Abstract
1. To address the questions of whether beta-adrenoreceptor stimulation can augment ATP-sensitive potassium current (IK(ATP)), and what the mechanism of such an effect might be, action potentials and whole-cell ionic currents were recorded from adult cat cardiac ventricular myocytes using a conventional whole-cell patch technique. 2. An outwardly directed, ohmic, non-inactivating, glyburide (10 microM)-sensitive current reversing near the reversal potential for potassium (EK) developed slowly (10-25 min) in cells dialysed with an ATP-free pipette (intracellular) solution. During this time, action potential duration markedly decreased while the resting membrane potential hyperpolarized closer to EK. Extended (> 30 min) periods of internal dialysis with ATP-free solution eventually resulted in run-down of the outward current. 3. Externally applied isoprenaline (1 microM) caused a rapidly developing (< or = 60 s), sustained enhancement of a glyburide (10 microM)-sensitive IK(ATP) in cells internally dialysed with ATP-free solution. IK(ATP) remained elevated even after the isoprenaline was removed, and subsequent applications of the beta-agonist failed to increase IK(ATP) further. Half-maximal isoprenaline stimulation of IK(ATP) occurred at a concentration of approximate of 1.5 nM. 4. Pretreatment with propranolol (1 microM) prevented the enhancement of IK(ATP) by a beta-agonist. 5. Isoprenaline-induced IK(ATP) could be blocked by either internal application of GDP-beta-S (2-5 mM) or pretreatment with cholera toxin (1-10 microgram ml-1, > 18 h). Pretreatment with pertussis toxin (1-2 microgram ml-1, > 18 h) did not attenuate the isoprenaline response, whereas internally applied GTP-gamma-S (100 microM) or F- (20 mM) caused IK(ATP) to increase rapidly in the absence of the beta-agonist. 6. Although externally applied forskolin (10 microM) also stimulated IK(ATP), neither 1,9-dideoxyforskolin (10 microM) nor 8-(4-chlorophenylthio)-cAMP (200 microM) had any effect on the current. Internal application of the adenylate cyclase inhibitor 2'-deoxyadenosine-3'-monophosphate (100 microM) resulted in a reduction in the response to isoprenaline, while internal application of a protein kinase A inhibitor (PKI5-24, 22.5 microM) did not attenuate the response to the beta-agonist. 7. IK(ATP) developed slowly during internal dialysis with ATP-free solution.(ABSTRACT TRUNCATED AT 400 WORDS)