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Department of Physiology, University of Bristol, School of Veterinary Science, UK. D.Mistry@bristol.ac.uk

1. Single Na+-activated K+ channels (K(Na)) were investigated by means of the inside-out patch clamp technique in ventricular myocytes isolated from the guinea-pig heart. 2. Na+-activated K+ channels were observed at very low density (< 9% of patches). In symmetrical (60/60 mM) K+ solutions, K(Na) channels had a mean slope conductance of 75 pS and in asymmetrical (150/70 mM; outside/inside) K+ solutions, they had a mean slope conductance of 220 pS. The reversal potentials obtained under these two ionic conditions were close to the equilibrium potential for K+, suggesting K+ selectivity. 3. In high (98 mM) [Na+]i, the channel showed two open states and up to four closed states, and K(Na) channels also displayed long closures (of the order of seconds). The opening probability (Po) was not voltage dependent. Transient sublevels between 8 and 86% of the main state were identified and appeared to be a common feature of K(Na) channels. 4. Decreasing the activating [Na+]i, reduced Po and this was associated with both an increase in mean closed times and a decrease in mean open times. Lowering [Na+]i also increased the longer closed-time constants and their relative proportions. The first open-time constant was more sensitive to alterations in [Na+]i. 5. Distributions of burst duration, between burst duration and openings within bursts were best described by the sum of two exponentials. Lowering [Na+]i decreased the burst duration and the duration of openings within burst. 6. These observations show that the Na+-activated K+ channel from guinea-pig ventricular myocytes has complex gating and bursting behaviour.

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