Spontaneous and receptor-controlled soluble guanylyl cyclase activity in anterior pituitary cells

Nitric oxide (NO)-dependent soluble guanylyl cyclase (sGC) is operative in mammalian cells, but its presence and the role in cGMP production in pituit ary cells have been incompletely characterized. Here we show that sGC is ex pressed in pituitary tissue and dispersed cells, enriched lactotrophs and s omatotrophs, and GH(3) immortalized cells, and that this enzyme is exclusiv ely responsible for cGMP production in unstimulated cells. Basal sGC activi ty was partially dependent on voltage-gated calcium influx, and both calciu m-sensitive NO synthases (NOS), neuronal and endothelial, were expressed in pituitary tissue and mixed cells, enriched lactotrophs and somatotrophs, a nd GH(3) cells. Calcium-independent inducible NOS was transiently expressed in cultured lactotrophs and somatotrophs after the dispersion of cells, bu t not in GH(3) cells and pituitary tissue. This enzyme participated in the control of basal sGC activity in cultured pituitary cells. The overexpressi on of inducible NOS by lipopolysaccharide + interferon-gamma further increa sed NO and cGMP levels, and the majority of de novo produced cGMP was rapid ly released. Addition of an NO donor to perifused pituitary cells also led to a rapid cGMP release. Calcium-mobilizing agonists TRH and GnRH slightly increased basal cGMP production, but only when added in high concentrations . In contrast, adenylyl cyclase agonists GHRH and CRF induced a robust incr ease in cGMP production, with EC(50)s in the physiological concentration ra nge. As in cells overexpressing inducible NOS, the stimulatory action of GH RH and CRF was preserved in cells bathed in calcium-deficient medium, but w as not associated with a measurable increase in NO production. These result s indicate that sGC is present in secretory anterior pituitary cells and is regulated in an NO-dependent manner through constitutively expressed neuro nal and endothelial NOS and transiently expressed inducible NOS, as well as independently of NO by adenylyl cyclase coupled-receptors.