Acute hypotension induced by aortic clamp vs. PTH provokes distinct proximal tubule Na+ transporter redistribution patterns

Renal parathyroid hormone (PTH) action is often studied at high doses (100 mug PTH/kg) that lower mean arterial pressure significantly, albeit transiently, complicating interpretation of studies. Little is known about the effect of acute hypotension on proximal tubule Na+ transporters. This study aimed to determine the effects of acute hypotension, induced by aortic clamp or by high-dose PTH (100 mug PTH/kg), on renal hemodynamics and proximal tubule Na/H exchanger isoform 3 (NHE3) and type IIa Na-P-i cotransporter protein (NaPi2) distribution. Subcellular distribution was analyzed in renal cortical membranes fractionated on sorbitol density gradients. Aortic clamp-induced acute hypotension ( from 100 +/- 3 to 78 +/- 2 mmHg) provoked a 62% decrease in urine output and a significant decrease in volume flow from the proximal tubule detected as a 66% decrease in endogenous lithium clearance. There was, however, no significant change in glomerular filtration rate (GFR) or subcellular distribution of NHE3 and NaPi2. In contrast, high-dose PTH rapidly (<2 min) decreased arterial blood pressure to 51 +/- 3 mmHg, decreased urine output, and shifted NHE3 and NaPi2 out of the low-density membranes enriched in apical markers. PTH at much lower doses (<1.4 mug . kg(-1) . h(-1)) did not change blood pressure and was diuretic. In conclusion, acute hypotension per se increases proximal tubule Na+ reabsorption without changing NHE3 or NaPi2 subcellular distribution, indicating that trafficking of transporters to the surface is not the likely mechanism; in comparison, hypotension secondary to high-dose PTH blocks the primary diuretic effect of PTH but does not inhibit the PTH-stimulated redistribution of NHE3 and NaPi2 to the base of the microvilli.