GLIBENCLAMIDE, AN ATP-SENSITIVE K+ CHANNEL BLOCKER, INHIBITS CARDIAC CAMP-ACTIVATED CL- CONDUCTANCE

Stimulation of the beta-adrenoceptor activates a time-independent Cl- conductance that is known to be regulated via phosphorylation by cAMP-dependent protein kinase in guinea pig ventricular myocytes. Since epitheliaI :cystic fibrosis transmembrane conductance regulator Cl- channels are known to be sensitive to an antidiabetic sulfonylurea, glibenclamide, we tested whether the drug modulates cardiac cAMP-activated Cl- conductance. Bath application of isoproterenol (1 mu mol/L, n=11) or forskolin (1 mu mol/L, n=17) or the intracellular application of cAMP (1 mmol/L, n=9) activated whole-cell Cl- currents recorded from single myocytes at 36 degrees C. External glibenclamide (greater than or equal to 10 mu mol/L, n=26) inhibited the Cl- current induced by either of the stimulants in a concentration dependent manner. The half-maximal inhibition concentration (IC50) Of glibenclamide and the Hill coefficient were 24.5 to 37.9 mu mol/L and 1.6 to 2.2, respectively. During current-clamp experiments, forskolin was found to shorten the action potential significantly (250+/-45 to 201+/-52 milliseconds, P<.05) in 7 of 11 cells tested. Glibenclamide antagonized the forskolin-induced shortening (to 243+/-54 milliseconds, n=7, P<.05). Intracellular administration of sodium orthovanadate (0.5 to approximate to 1 mmol/L n=6) brought about persistent activation of Cl- current after brief bath application of forskolin, This Cl- was not affected by H-89 (100 mu mol/L, n=3), a specific inhibitor of cAMP-dependent protein kinase, and was suppressed by glibenclamide similarly, with an IC50 of 29.7 mu mol/L. Thus, it is concluded that glibenclamide inhibits cardiac cAMP-activated Cl- channels at some step(s) downstream from the phosphorylation/dephosphorylation process.