Site of restoration of the effect of PTH by propranolol in respiratory alkalosis.

Respiratory alkalosis decreases the phosphaturic response to parathyroid hormone (PTH). beta-adrenoreceptor blockade by propranolol infusion restores the phosphaturic effect of PTH in respiratory alkalotic rats; however, the nephron site of these changes in phosphate reabsorption by propranolol is not known. The present study was performed to localize the nephron segment(s) involved in the restoration of the phosphaturic effect of PTH by propranolol infusion in respiratory alkalotic rats. PTH infusion increased the fractional delivery of phosphate (FDPi) to the late proximal tubule to similar levels in the propranolol and vehicle-infused respiratory alkalotic rats (FDPi 46.6 +/- 4.4% and 48.6 +/- 4.2%, respectively). In contrast, PTH only increased FDPi to the early distal tubules (to 17.1 +/- 0.9%) in the absence of propranolol compared to FDPi 41.9 +/- 2.5% in the presence of propranolol in respiratory alkalotic animals. We conclude that the restoration of the phosphaturic effect of PTH in respiratory alkalotic rats by propranolol infusion is due primarily to decreased reabsorption of phosphate by the straight segment of the proximal tubule.