Dependence of Electrical Activity and Calcium Influx-Controlled Prolactin Release on Adenylyl Cyclase Signaling Pathway in Pituitary Lactotrophs

Abstract

Pituitary lactotrophs in vitro fire extracellular Ca2+-dependent action potentials spontaneously through still unidentified pacemaking channels, and the associated voltage-gated Ca2+influx (VGCI) is sufficient to maintain basal prolactin (PRL) secretion high and steady. Numerous plasma membrane channels have been characterized in these cells, but the mechanism underlying their pacemaking activity is still not known. Here we studied the relevance of cyclic nucleotide signaling pathways in control of pacemaking, VGCI, and PRL release. In mixed anterior pituitary cells, both VGCI-inhibitable and -insensitive adenylyl cyclase (AC) subtypes contributed to the basal cAMP production, and soluble guanylyl cyclase was exclusively responsible for basal cGMP production. Inhibition of basal AC activity, but not soluble guanylyl cyclase activity, reduced PRL release. In contrast, forskolin stimulated cAMP and cGMP production as well as pacemaking, VGCI, and PRL secretion. Elevation in cAMP and cGMP levels by inhibition of phosphodiesterase activity was also accompanied with increased PRL release. The AC inhibitors attenuated forskolin-stimulated cyclic nucleotide production, VGCI, and PRL release. The cell-permeable 8-bromo-cAMP stimulated firing of action potentials and PRL release and rescued hormone secretion in cells with inhibited ACs in an extracellular Ca2+-dependent manner, whereas 8-bromo-cGMP and 8-(4-chlorophenylthio)-2'-O-methyl-cAMP were ineffective. Protein kinase A inhibitors did not stop spontaneous and forskolin-stimulated pacemaking, VGCI, and PRL release. These results indicate that cAMP facilitates pacemaking, VGCI, and PRL release in lactotrophs predominantly in a protein kinase A- and Epac cAMP receptor-independent manner.

The Regulation of Pituitary Prolactin Secretion: Hypothalamic, Intrapituitary and Intracellular Factors and Signaling Mechanisms

“…The Gi/o signaling pathway is involved in DA-related inhibition, since resulted in depressed cAMP production. On the other hand, inhibition of basal cAMP production in pituitary lactotrophs does not completely abolish the spontaneous firing of APs and only partially inhibits basal PRL release (Gonzalez-Iglesias 2006). Similar to D2 DA receptor coupled subunits, the endothelin (ETA) receptors are also connected to both stimulatory and inhibitory (Gs, Gi/o, Gz) Gprotein pathways.…”

“…Involvement of different membrane enzymes in the context of varied levels of activation of G-proteins as well as Ca 2+ -and protein kinase C-dependent processes are equally important to generate the characteristic cAMP signal and cut-off at certain level by the cAMP-degrading phosphodiesterases. (Antony, 2000;Diamond 1999;Gonzalez-Iglesias, 2006) The level of Ca 2+ concentration in cytosol and the cyclic nucleotide signaling pathways are connected in regulation of physiological processes through messenger generation and also influencing additional other signaling pathways and vice versa, there is a distinct molecular machinery responsible to modulate the calcium concentration ([Ca 2+ ]i) (Bruce 2003).…”

“…(Gonzalez-Iglesias 2006) This machinery of the cyclic nucleotides and the kinases may serve as a regulator or a pacemaker for hormone secretion of pituitary cells. It was demonstrated by in vitro models, that:…”

“…Partial dependence of basal PRL release on cyclic nucleotides and partial inhibition of AC activity by spontaneous VGCI are the findings that consistent with reciprocal modulation of cyclic nucleotides and VGCI in spontaneously firing pituitary lactotrophs. (Gonzalez-Iglesias 2006).…”

Phosphodiesterases in endocrine physiology and disease

“…First, it was shown that the addition of 3-isobutyl1methylxanthine (IBMX), a common inhibitor of PDE subtypes, increased cyclic nucleotides in primary culture of rats anterior pituitary cells (41). Moreover, application of IBMX caused a dose-dependent increase in basal prolactin release.…”