Evidence for the mobilization of cellular calcium by prostacyclin in cat adrenocortical cells: The effect of TMB-8

Stimulation of steroid production by isolated cat adrenocortical cells is partially dependent upon the presence of extracellular Ca²⁺ when elicited by prostacyclin (PGI₂) and completely dependent upon extracellular Ca²⁺ when elicited by corticotropin. TMB-8, an intracellular Ca²⁺ antagonist, completely blocked PGI₂-evoked steroid output in the absence of external Ca²⁺; this inhibition was partially reversed by the addition of Ca²⁺. The increase in secretion caused by corticotropin or PGI₂ in the presence of Ca²⁺ was also reduced in a dose-dependent manner by TMB-8. The steroidogenic action of pregnenolone, which is induced by a Ca²⁺ independent mechanism, was not blocked by TMB-8, either in the presence or absence of Ca²⁺. Corticotropin significantly potentiated the Ca²⁺-independent aspect of PGI₂ action. These studies provide evidence for an internal, PGI₂-sensitive Ca²⁺ store in cat adrenocortical cells.