Alpha-latrotoxin as a secretagogue of endocrine cells

{dollar}alpha{dollar}-latrotoxin ({dollar}alpha{dollar}-LT), the component of the venom of black widow spider (Latrodectus mactans) that targets vertebrate nerve terminals, enhances catecholamine release from neurectoderm-derived adrenal chromaffin cells, as well as insulin and glucagon release, from {dollar}beta{dollar}- and {dollar}alpha{dollar}-cells respectively, of endoderm-derived pancreatic islets of Langerhans. Applying capacitance tracking, and/or amperometry to single patch clamped chromaffin and {dollar}beta{dollar}-cells, often in combination with ratiometric cytosolic Ca{dollar}sp{lcub}2+{rcub}{dollar} measurement, we observe that {dollar}alpha{dollar}-LT enhances both "spontaneous" and "depolarization-evoked" exocytosis.; Micromolar concentrations of extracellular Ca{dollar}sp{lcub}2+{rcub}{dollar} are crucial to the secretagogue effects of {dollar}alpha{dollar}-LT on these endocrine cells. "Spontaneous" exocytosis, occurring at subnanomolar concentrations of toxin, is preceded by progressive development of novel extraordinarily large conductance (300-400 pS), Ca{dollar}sp{lcub}2+{rcub}{dollar}-permeable ion channels, as well as a progressive increase in cytosolic Ca{dollar}sp{lcub}2+{rcub}{dollar} to {dollar}mu{dollar}M range. This range of cytosolic Ca{dollar}sp{lcub}2+{rcub}{dollar} is sufficient to evoke quantal release of hormone during cell membrane permeabilization with detergents, or dialysis of the cell cytoplasm against Ca{dollar}sp{lcub}2+{rcub}{dollar}-containing solutions. The latency of onset of "spontaneous release" decreases, while the intensity of initial discharge increases, with increasing toxin concentration at fixed (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm o{rcub}{dollar} or with increasing (Ca{dollar}sp{lcub}2+{rcub}rbracksb{lcub}rm o{rcub}{dollar} at fixed toxin concentration. Neither toxin-induced channel activity nor release is abolished by potent blockers of native Na{dollar}sp+{dollar} or Ca{dollar}sp{lcub}2+{rcub}{dollar} channels. However, toxin-induced "spontaneous" release is often accompanied by block of native voltage-dependent Na{dollar}sp+{dollar} and Ca{dollar}sp{lcub}2+{rcub}{dollar} currents. At moderate doses of toxin, toxin-induced exocytosis is often followed by endocytosis.; Enhancement of "depolarization-evoked" release is best seen at a low range of toxin concentrations where "spontaneous" release is, at best, very modest and voltage dependent Na{dollar}sp+{dollar} and Ca{dollar}sp{lcub}2+{rcub}{dollar} currents remain stable. Though sometimes accompanied by toxin-induced channel activity and a modest rise in cytosolic Ca{dollar}sp{lcub}2+{rcub}{dollar} (up to a few hundred nM), at other times, enhancement of "depolarization-evoked" release occurs in the absence of either change.; These results suggest that toxin-enhanced quantal release from excitable endocrine cells, is strikingly similar to release from nerve terminals. While the channel-forming ability of {dollar}alpha{dollar}-latrotoxin is the major cause of toxin-induced "spontaneous" release. the ability of the toxin to modulate the secretory machinery, via its receptor, may contribute to enhancement by toxin of depolarization-evoked release.