CAMP-DEPENDENT CONTROL OF NA+/H+ ANTIPORT BY AVP, PTH, AND PGE2 IN RAT MEDULLARY THICK ASCENDING LIMB CELLS

Antidiuretic hormone and parathyroid hormone (PTH) inhibit HCO3- absorption by the rat medullary thick ascending limb (MTAL). Studies were performed on rat MTAL tubule suspension to specify the H+-HCO3- membrane transporters affected by these hormones and the implicated intracellular second messengers. Arginine vasopressin (AVP) and PTH stimulated cell adenosine 3',5'-cyclic monophosphate (cAMP) production with a relative rank order potency of AVP > rat PTH-(1-34) > bovine PTH-(1-84). Significant cell acidification in HCO3--CO2-free medium, monitored in 2'7'-bis(carboxyethyl)-5(6')-carboxyfluorescein-loaded cells, was caused by 0.1 nM AVP, 1 nM rat PTH-(1-34), but not by <100 nM bovine PTH-(1-84), as well as by 10(-4) M 8-bromo-cAMP and 2 x 10(-5) M forskolin; 10 nM AVP or rat PTH(1 - 34) did not alter the intracellular pH when Na+/H+ anti-port was inhibited by 2 mM amiloride. Prostaglandin E2 (PGE2, 10(-6) M), which inhibited AVP-stimulated cell cAMP production, reduced by 35% the cell acidification response to 10 nM AVP. AVP and 8-bromo-cAMP inhibited Na+/H+ antiport-dependent cell pH recovery from intracellular acidification, which was explained by a decrease in the V(max) of the antiporter. AVP did not directly affect K+-HCO3- cotransport and plasma membrane H+-ATPase of rat MTAL cells. Cytosolic calcium ([Ca2+]i), monitored in fura-2-loaded cells, was unaffected by up to 1 nM AVP, 100 nM PTH, glucagon, calcitonin, and oxytocin, and 1 muM PGE2; however, 100 nM AVP, but not 1-desamino-8-D-AVP (dDAVP), caused a peak increase in [Ca2+]i, even in the absence of extracellular Ca2+, and stimulated cell accumulation of [H-3]inositol phosphates. We conclude that in rat MTAL 1) AVP and PTH inhibit Na+/H+ antiport through cAMP but do not directly affect K+-HCO3- cotransport and H+-ATPase; 2) PGE2 reduces AVP effects, likely through an interaction on cAMP production; and 3) AVP also stimulates at high concentrations the phosphoinositide-Ca2+ transduction pathway through V1 receptors.