alpha-Latrotoxin-induced quantal release of catecholamines from rat adrenal chromaffin cells

alpha-Latrotoxin (alpha-LT) potently enhances quantal release of neurotransmitter from nerve terminals. To develop the adrenal chromaffin cell as a 'model' system for the study of mechanisms of toxin action, we used amperometry to examine secretion of catecholamines and spectrofluorometry to measure cytosolic Ca. Several key features of toxin action emerged. (1) Release occurs at concentrations of toxin >35 pM and the pattern of release changes from repeated brief bursts to more continuous discharges of varying duration as the toxin concentration increases. (2) Release requires extracellular calcium in the micromolar range, but not the activity of native voltage-dependent calcium channels. (3) Release is associated with a rise in cytosolic calcium to near micromolar range. (4) Provided calcium is later restored, release can be induced even though the toxin is applied and washed away in calcium-free saline. These features largely resemble those of alpha-LT action on nerve terminals. They suggest that in chromaffin cells, as in neurons, the Ca-dependence of toxin-enhanced quantal release is based on Ca entry through toxin-induced channels rather than a requirement of extracellular Ca for toxin binding.