Abstract
Arginine vasopressin (AVP) transiently stimulates Na+ transport in the rabbit cortical collecting duct (CCD). However, the sustained effect of both AVP and its putative second messenger, cyclic adenosine monophosphate (cAMP), on Na+ transport in the rabbit CCD is inhibitory. Because maneuvers that increase [Ca++]i inhibit Na+ transport, the effects of AVP and cell-permeable cAMP analogues, on [Ca++]i were investigated in fura-2-loaded in vitro microperfused rabbit CCDs. Low-dose AVP (23-230 pM) selectively stimulated Ca++ influx, whereas 23 nM AVP additionally released calcium from intracellular stores. 8-chlorophenylthio-cAMP (8CPTcAMP) and 8-bromo-cAMP (8-Br-cAMP) also increased CCD [Ca++]i. The 8CPTcAMP-stimulated [Ca++]i increase was totally dependent on basolateral [Ca++]. In the absence of cAMP, peritubular Na+ removal produced a marked increase in [Ca++]i, which was also dependent on bath [Ca++], suggesting the existence of basolateral Na+/Ca++ exchange. Luminal Na+ removal in the absence of cAMP did not alter CCD [Ca++]i, but it completely blocked the cAMP-stimulated [Ca++]i increase. Thus the cAMP-dependent Ca++ increase is totally dependent on both luminal Na+ and basolateral Ca++, suggesting the [Ca++]i increase is secondary to cAMP effects on luminal Na+ entry and its coupling to basolateral Na+/Ca++ exchange. 8CPTcAMP inhibits lumen-to-bath 22Na flux [JNa(l-b)] in CCDs bathed in a normal Ca++ bath (2.4 mM). However, when bath Ca++ was lowered to 100 nM, a maneuver that also blocks the 8CPTcAMP [Ca++]i increase, 8CPTcAMP stimulated, rather than inhibited JNa(l-b). These results suggest that cAMP formation initially stimulates CCD Na+ transport, and that increased apical Na+ entry secondarily activates basolateral Ca++ entry. The cAMP-dependent [Ca++]i increase leads to inhibition Na+ transport in the rabbit CCD.
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