INTRINSIC VOLTAGE-DEPENDENCE AND CA2-CONDUCTANCE CA-ACTIVATED K+ CHANNELS( REGULATION OF MSLO LARGE)

The kinetic and steady-state properties of macroscopic mslo Ca-activat ed K+ currents were studied in excised patches from Xenopus oocytes. I n response to voltage steps, the timecourse of both activation and dea ctivation, but for a brief delay in activation, could be approximated by a single exponential function over a wide range of voltages and int ernal Ca2+ concentrations ([Ca](i)). Activation rates increased with v oltage and with [Ca](i), and approached saturation at high [Ca](i). De activation rates generally decreased with [Ca](i) and voltage, and app roached saturation at high [Ca](i). Plots of the macroscopic conductan ce as a function of voltage (G-V) and the time constant of activation and deactivation shifted left ward along the voltage axis with increas ing [Ca](i). G-V relations could be approximated by a Boltzmann functi on with an equivalent gating charge which ranged between 1.1 and 1.8 e as [Ca](i) varied between 0.84 and 1,000 mu M. Hill analysis indicate s that at least three Ca2+ binding sites can contribute to channel act ivation. Three lines of evidence indicate that there is at least one v oltage-dependent unimolecular conformational change associated with ms lo gating that is separate from Ca2+ binding. (a) The position of the mslo G-V relation does not vary logarithmically with [Ca](i). (b) The macroscopic rate constant of activation approaches saturation at high [Ca](i) but remains voltage dependent. (c) With strong depolarizations mslo currents can be nearly maximally activated without binding Ca2+. These results can be understood in terms of a channel which must unde rgo a central voltage-dependent rate limiting conformational change in order to move from closed to open, with rapid Ca2+ binding to both op en and closed states modulating this central step.