ENHANCEMENT OF ATP-SENSITIVE POTASSIUM CURRENT IN CAT VENTRICULAR MYOCYTES BY BETA-ADRENOCEPTOR STIMULATION

To address the questions of whether beta-adrenoreceptor stimulation ca n augment ATP-sensitive potassium current (I-K(ATP)), and what the mec hanism of such an effect might be, action potentials and whole-cell io nic currents were recorded from adult cat cardiac ventricular myocytes using a conventional whole-cell patch technique. 2. An outwardly dire cted, ohmic, non-inactivating, glyburide (10 mu M)-sensitive current r eversing near the reversal potential for potassium (E(K)) developed sl owly (10-25 min) in cells dialysed with an ATP-free pipette (intracell ular) solution. During this time, action potential duration markedly d ecreased while the resting membrane potential hyperpolarized closer to E(K). Extended (> 30 min) periods of internal dialysis with ATP-free solution eventually resulted in run-down of the outward current. 3. Ex ternally applied isoprenaline (1 mu M) caused a rapidly developing (le ss than or equal to 60 s), sustained enhancement of a glyburide (10 mu M)-sensitive I-K(ATP) in cells internally dialysed with ATP-free solu tion. IK(ATP) remained elevated even after the isoprenaline was remove d, and subsequent applications of the beta-agonist failed to increase I-K(ATP) further. Half-maximal isoprenaline stimulation of I-K(ATP) oc curred at a concentration of similar to 1.5 nM. 4. Pretreatment with p ropranolol (1 mu M) prevented the enhancement of I-K(ATP) by a beta-ag onist. 5. Isoprenaline-induced I-K(ATP) could be blocked by either int ernal application of GDP-beta-S (2-5 mM) or pretreatment with cholera toxin (1-10 mu g ml(-1), > 18 h). Pretreatment with pertussis toxin (1 -2 mu g ml(-1), > 18 h) did not attenuate the isoprenaline response, w hereas internally applied GTP-gamma-S (100 mu M) or F- (20 mM) caused I-K(ATP) to increase rapidly in the absence of the beta-agonist. 6. Al though externally applied forskolin (10 mu M) also stimulated I-K(ATP) neither 1,9-dideoxyforskolin (10 mu M) nor 8-(4-chlorophenylthio)-cAM P (200 mu M) had any effect on the current. Internal application of th e adenylate cyclase inhibitor 2'-deoxyadenosine-3'monophosphate (100 m u M) resulted in a reduction in the response to isoprenaline, while in ternal application of a protein kinase A inhibitor (PKI5-24, 22.5 mu M ) did not attenuate the response to the beta-agonist. 7. I-K(ATP) deve loped slowly during internal dialysis with ATP-free solution. In contr ast, a large, more rapidly developing I-K(ATP) was observed during int ernal dialysis with ATP-free solution in cells that had been treated w ith the metabolic inhibitor 2-deoxyglucose (2-DOG, 10 mM), suggesting that glycolytically (intrinsically) produced ATP might be present at s ubsarcolemmal sites to suppress I-K(ATP) at least partially even after prolonged internal dialysis with ATP-free solution. 8. The magnitude of the isoprenaline-induced increase in I-K(ATP) diminished progressiv ely as the intracellular ATP concentration was increased from 0.1 to 1 .0 mM, with a half-maximal inhibitory ATP concentration of 0.4 mM. 9. These results indicate that beta-agonists can stimulate I-K(ATP) in ce lls dialysed with ATP-free solution via a G(s)- and adenylate cyclase- dependent, cAMP- and protein kinase A independent pathway. Therefore, beta-adrenoreceptor stimulation may act through G(s) to stimulate aden ylate cyclase, which in turn causes further depletion of subsarcolemma l ATP sufficient to effect enhancement of I-K(ATP).