Whole-cell recordings of macroscopic K+ currents were made from acutely dissociated and cultured melanotrophs isolated from the pars intermedia of the adult rat pituitary. 2. In acutely dissociated cells, external capsaicin reversibly decreased the amplitude both of the fast-activating, fast-inactivating potassium current IK(f) and the slowly-activating, slowly-inactivating potassium current IK(s). To simplify the investigation of the mechanism of action of capsaicin experiments were conducted on cultured melanotrophs that express only IK(s). 3. In control cells the activation rate and the amplitude of IK(s) increased with depolarization and the current showed very little inactivation at any voltage during pulses lasting for 100-300 ms. In capsaicin, the decrease of the current amplitude was associated with an increased rate of current decay ('inactivation'). At a given voltage, the extent and the rate of the capsaicin-induced inactivation was proportional to the capsaicin concentration; and, at a given concentration, the extent and rate of the inactivation increased with membrane depolarization. 4. The fit of the Hill equation to data derived from the steady-state block of IK(s) evoked at 10 mV indicated an equilibrium dissociation constant (KD) of 17.4 microM (95% confidence limits 15.8-19.0) and a Hill coefficient of 1.8 (95% Cl 1.5-2.2) suggesting that at least two molecules of capsaicin must bind to the channel to block it. 5. Analysis of the voltage-dependence of the steady-state block in 100 microM capsaicin showed that half-maximal block occurred at -29 +/- 2 mV (n = 10). Two-pulse experiments designed to study the time-dependence of channel block in 100 MicroM capsaicin indicated that the blocking kinetics were well fitted by a single exponential and that the rate of block increased with depolarization. The value for Tblock at 0mV was 24 +/-7ms (n=4).6. Recovery from block in 100 MicroM capsaicin was also well fitted by a single exponential. The recovery time constant ( was 708 +/- 140 ms at - 50 mV, 70 +/- 6 ms at - 70 mV and 19 +/- 1.3 ms at-90 mV (n = 4).7. In 50-100 MicroM capsaicin, the decay of the tail current was biexponential, the values for fast and Tslow being, respectively, less than and greater than the single time constant fitted to the control tail current.Inward and outward K+ currents were equally affected by capsaicin.8. Most of these effects of capsaicin on the IK(S) of melanotrophs can be accounted for by a kinetic scheme in which capsaicin binds to and blocks open K+ channels.
