Characterization of the driving force as a modulator of gating in cardiac ATP-sensitive K+ channels -: Evidence for specific elementary properties

Single cardiac ATP-sensitive K+ channels and, comparatively, two other members of the inwardly rectifying K+ channel family, cardiac K-(ir(+)) and K-(ACh(+)) channels, were studied in the inside-out recording mode in order to analyze influence and significance of the electrochemical K+ gradient for open-state kinetics of these K+ channels. The conductive state of K-(ATP(+)) channels was defined as a function of the electrochemical K+ gradient in that increased driving force correlates with shortened open-channel lifetime. Flux coupling of gating can be largely excluded as the underlying mechanism for two reasons: (i) tau(open) proved identical in 23 pS, 56 pS and 80 pS channels; (ii) K-(ATP(+)) channel protonation by an external pH shift from 9.5 to 5.5 reduced conductance without a concomitant detectable change of tau(open). Since gating continued to operate at E-K, i.e., in the absence of K+ permeation through the pore, K+ driving force cannot be causally involved in gating. Rather the driving force acts to modulate the gating process similar to Rb+ whose interference with an externally located binding site stabilizes the open state. In K-(ir(+)) and K-(ACh(+)) channels, the open state is essentially independent on driving force meaning that their gating apparatus does not sense the electrochemical K+ gradient. Thus, K-(ATP(+)) channels differ in an important functional aspect which may be tentatively explained by a structural peculiarity of their gating apparatus.