An estimate of the number of Ca 2+-dependent K + channels in the human red cell

Abstract

An original approach has been designed to count Ca 2+-dependent K + channels in the human red cell using a preparation of inside-out vesicles. The relative frequency of vesicles having no K + channels is estimated from the fraction of 42K + (or 86Rb +) which is not released from loaded vesicles on maximal stimulation with Ca 2+. The mean number of channels per vesicle is then calculated from this figure assuming a Poisson distribution for the K + channels. From this value and the mean vesicular radius, computed from the volume / surface ratio, the mean number of channels per cell can be estimated. A value of 142 ± 27 ( mean ± S.E. ) was obtained, which is well above that estimated by comparison of unitary conductance and tracer equilibration rate measurements (about 10 channels / cell, Grygorczyk, R. Schwarz, W. and Passow, H. (1984) Biophys. J. 45, 693–698), but compares favourably with the channel density inferred from comparison with the number of Na + pumps in a similar preparation of inside-out vesicles (100–200 / cell, Lew, V.L., Muallem, S. and Seymour, C.A. (1982) Nature 296, 742–744). The procedure described here can be considered for general application as an alternative to other known procedures.