Acute arterial hypertension provokes a rapid decrease in proximal tubule (PT) Na+ reabsorption, increasing flow to the macula densa, the signal for tubuloglomerular feedback. We tested the hypothesis, in rats, that Na+ transport is decreased due to rapid redistribution of apical Na+/H+ exchangers and basolateral Na+ pumps to internal membranes. Arterial pressure was increased 50 mmHg by constricting various arteries. We also tested whether transporter internalization occurred when PT Na+ reabsorption was inhibited with the carbonic anhydrase inhibitor benzolamide. Five minutes after initiating either natriuretic stimuli, cortex was removed, and membranes were fractionated by density gradient centrifugation. Urine output and endogenous lithium clearance increased threefold in response to either stimuli. Acute hypertension provoked a redistribution of apical Na+/H+ exchanger NHE3, alkaline phosphatase, and dipeptidyl peptidase IV to higher density membranes enriched in the intracellular membrane markers. Basolateral membrane Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) activity decreased 50%, 25-30% of the alpha 1-and beta 1-subunits redistributed to higher density membranes, and the remainder is attributed to decreased activity of the transporters. Benzolamide did not alter Na+ transporter activity or distribution, implying that decreasing apical Na+ uptake does not initiate redistribution or inhibition of basolateral Na(+)-K(+)-ATPase. We conclude that PT natriuresis provoked by acute arterial pressure is mediated by both endocytic removal of apical Na+/H+ exchangers and basolateral Na+ pumps as well as decreased total Na+ pump activity.

The purpose of this study was to determine the pattern of thyroid hormone (triiodothyronine, T3) regulation of the Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) alpha- and beta-subunit expression in skeletal muscle, which expresses alpha 1-, alpha 2-, beta 1-, and beta 2-subunits, and compare it with that seen in kidney, which expresses only alpha 1 and beta 1. Three steady states were studied: hypothyroid, euthyroid, and hyperthyroid (hypothyroids injected daily with 1 microgram T3/g body wt for 2-16 days). Protein and mRNA abundance, determined by Western and Northern analysis, were normalized to a constant amount of homogenate protein and total RNA, respectively. In skeletal muscle, there was no change in alpha 1- or beta 1-mRNA or protein levels in the transition from hypothyroid to hyperthyroid. However, alpha 2 was highly regulated; mRNA reached a new steady-state level of fivefold over hypothyroid by 8 days of T3 treatment and protein abundance increased threefold. In addition, beta 2-mRNA and protein were detected in skeletal muscle and were also highly regulated by T3; beta 2-mRNA increased nearly fourfold over hypothyroid level, and beta 2-protein abundance increased over twofold. In kidney in the transition from hypothyroid to hyperthyroid, there were coordinate 1.6-fold increases in both alpha 1- and beta 1-mRNA abundance that predicted the observed changes in alpha 1- and beta 1-protein levels and Na(+)-K(+)-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Parathyroid hormone (PTH) is believed to inhibit bicarbonate reabsorption by inhibiting Na-H antiport activity in proximal tubular brush-border membranes. The sequence of events triggered by PTH was investigated in a crude preparation of proximal tubules obtained by mechanical disruption and filtration through nylon mesh filters. Tubule samples were subjected to analytical subcellular fractionation after 2-, 5-, and 30-min treatments with 1 IU/ml PTH. These PTH-treatment intervals caused 54, 63, and 68% decreases in the Na-H antiport activity of a population of brush-border membrane vesicles that was resolved from a PTH-unresponsive brush-border population by density-gradient centrifugation. The rapid loss of Na-H antiport activity from the responsive population was accompanied by a transient increase in the Na-H antiport activity of a region of the density gradient, designated density window III, which was shown to contain two distinct membrane populations; these populations were both enriched in acid phosphatase activity, and one of them was also an important locus of galactosyltransferase activity. The increase in the Na-H antiport activity of window III accounted for 52% of the activity lost from the PTH-responsive population after 2 min, and for 43% of the activity lost after 5 min, but it was completely abolished after 25 more minutes in the presence of PTH. These observations suggest that PTH triggers a rapid translocation of Na-H antiporters from the microvillus membrane to a distinct membrane domain, where they are subsequently inactivated.

K+ deficiency has been linked to a loss of K+ from muscle associated with a decrease in ouabain binding and K(+)-dependent phosphatase activity. This study aimed to quantitate the Na(+)-K(+)-ATPase alpha- and beta-isoform-specific responses to hypokalemia in vivo in heart, skeletal muscle, and brain at pre- and posttranslational levels. Two-week dietary K+ restriction resulted in decreases in alpha 2-mRNA in heart and skeletal muscle to 0.60 and 0.65, and in alpha 2-protein abundance to 0.38 and 0.18 of control, respectively. The decrease in alpha 2-protein was greater than the decrease in mRNA in both tissues, suggesting translational and/or posttranslational mechanism(s) of regulation as well as pretranslational regulation in response to hypokalemia. K(+)-dependent p-nitrophenyl phosphatase (pNPPase) activity decreased in heart and skeletal muscle to 0.67 and 0.58, respectively. There were no changes in alpha 1-. or beta-mRNA or protein levels in skeletal muscle or heart. In brain, there was a similar pattern of regulation. While brain alpha 2-mRNA did not change in hypokalemia, protein levels decreased to 0.72 of control. In conclusion, hypokalemia is associated with a large decrease in expression of the alpha 2-isoform of Na(+)-K(+)-ATPase. These results support the hypothesis that in skeletal and heart muscle hypokalemia induces a decrease in Na(+)-K(+)-ATPase activity (measured as K(+)-dependent pNPPase activity) by specifically decreasing the expression of the alpha 2-isoform of Na(+)-K(+)-ATPase.(ABSTRACT TRUNCATED AT 250 WORDS)

In this study, we measured the time courses of change in Na(+)-K(+)-ATPase alpha 1-, alpha 2-, and beta 1-subunit mRNA and protein abundance in cardiac myocytes isolated from euthyroid, hypothyroid, and hyperthyroid (hypothyroids injected daily with 1 microgram T3/g body wt) rats. In hypothyroids, alpha 1-, alpha 2-, and beta 1-protein levels were decreased to 0.55, 0.42, and 0.57 of euthyroids, predicting the decrease in Na(+)-K(+)-ATPase activity to 0.53 of control. There was no change in these subunits' mRNA levels, indicating that the decreases in protein levels were not due to decreased subunit transcription rates. In hyperthyroids, the alpha 1-mRNA increased to a steady state of threefold over hypothyroid by 1 day of T3 treatment, and the alpha 1-protein levels increased to twofold over hypothyroid by 4 days of T3. alpha 2-mRNA increased to 5-fold over hypothyroid by 2 days, whereas the alpha 2-protein levels increased to 14-fold above hypothyroid by 4 days of T3. Beta 1-mRNA increased to 12-fold above hypothyroid by 1 day of T3 treatment, whereas beta 1-protein increased to only 2.5-fold over hypothyroid by 4 days of T3. The discoordinate changes in alpha 2- and beta 1-mRNA vs. protein can be reconciled with the hypothesis that beta 1 is rate limiting for assembly in eu- and hypothyroids, and favors assembly with alpha 1, while excess unassembled alpha 2 is degraded. In the hyperthyroids we predict beta 1 is not rate limiting and there is increased alpha 2 beta 1-assembly. We calculate that T3 decreases the alpha 1-to-alpha 2 ratio from 24:1 in hypothyroid to 3.4:1 in hyperthyroid cardiomyocytes.