STIMULATION BY CGMP OF APICAL NA CHANNELS AND CATION CHANNELS IN TOAD URINARY-BLADDER

The effects of 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) on apical membrane cation conductances in the toad urinary bladder were investigated. 8-BrcGMP (1 mM) added to the serosal solution increased the amiloride-sensitive short-circuit current (I(Na)) after a delay of 5 min to a steady-state value 1.8 times that of controls achieved after 30 min. Similar effects were seen when the bladders were bathed on the serosal side with a normal NaCl Ringer solution and with a high-K sucrose solution to depolarize the basolateral membrane. Under the latter conditions, the amiloride-sensitive transepithelial conductance increased in parallel with the short-circuit current, indicating stimulation of apical membrane Na channels. The threshold concentration for observing the stimulation of I(Na) was 100-mu-M, 10-100 times larger than the concentration of 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) required to elicit an increase in I(Na). Currents through an outwardly rectifying Ca-sensitive cation conductance (I(out)) were also increased by 1.8-fold relative to controls. This stimulatory effect occurred after a delay of 15 min and reached maximal levels 90-120 min after addition of the nucleotide. The effects of cGMP on I(Na) were not additive with those of 8-BrcAMP or with antidiuretic hormone, an agent known to act by increasing cAMP within the cell. Addition of 1 mM 3-isobutyl-1-methylxanthine to the serosal side of the bladders stimulated I(Na) by 1.3-fold and I(out) by 2.4-fold. In both cases, subsequent addition of cGMP produced no further activation of either conductance. The results suggest that cGMP is modulating a pathway that is the same as or closely related to that involving cAMP.