Fig. 4.

8-Br-cGMP but not 8-Br-cAMP reduces IR_K⁺. A, 8-Br-cGMP mimicked the action of APB or glutamate by suppressing IR_K⁺. 8-Br-cGMP (1 mm) in the pipette eliminated IR_K⁺ within 5 min of membrane rupture. B, 8-Br-cAMP (1 mm) failed to affect IR_K⁺ but did influence the calcium current. In this example, even after 25 min, IR_K⁺ is unchanged, whereas the calcium current is effectively eliminated. C, IBMX (100 μm) included in the patch pipette led to a slow, progressive suppression of IR_K⁺. In all of these experiments, the response labeled Control was recorded within 30 sec of membrane rupture; the experimental traces were recorded as indicated on the figure. Alltraces are averages of four successive ramps.D, Time course for the effects of cyclic nucleotides is shown. Each symbol represents one experiment;open symbols are for 8-Br-cGMP (n = 5), and filled symbols are for 8-Br-cAMP (n = 5). Current amplitude represents the percent of control (time, 30 sec) measured at −90 mV for the varioustime points. Complete IR_K⁺ suppression was determined by subsequent application of 1 mm Ba²⁺. Some values exceed 100% limits because the Ba²⁺treatment often shifted the entire I–V relation slightly more negative (see Fig. 1).