The present studies probe the role of Ca2+ and Na+ in the stimulation-permeability coupling sequences by which antidiuretic hormone (ADH) induces a cyclic AMP (cAMP)-mediated increase in urea permeability in toad urinary bladder. The following results were obtained: (a) Removal of mucosal Na+ or Ca2+ or deletion of serosal Ca2+ did not modify ADH action. (b) Reduction of the serosal Na+ concentration to less than 50 mM inhibited the effects of both ADH and cAMP. The minimal concentration of serosal Na+ needed for the hormone to elicit its maximal effect was reduced to approximately 10 mM if serosal Ca2+ was concomitantly deleted. (c) The Na+ ionophore monensin produced an inhibition of ADH and cAMP actions that was dependent on the presence of Na+ and Ca2+ in the serosa. (d) The Ca2+ ionophore A23187 produced a serosal Ca2+-dependent inhibition of ADH effect and did not modify cAMP action. (e) Carbachol, which increases Ca2+ uptake to the same extent that A23187 does, had no effect on ADH action. (f) Quinidine, which releases Ca2+ from intracellular stores, produced a large inhibition of the action of ADH but not that of cAMP; the inhibition was greatly reduced if serosal Ca2+ was deleted. (g) Dinitrophenol and iodoacetate, which also release Ca2+ from intracellular pools, had no effect on ADH action. (h) The Ca2+ channel blocker diltiazem had no effect on ADH action and did not modify the inhibitions produced by deletion of serosal Na+ or monensin. (i) The cyclooxygenase inhibitor indomethacin partially removed the inhibition produced by deletion of serosal Na+ and almost completely impeded the inhibitions produced by either monensin or A23187. It is concluded: (a) Extracellular Ca2+, Na+ transport rates, and serosal Na+, in concentrations between 10 and 110 mM, have no participation in modulating the increase in urea permeability produced by ADH. (b) Increases in cytosolic Ca2+ activity, which are capable of inhibiting the effect of ADH on urea permeability at pre- and/or post-cAMP steps, seem to be highly compartmentalized. (c) Endogenous prostaglandins might play a role in the inhibitions produced by absence of serosal Na+, monensin, or A23187.
M A Hardy, H M Ware