COMPARISON OF SECRETORY RESPONSES AS MEASURED BY MEMBRANE CAPACITANCEAND BY AMPEROMETRY

We have compared capacitance and amperometric measurements in bovine c hromaffin cells when secretion was elicited by flash photolysis of cag ed-calcium or step depolarizations. Total amperometric charge depended linearly on the amount of capacitance increase in both types of exper iments. Furthermore, the properties of resolvable amperometric spikes after flashes were comparable to those observed after depolarizations, and their timing was compatible with the rate of capacitance increase . For a more detailed comparison, we used Monte Carlo simulations of m ultiple amperometric events occurring randomly over the surface of a s phere and summing together, to generate a reference amperometric signa l for a given measured capacitance increase. Even after correction for endocytotic processes, the time courses of the integrated experimenta l records lagged behind the integrated Monte Carlo records by similar to 50 ms in flash and depolarization experiments. This delay was large r by similar to 40 ms than what can be expected from the ''pre-foot de lay'' or the foot duration. Possible sources for the remaining delay c ould be diffusional barriers like the patch-pipette and the chamber bo ttom, which are not taken into account in the model. We also applied a novel type of fluctuation analysis to estimate the relative quantum s ize of an amperometric event. On average the estimates from experiment al amperometric traces, in both flash and depolarization experiments, were 3-5 times smaller than estimates from simulated ones. This discre pancy can be due to contributions to the amperometric current from sma ll vesicles, preferred release from cellular regions orientated toward the chamber bottom, or abundance of ''foot-only'' events. In conclusi on, amperometric signals in flash and depolarization experiments displ ayed similar delayed average time courses and a lower estimate for the relative quantum size compared to the modeled amperometric signals. H owever, individual amperometric spikes were in agreement with expectat ions derived from capacitance signals.