ACUTE SIMULATED ISCHEMIA PRODUCES BOTH INHIBITION AND ACTIVATION OF K+ CURRENTS IN ISOLATED VENTRICULAR MYOCYTES

Objective: The aim was to investigate the effects of acute ischaemia o n cardiac repolarizing K+ currents. Methods: We developed a model of a cute ischaemia in isolated rat ventricular myocytes transiently surrou nded with a mineral oil droplet. During ischaemic challenges, we recor ded intracellular pH using the fluorescent probe seminaphthorhodafluor -1 (SNARF-1) and whole-cell K+ currents using the patch-damp technique . Results: Decrease in intracellular pH (pH(i)) during simulated ischa emia was dependent upon the extracellular proton buffer used (pH, decr eased from 7.44 +/- 0.02 to 7.16 +/- 0.04 in a Hepes-buffered medium a nd from 7.08 +/- 0.04 to 6.56 +/- 0.07 with bicarbonate buffer). In He pes, action potential duration initially lengthened and then shortened under the effects of ischaemia. Initial action potential duration len gthening was concomitant with a block of the inward rectifier K+ curre nt, whereas late shortening corresponded with the activation of the AT P-sensitive K+ current. Similar changes occurred in bicarbonate buffer although with different amplitudes and kinetics. Patch-clamp experime nts also showed inhibition of the transient outward K+ current. Brief transient episodes of ischaemia activated ATP-sensitive K+ current in only 20% of control cells (n = 21) but in 100% of cells treated with 1 5 mu M cromakalim (n = 9). Conclusions: (i) Simulated ischaemia produc es complex effects on repolarizing K+ currents including both inhibiti on and activation; (ii) cromakalim accelerates activation of ATP-sensi tive K+ current during simulated ischaemia.