Maitotoxin stimulates steroid and peptide hormone secretion by swine luteal tissue

The impact of calcium influx on the secretion of steroidal and nonsteroidal ovarian products was tested using a novel marine toxin, maitotoxin, a putative activator of endogenous calcium channels. In incubations of swine luteal minces in balanced salt solution, maitotoxin induced dose-dependent (3-12.5 ng/ml) and time-dependent (15-60 min) secretion of progesterone and relaxin. The magnitude of these stimulatory effects approached that observed for luteinizing hormone (LH) and was explicitly dependent on the presence of extracellular calcium ions. Maitotoxin's actions were not significantly impeded by substitution of choline for sodium ions in the incubation medium. Moreover, measurements of tissue calcium content by atomic absorption spectrometry indicated that maitotoxin augmented the total cellular content of calcium by 44 ± 3.1% within 1 min. This time course was distinct from that observed for LH. The specificity of these actions on the calcium pathway was further suggested by the absence of any measurable effects of maitotoxin on luteal cAMP generation. We conclude that maitotoxin transiently increases luteal calcium content without altering cAMP generation and elicits concomitant release of both a steroidal (progesterone) and nonsteroidal (relaxin) product of the corpus luteum. Such actions are dependent on extracellular calcium but not sodium ions. These observations provide evidence for a calcium-dependent stimulatory pathway that activates both steroidal and nonsteroidal mechanisms of secretion in the swine corpus luteum.