Tetracycline-induced inhibition of Na+ transport in the toad urinary bladder

short-circuit current) were examined in toad urinary bladders mounted in modified Ussing chambers. During a l-h incuba-tion period serosal DMC (but not MNC or OTC) inhibited basal Nat transport, whereas MNC (but not DMC or OTC) inhibited ADH-stimulated Na+ transport. MNC also in-hibited cyclic AMP-stimulated Na+ transport. During longer incubation periods all three drugs inhibited basal Na+ trans-port. The DMC-induced inhibition of basal Na+ transport and the MNC-induced inhibition of ADH-stimulated Na+ trans-port were paralleled by an inhibition of the active conduct-ance of the bladders. Thus, although all three drugs inhibit basal Nat transport, only MNC inhibits ADH-stimulated Na+ transport. This effect does not correlate with the known effects of the tetracyclines on ADH-stimulated water flow or with drug-protein binding, and may be related to the greate