CALCIUM-ACTIVATED POTASSIUM CHANNELS IN GOLDFISH HAIR-CELLS

1. Characteristics of Ca2+-activated K+ channels in the basolateral membrane of hair cells isolated from the caudal part of the goldfish saccular macula were studied mainly with the inside-out mode of the patch clamp method. 2. Several types of Ca2+-activated K+ channels differing in unitary conductance were identified. The conductances (n = 156) ranged from 130 to 320 ps (when measured in symmetrical 125 mM KCI) and could be roughly separated into four groups, centred on values of 150, 200, 250 and 300 pS. The pharmacological profile, assessed by, for example, tetraethylammonium blockade, and the relatively large conductance indicated that these channels can be classified as large-conductance Ca2+-activated K+ channels (BK channels). The relative permeability of these channels to different ion species was in the order K+ (1.0)>Rb+(0.8)>NH4+(0.14)>Na+, Cs+(< 0.05). 3. Curves relating open state probability to [Ca2+](i), for membrane potentials between -50 and +50 mV, were similar to those observed for BK channels of rat muscle. However, the maximum open state probability (100-1000 mu M [Ca2+](i) and 50 mV membrane potential) was 0.4-0.9, and always less than 1. 4. These channels had a short arithmetic mean open time ranging from 0.08 to 1.2 ms (0.08-0.5 ms in 88 % of cases) and an arithmetic mean shut time ranging from 0.24 to 1.2 ms (10 mu M [Ca2+](i) and 50 mV membrane potential). The shut intervals were more sensitive to changes in [Ca2+](i) and membrane potential than were the open intervals. 5. The distribution of individual open and shut intervals was fitted with the sum of exponential functions. Except for the slowest shut component, which only accounted for less than 1 % of shut events, all other components had time constants shorter than 1 ms. As a result of these short open and shut intervals, the current trace had a flickery pattern rather than a burst-interburst pattern. 6. There was a rough correlation between unitary conductance and mean open time, i.e. channels with a large unitary conductance had a longer mean open time. 7. The sensitivity to [Ca2+](i) of the Ca2+-activated K+ channel in goldfish hair cells was one to two orders of magnitude lower than that of BK channels in rat muscle. Channels with a longer mean open time had a higher Ca2+ sensitivity. 8. The stability of the single Ca2+-activated K+ channel kinetics was studied by measuring the 'moving' mean duration of open and shut intervals. The moving mean duration of the open intervals was more stable (0.17-0.3 ms) than that of shut intervals (0.2-0.8 ms). Transitions to another mode with a longer mean open time occurred. 9. When depolarizing potential jumps were applied, the averaged single channel current activated with an exponential-like time course (time constant, 1.1-4.5 ms). The tail current following the repolarizing potential jump decayed much more quickly (time constant, 0.05-0.3 ms). The mean open times of channels were correlated with the time constant of the tail current decay.