Suppression of the vascular sodium-potassium pump, with ouabain for example, increases contractile activity and the contractile responses to vasoactive agents. Thus the humoral pump inhibitor may be involved in the genesis and maintenance of experimental low renin hypertension.

Myocardial Na,K-ATPase activity in rats with steroid and spontaneous hypertension.

Vascular Na-K pump activity (ouabain-sensitive 86Rb uptake) and cardiac Na,K-ATPase activity are decreased in rats with one-kidney, one clip and reduced renal mass-saline hypertension. In the present study we measured left ventricular, microsomal Na,K-ATPase activity in rats with two steroid forms of hypertension; one-kidney, deoxycorticosterone acetate-saline (1-K, DOCA-saline) and one-kidney, dexamethasone (1-K, DEXA) hypertension and also in spontaneously hypertensive rats (SHR). Relative to one-kidney, normotensive (1-K, NT) control rats, cardiac Na,K-ATPase activity was decreased and Mg-ATPase activity was increased in rats with 1-K, DOCA-saline hypertension (systolic BP = 175 +/- 2 mmHg, sustained eight to 10 weeks, n = 11). The apparent dissociation constant of cardiac Na,K-ATPase for K was unchanged in these hypertensive rats. In rats with 1-K, DEXA hypertension (systolic BP = 171 +/- 1 mmHg, sustained eight to 10 weeks, n = 9), cardiac Na,K-ATPase activity was increased and Mg-ATPase activity was unchanged relative to 1-K, NT control rats. We observed no change in either cardiac Na,K-ATPase activity or Mg-ATPase activity in SHR (36-38 weeks of age, systolic BP = 186 +/- 7 mmHg, n = 12) relative to either age-matched normotensive Wistar-Kyoto or Wistar control rats. These studies therefore suggest that cardiac Na,K-ATPase activity is decreased in 1-K, DOCA-saline hypertensive rats, increased in 1-K, DEXA hypertensive rats and unchanged in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial (Na+, K+)-ATPase activity in dahl salt-sensitive and resistant rats

Vascular (Na+,K+)-pump activity (ouabain-sensitive 86Rb+ uptake) and myocardial (Na+,K+)-ATPase activity are reduced in animals with various forms of low renin, experimental hypertension. On the other hand, vascular (Na+,K+)-pump activity is increased in Dahl salt-sensitive relative to resistant rats (a genetic model of hypertension), regardless of salt intake or blood pressure and it is also increased in Dahl salt-sensitive rats on high salt (8% NaCl) relative to low salt (0.4% NaCl) diets. It has been suggested that this increase in vascular (Na+,K+)-pump activity may be secondary to an increase in the vascular sarcolemmal permeability to Na+ in these salt-sensitive rats. In the present study, (Na+,K+)-ATPase activity of left ventricular microsomal fractions, was increased in Dahl salt-sensitive relative to resistant rats on low salt diets; however, this difference disappeared when these salt-sensitive and resistant rats were placed on high salt diets. In contrast, myocardial (Na+,K+)-ATPase activity was decreased in Dahl salt-sensitive rats on high relative to low salt diets. Evidence that this decrease in (Na+,K+)-ATPase activity is not secondary to myocardial hypertrophy in the hypertensive salt-sensitive rats, and mechanisms by which decreased cardiovascular (Na+,K+)-pump activity, increased sarcolemmal permeability or both, might contribute to elevated blood pressure, are discussed.

Greater sensitivity to vanadate of rat renal relative to cardiac (Na++K+)-ATPase

Vanadate (VO4(-3] produces a positive inotropic effect in rats and also promotes diuresis as well as natriuresis. Although the mechanism(s) of these effects is uncertain, in the kidney, VO4(-3) may act through inhibition of (Na+ + K+)-ATPase activity, whereas in the heart, other or additional mechanisms are likely. Under the assay conditions used in the present study, microsomal (Na+ + K+)-ATPase activities from rat kidney cortex and medulla were inhibited to a greater extent than was left ventricular (Na+ + K+)-ATPase activity over a range of VO4(-3) concentrations. The apparent dissociation constant for left ventricular (Na+ + K+)-ATPase (10.95 +/- 1.26 X 10(-7)M VO4(-3] was significantly greater than that of (Na+ + K+)-ATPase from the cortex (3.46 +/- 0.96 X 10(-7)M VO4(-3] or the medulla (3.32 +/- 0.7 X 10(-7)M VO4(-3), N = 6, P less than .05) whereas there were no significant differences between the effects of VO4(-3) on (Na+ + K+)-ATPase from the cortex and medulla. The greater inhibition by VO4, of (Na+ + K+)-ATPase from the cortex relative to that of the left ventricle, occurred over a range of Na+ and K+ concentrations, and K+ enhanced the inhibition by VO4(-3) to a greater extent for (Na+ + K+)-ATPase from the cortex than the left ventricle. These results suggest that renal (Na+ + K+)-ATPase is more sensitive than left ventricular (Na+ + K+)-ATPase to inhibition by VO4(-3) and would, therefore, be more likely to be modulated in vivo.

Position Paper: Sodium Metabolism: The Sodium-Potassium Membrane Pump and Volume Overload Hypertension

The mechanism of salt-dependent, volume-overloaded hypertension is particularly troublesome (1–3). Renin levels are low, and the responses to converting enzyme inhibitors and angiotensin antagonists are minimal. Catecholamine levels are not helpful; in fact, plasma catecholamine levels decrease as a function of salt intake in normal subjects. Long-term autoregulation subsequent to increased cardiac output and overperfusion of tissues has been considered, but increase in total peripheral resistance and blood pressure have been observed in the absence of increased cardiac output.