Calcium regulation by and buffer capacity of molluscan neurons during calcium transients

The properties of Ca-regulation and -buffering of physiological levels of Ca-transients were examined in the soma of Archidoris monteryensis neurons. The rate of recovery from a Ca-transient was examined with two experimental protocols; in one the pulse duration was kept constant and its amplitude was varied, and in the other the duration was varied while the amplitude was kept constant. These experiments revealed that the recovery from a Ca-transient was approximately a first order process and the apparent first order rate constant was dependent on the duration of Ca-influx. The calcium buffer capacity of the cytoplasm was determined by an indirect method which utilised measured amounts of intracellular EGTA to reduce transient changes in free calcium. An equation for the cytoplasmic buffer capacity was derived on the assumption that the capacities of exogenous and endogenous Ca buffers summate linearly. The resting cytoplasmic Ca buffer capacity was 45.2 microM/delta pCa, when it was assumed that the incoming Ca diffuses a distance of 10 microns into the cytoplasm. For a diffusion distance of 5 microns it was 34.5 microM/delta pCa. In both cases, the buffer capacity increased with an increase in the size of Ca transient.