Purinergic-mediated inhibition of Na+-K+-ATPase in proximal tubule cells: elevated cytosolic Ca2+ is not required.

The involvement of cytosolic Ca2+ concentration ([Ca2+]i) as messenger for the regulation of Na+-K+-ATPase activity was investigated in a renal cell line recently developed by immortalization of early proximal tubule primary cultures from the Wistar-Kyoto rat strain. Na+-K+-ATPase was measured as short-circuit current (Isc) in intact monolayers after permeabilization of the apical plasma membrane with amphotericin B. With symmetrical solutions, Isc quantitatively reflects Na+-K+-ATPase activity as judged by ouabain inhibition and dependence on Na+ and K+. Extracellular ATP (50% effective concentration = 0.32 mM) on the apical side produced acute inhibition of Na+-K+-ATPase-generated Isc of up to 50%. The inhibition peaked within 1 min and lasted approximately 5 min. The potency order was ATP > ADP >> beta,gamma-methyleneadenosine 5'-triphosphate = UTP, consistent with a P2y receptor. Extracellular ATP also stimulated a transient increase in [Ca2+]i. This increase had a similar time course as the inhibition of ATPase and reached a peak change of approximately 120 nM. However, the elevation of [Ca2+]i is not required in the purinergic inhibition of the Na+-K+-ATPase, since, first, increases in [Ca2+]i produced with a Ca2+ ionophore (ionomycin) failed to mimic the purinergic inhibition and, second, 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, which abolished the [Ca2+]i elevation, failed to block the purinergic inhibition.