Fig. 1. Bicarbonate enhances the rate of depolarization-evoked Ca²⁺ entry. (A,B) Sperm were perfused locally with medium Na7.4 that contained 0 or 15 mM bicarbonate except during brief (10 second) stimuli with the depolarizing medium K8.6 as shown. [Ca²⁺]_i was monitored by indo-1 emission ratio photometry. (C) The initial portions of the averaged responses evoked in these experiments before (,•) and after () exposure to 15 mM HCO₃^-. The slope of the best-fit regression line with the indicated rate of rise is a measure of the number of Ca²⁺ channels open during the KCl depolarization.