Thermodynamically specific gating kinetics of cardiac mammalian K+(ATP) channels in a physiological environment near 37 degrees C.

Elementary K+ currents through isolated ATP-sensitive K+ channels from neonatal rat cardiocytes were recorded to study their temperature dependence between 9 degrees C and 39 degrees C. Elementary current size and, thus, K+ permeation through the open pore varied monotonically with temperature with a Q10 of 1.25 corresponding to a low activation energy of 3.9 kcal/mol. Open-state kinetics showed a complicated temperature dependence with Q10 values of up to 2.94. Arrhenius anomalies of tau(open)(1) and tau(open)(2) indicate the occurrence of thermally-induced perturbations with a dominating influence on channel portions that are involved in gating but are obviously ineffective in altering pore-forming segments. At 39 degrees C, open-state exit reactions were associated with the highest activation energy (O2 exit reaction: 12.1 kcal/mol) and the largest amount of entropy. A transition from 19 degrees C to 9 degrees C elucidated a paradoxical kinetic response, shortening of both O-states, irrespective of the absence or presence of cAMP-dependent phosphorylation. Another member of the K+ channel family and also a constituent of neonatal rat cardiocyte membranes, 66 pS outwardly-rectifying channels, was found to react predictably since tau(open) increased on cooling. Obviously, cardiac K+(ATP) channels do not share this exceptional kinetic responsiveness to a temperature transition from 19 degrees C to 9 degrees C with other K+ channels and have a unique sensitivity to thermally-induced perturbations.