CALCIUM BUFFERING IN BURSTING HELIX PACEMAKER NEURONS

Bursting pacemaker neurons of the snail Helix pomatia were voltage-cla mped and Ca currents in response to depolarizing steps were recorded. Simultaneously, changes in intracellular Ca concentrations were measur ed using the fluorescent dye fura-2 and a highly sensitive digital cam era. Ca influx through voltage-gated channels induced a spatially non- uniform increase in intracellular Ca. The Ca signals decayed with a ti me constant of about 5 s. By increasing the concentration of the indic ator dye, its Ca-buffering capacity was enhanced and Ca transients in response to depolarization were diminished. Thereby, the endogenous Ca buffer capacity could be determined and was calculated to be about 48 0 buffered ions for every free Ca ion. The buffer capacity did not var y significantly with the amount of Ca influx within the range tested, suggesting that the buffer is not saturated at Ca concentrations of up to 1 mu M.