Parathyroid hormone and dibutyryl cAMP inhibit Na+/H+ exchange in renal brush border vesicles

Parathyroid hormone (PTH) and cAMP inhibit sodium, water, and bicarbonate reabsorption in the proximal tubule. We wished to determine whether these agents directly inhibit proximal tubular Na+/H+ exchange. A suspension of rabbit proximal tubules was prepared by enzymatic digestion and Ficoll gradient centrifugation. Oxygen consumption at 37 degrees C was stable over 60 min, averaged 20 nmol X mg protein-1 X min-1, and was inhibited 60% by ouabain. Over 96% of cells excluded trypan blue. From this suspension, brush border membrane vesicles were isolated. The vesicles were enriched 12.7 times in alkaline phosphatase relative to a cortical homogenate and demonstrated pH gradient-stimulated, amiloride-sensitive Na+/H+ countertransport and sodium-phosphate and sodium-D-glucose cotransport. When the tubule suspension was exposed to PTH or dibutyryl cAMP, the activity of Na+/H+ countertransport in the resultant brush border vesicles was inhibited. Neither PTH nor dibutyryl cAMP affected the amiloride-insensitive component of sodium transport or sodium-phosphate or sodium-D-glucose cotransport. The effect of PTH on Na+/H+ counter-transport could not be explained by an alteration in fluidity of the brush border membrane. These experiments demonstrate that PTH and dibutyryl cAMP directly inhibit Na+/H+ countertransport in the brush border membrane of the rabbit proximal tubule.