Mechanisms of activation of renal (Na + + K +)-ATPase in the rat Effects of acute and chronic administration of dexamethasone

Abstract

The mechanisms of activation of renal (Na + + K +)-ATPase by administration of the synthetic glucocorticoid hormone, dexamethasone, have been investigated in adrenalectomized rats. Chronic treatment with dexamethasone (1–5 mg/100 g body wt. daily for 5 days) stimulated (Na + + K +)-ATPase specific activity in crude homogenated and microsomal fractions of renal cortex (by approx. 100–150%) and renal medulla (by approx. 100%). Acute treatment with dexamethasone (0.5–10 mg/100 g body wt.) also stimulated enzyme activity in crude homogenates and microsomal fractions of renal cortex and medulla (by approx. 40–50%). Stimulation was dose dependent and occurred within 2h after hormone treatment. In vitro addition of dexamethasone (10 −4–10 −8 M) to microsomal fractions did not modify the specific activity of (Na + + K +)-ATPase. Stimulation of (Na + + K +)-ATPase activity by acute and chronic administration of the hormone was demonstrated whether specific activities were expressed as a function of cellular protein or cellular DNA. Dexamethasone treatment increased the ratios protein:DNA and, to a lesser extent, the ratios RNA:DNA. However, these effects were mainly due to a reduction in the renal contents of DNA, which suggests that the observed enzyme activation is not due to an action of the hormone on renal hypertrophy. Dexamethasone also reduced cellular DNA contents in the liver. The characteristics of the activation process were essentially similar after treatment with single or multiple doses of the hormone. There were increases in the value for Na + (approx. 100%), K + (approx. 40%) and ATP (approx. 160%). The K m values for Na + (approx. 17 mM) and K + (approx. 1.8 mM) were unchanged and there was a small increase in the K m value for ATP (0.7 mM as against 1.7 mM). There was no difference in the Hill coefficients for the three substrates. The levels of the high-energy P i intermediate of the (Na + + K +)-ATPase reaction were augmented by dexamethasone treatment and the increased levels were quantitatively correlated with the observed stimulation of (Na + + K +)-ATPase specific activity. The apparent turnover numbers of the reaction remained unchanged. The specific activity of the ouabain-sensitive p-nitrophenylphosphatase increased proportionally to the increase in (Na + + K +)-ATPase specific activity. Enzyme activation by acute dexamethasone treatment occurred in the absence of changes in glomerular filtration rate and tubular Na + excretion. These results indicate that (Na + + K +)-ATPase activation by acute and chronic dexamethasone treatment represents an increase in the number of enzyme units with little or no change in the kinetic properties (affinity, cooperativity) of the enzyme. In addition, the information presented suggests a direct regulatory effect of glucocorticoid hormones on the activity of renal (Na + + K +)-ATPase and is inconsistent with the concept that changes in Na + loads mediate the effects of these hormones on enzyme activity. Instead, the results suggests a primary role for glucocorticoid hormones in the renal regulation of Na + homeostasis.