Extracellular Ca2+ regulates the stimulation of Na+ transport in A6 renal epithelia

Abstract

We investigated the involvement of intracellular and extracellular Ca2+ in the stimulation of Na+ transport during hyposmotic treatment of A6 renal epithelia. A sudden osmotic decrease elicits a biphasic stimulation of Na+ transport, recorded as increase in amiloride-sensitive short-circuit current (I-sc) from 3.4+/-0.4 to 24.0+/-1.3 muA/cm(2) (n=6). Changes in intracellular Ca2+ concentration ([Ca2+](i)) were prevented by blocking basolateral Ca2+ entry with Mg2+ and emptying the intracellular Ca2+ stores before the hyposmotic challenge. This treatment did not noticeably affect the hypotonicity-induced stimulation of I-sc. However, the absence of extracellular Ca2+ severely attenuated Na+ transport stimulation by the hyposmotic shock, and I-sc merely increased from 2.2+/-0.3 to 4.8+/-0.7 muA/cm(2). Interestingly, several agonists of the Ca2+-sensing receptor, Mg2+ (2 mM), Gd3+ (0.1 mM), neomycin (0.1 mM), and spermine (1 mM) were able to substitute for extracellular Ca2+. When added to the basolateral solution, these agents restored the stimulatory effect of the hyposmotic solutions on I-sc in the absence of extracellular Ca2+ to levels that were comparable to control conditions. None of the above-mentioned agonists induced a change in [Ca2+](i). Quinacrine, an inhibitor of PLA(2), overruled the effect of the agonists on Na+ transport. In conclusion, we suggest that a Ca2+-sensing receptor in A6 epithelia mediates the stimulation of Na+ transport without the interference of changes in [Ca2+](i).