Different sites control voltage dependence and conductance of sarcoball anion channel.

Single anion-selective channels from frog skeletal muscle SR were recorded using the sarcoball technique (Stein, P., and P. T. Palade. 1988. Biophys. J. 54:357-363). The voltage dependence of the open probability (Po) was found to be dependent on the concentration of permeant anions on either side of the patch membrane. With 50 mM or greater permeant anions present on both sides of the membrane, the Po vs. voltage plot yielded a bell-shaped curve centered around 0 mV (Hals, G. D., P. G. Stein, and P. T. Palade. 1989. J. Gen. Physiol. 93:385-410). When permeant anions in the bath (Cl-) were replaced with relatively impermeant anions (gluconate, MOPS, propionate, or Hepes), the Po vs. voltage relationship was shifted by approximately -35 mV. Similarly, analogous experiments with the pipette solution produced a shift of comparable magnitude, but opposite polarity (approximately +35 mV). The stilbene derivative DIDS also shifted the voltage dependence, which suggests that amino groups may be involved in the shifts in voltage dependence. Other amino group modifiers reduced the single-channel conductance, and these data more strongly support the notion that amino groups are involved in conduction as well. The results indicate that amino groups involved in the conductance decrease are separate from those related to voltage sensitivity.