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Department of Physiology, University of British Columbia, Vancouver, Canada.

1. Whole-cell voltage-clamp recordings were made from acutely dissociated melanotrophs obtained from adult rats. 2. In the presence of external Na+ and Ca2+ channel blockers and 20 mM-tetraethylammonium (TEA) depolarizations to -40 mV or more evoked a fast-activating fast-inactivating outward K+ current (IK(f)). Double-pulse experiments showed that steady-state half-inactivation occurred near -37 mV; half-maximal activation of IK(f) occurred at -15 mV. Recovery from inactivation in most cells fitted a single exponential with a time constant of 40-50 ms. 3. When applied either internally or externally, 1-2.5 mM-4-aminopyridine (4-AP) substantially reduced IK(f) but the degree of block was affected by the intensity, duration and frequency of depolarizing commands. 4. Analysis of the steady-state voltage dependence of the block by 4-AP showed that half-maximal blocking occurred at approximately -31 mV. This implied that 4-AP binds to the resting state of the IK(f) channel. 5. Studies of the time dependence for the blocking or unblocking of IK(f) showed that both processes were exponential with mean time constants of 1942 ms (at -70 mV) and 726 ms (at 20 mV), respectively. Recovery from inactivation was apparently unaffected by 4-AP. 6. A four-state sequential model in which 4-AP reversibly binds to the resting state of the channel replicates the frequency dependence of the 4-AP blockade.

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