THERMODYNAMICALLY SPECIFIC GATING KINETICS OF CARDIAC MAMMALIAN K-ATP(-DEGREES-C() CHANNELS IN A PHYSIOLOGICAL ENVIRONMENT NEAR 37)

Elementary K+ currents through isolated ATP-sensitive K+ channels from neonatal rat cardiocytes were recorded to study their temperature dep endence between 9 degrees C and 39 degrees C. Elementary current size and, thus, K+ permeation through the open pore varied monotonically wi th temperature with a Q(10) of 1.25 corresponding to a low activation energy of 3.9 kcal/mol. Open-state kinetics showed a complicated tempe rature dependence with Q(10) 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 wer e associated with the highest activation energy (O-2 exit reaction: 12 .1 kcal/ mel) and the largest amount of entropy. A transition from 19 degrees C to 9 degrees C elucidated a paradoxical kinetic response, sh ortening of both O-states, irrespective of the absence or presence of cAMP-dependent phosphorylation. Another member of the K+ channel famil y and also a constituent of neonatal rat cardiocyte membranes, 66 pS o utwardly-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 tr ansition from 19 degrees C to 9 degrees C with other K+ channels and h ave a unique sensitivity to thermally-induced perturbations.