CALCIUM-CHANNEL BLOCKERS INHIBIT AMILORIDE-STIMULATED SHORT-CIRCUIT CURRENT IN FROG TADPOLE SKIN

The larval frog skin has a very high electrical resistance and a corresponding low rate of transepithelial ion transport. Amiloride, a blocker of sodium transport in adult skin, transiently stimulates rather than inhibits short-circuit current (I(sc)) across larval skin. The time course and concentration response to amiloride and the effects of calcium channel blockers on I(sc) were studied with larval frog skin mounted in modified Ussing chambers. The amiloride (1 mM) transient was markedly blunted if the skin was previously exposed to low amiloride (0.01-0.1 mM) concentrations. The calcium channel blockers verapamil, nitrendipine, diltiazem, W-7, and lanthanum all blocked the amiloride transient. Diltiazem itself caused a rapid transient in I(sc), indicating that it may be a partial agonist. These data suggest that the amiloride-stimulated cation channels rapidly desensitize in a manner similar to the acetylcholine receptor. The decline in I(sc) after amiloride stimulation could be caused by amiloride block of the open channel. Blockade of amiloride stimulation by well-known calcium channel blockers suggests that these larval cation channels may have some characteristics in common with calcium channels.