Possible role of Ca2+-binding sites in the regulation of Na+ transport in toad urinary bladder

La3+ was used to assess the role of membrane-bound Ca2+ in the regulation of basal and antidiuretic hormone (ADH)-induced Na+ transport by the isolated toad urinary bladder. Na+ transport was monitored by means of a short-circuit current (Isc) device. Mucosal La3+ (0.5-5 mM) increased Isc, while serosal La3+ (5 mM) produced a biphasic response (stimulation followed by inhibition). The stimulatory effects of La3+ were additive when present on both sides and were suppressed by mucosal amiloride or serosal ouabain. The action of mucosal La+ was reversible but the inhibition produced by serosal La3+ was not. In the presence of serosal La3+ the natriferic effect of ADH was abolished, but Theophylline, dibutyryl-cAMP, Amphotericin B, mucosal La3+, mucosal low pH, and phospho(enol) pyruvate, were able to increase Isc. These results suggest that Ca2+ binding sites in apical and basolateral membranes may play a key role in the modulation of both basal and ADH-induced Na+ transport. Serosal La3+ apparently inactivates the hormone-receptor interaction and/or the link between the ADH-receptor complex and the activation of adenylate cyclase, but does not interfere with the operation of the Na+ "pump", the basal activity of adenylate cyclase or any of the intracellular events that mediate the effect of ADH on Na+ transport.