Abstract
1. We have used patch-clamp methods to study the effects of pH at the cytoplasmic surface of the membrane on ATP-dependent K+ channels (KATP channels) in patches excised from frog (Rana temporaria) skeletal muscle, and to study the kinetics of ATP binding. 2. In the absence of ATP, a reduction in pH led to a slight decrease in single-channel current amplitude, an increase in the number of very brief closings, an increase in the apparent mean open time, and an increase in burst duration. After correction for missed closings, the change in mean open time was slight. Despite these changes in detailed kinetics, the channel open-state probability, Popen, changed little with changes in pH in the absence of ATP. 3. In the presence of ATP, a decrease in internal pH (pHi) reduced the degree of channel inhibition by ATP, shifting the curve relating Popen and [ATP] to higher concentrations of ATP without altering its steepness. The ATP concentration for half-inhibition of channel activity (Ki) was 17 microM at pH 7.2 and 260 microM at pH 6.3. 4. The effect of pH could be modelled by assuming that one or two protons bind to the channel and prevent ATP binding to exert its effect of causing channel closure. The predicted dissociation constants for ATP and H+ respectively were 5.4 and 0.11 microM. 5. The rate constants for binding and unbinding of ATP were estimated from the dependence of the mean open time on [ATP] and from the Ki. The apparent rate constants for ATP binding were 0.6 and 0.04 mM-1 ms-1 at pH 7.2 and 6.3 respectively, while the rate constant for unbinding was 0.01 ms-1. In terms of our model the calculated true rate constant for ATP binding was 1.85 mM-1 ms-1. ATP binding also led to a reduction in burst duration. 6. The effect of pH described here differs from findings in cardiac muscle and pancreatic B-cells. The results are discussed in relation to the possible function of KATP channels in skeletal muscle during exercise.