The Stimulatory Effect of Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) on LH Release from Rat Pituitary Cells in Vitro Does not Involve Calcium Mobilization

Summary

The hypothalamic peptide “pituitary adenylate cyclase activating polypeptide (PACAP)” stimulates cAMP production in cultured rat pituitary cells and enhances LH release. It has been suggested that the stimulation of LH release by PACAP comprises two distinct mechanisms: a direct stimulatory action on LH secretion and a potentiation of the response of the gonadotrophes to LHRH. Thus the possibility exists that PACAP may enhance LH secretion not only by increased cAMP production but also by increasing cytosolic Ca²⁺ concentrations ([Ca²⁺]). In the present study we examined whether PACAP affects cytosolic [Ca²⁺] in identified rat gonadotrophes (as determined by the fura-method) and whether the suggested potentiating effect of PACAP on LHRH induced LH release is dependent on Ca²⁺. PACAP (lnM) and 0.1 nM LHRH significantly increased LH concentrations in the culture medium after 5 hrs of incubation. Coincubation of cells with both peptides resulted in an additive increase of LH release. While the stimulatory effect LHRH was blunted in Ca²⁺-free medium, PACAP remained stimulatory to LH release. PACAP stimulated cAMP formation regardless whether the culture medium contained Ca²⁺ or not. Gonadotrophes were selected by their response to LHRH (1 µM) and were subsequently challenged with PACAP (lμM). About 75% of gonadotrophes responded also to PACAP with an increase of cytosolic [Ca²⁺] which was blunted by removal of extracellular Ca²⁺. We suggest that in the rat pituitary the majority of the gonadotrophes are PACAP responsive as determined by an increase of cytosolic [Ca²⁺]. However, it appears that the stimulatory action of PACAP on LH release is primarily due to enhanced cAMP production since LH secretion and cAMP formation were elevated by an amount of PACAP ineffective to modulate cytosolic [Ca²⁺]. Our data do not support the assumption that PACAP potentiates the action of LHRH as we found an additive effect of both peptides on LH secretion.

Key words

PACAP - LH secretion - intracellular calcium