Journal of Physiology (1994), 481.2 Regulation of intracellular calcium and calcium buffering properties of rat isolated neurohypophysial nerve endings

1. Electrophysiological measurements of Ca21 influx using patch clamp methodology were combined with fluorescent monitoring of the free intracellular calcium concentration ([Ca2+]i) to determine mechanisms of Ca2 ' regulation in isolated nerve endings from the rat neurohypophysis. 2. Application of step depolarizations under voltage clamp resulted in voltage-dependent calcium influx (Ic') and increase in the [Ca2+]1. The increase in [Ca2+]i was proportional to the time-integrated ICa for low calcium loads but approached an asymptote of [Ca2+]i at large Ca2+ loads. These data indicate the presence of two distinct rapid Ca2+ buffering mechanisms. 3. Dialysis of fura-2, which competes for Ca2+ binding with the endogenous Ca2+ buffers, reduced the amplitude and increased the duration of the step depolarization-evoked Ca2+ transients. More than 99 % of Ca2+ influx at low Ca2+ loads is immediately buffered by this endogenous buffer component, which probably consists of intracellular Ca2+ binding proteins. 4. The capacity of the endogenous bufer for binding Ca2+ remained stable during 300 s o