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-Latrotoxin Stimulates Inward Current, Rise in Cytosolic Calcium Concentration, and Exocytosis in at Pituitary Gonadotropes¹

Department of Pharmacology and Division of Neuroscience, University of Alberta, Edmonton, Alberta, Canada T6G 2H7

Address all correspondence and requests for reprints to: Dr. Frederick W. Tse, Department of Pharmacology, 9–70 Medical Science Building, University of Alberta, Edmonton, Alberta, Canada T6G 2H7. E-mail: fred.tse{at}ualberta.ca

-Latrotoxin (LTX) from the black widow spider venom, stimulates neurotransmitter release from neuronal cells via Ca²⁺-dependent as well as Ca²⁺-independent mechanisms. In some peptide-secreting endocrine cells, however, LTX stimulates hormone release mainly via a Ca²⁺-independent mechanism. Here we investigated the action of LTX in rat pituitary gonadotropes that secrete the peptide, LH. Using the patch-clamp technique in conjunction with the fluorescent Ca²⁺ indicator (indo-1) to simultaneously measure the cytosolic Ca²⁺ concentration ([Ca²⁺]_i) and ionic current, we showed that LTX elicited bursts of inward current that were accompanied by [Ca²⁺]_i elevations. In the presence of a physiological concentration of extracellular Ca²⁺, the unitary conductance of the LTX-induced current was about 300 pS, and only about 6.4% of the current was carried by Ca²⁺. The LTX-induced current was occasionally followed by intracellular Ca²⁺ release. At [Ca²⁺]_i of 1 µM or more, exocytosis (detected by membrane capacitance measurement) was consistently triggered, and it was frequently followed by endocytosis. Thus, LTX triggers Ca²⁺-dependent exocytosis in gonadotropes via extracellular Ca²⁺ entry as well as intracellular Ca²⁺ release. In approximately 25% of the cells, LTX could also trigger a slow exocytosis in the absence of [Ca²⁺]_i elevation. Therefore, LTX has both Ca²⁺-dependent and Ca²⁺-independent actions in gonadotropes.

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