Somatostatin-induced control of cytosolic free calcium in pituitary tumour cells

1. In rat pituitary tumour cells (GC cells), spontaneous oscillations of the intracellular concentration of Ca(2+) ([Ca(2+)](i)) induce growth hormone (GH) secretion that is inhibited by octreotide, a somatostatin (SRIF) agonist which binds to SRIF subtype (sst) receptor 2. The effects of its functional activation on the control of [Ca(2+)](i) were investigated using fluorimetric measurements of [Ca(2+)](i). 2. SRIF decreases the basal [Ca(2+)](i) and the [Ca(2+)](i) rise in response to forskolin (FSK) through the inhibition of L-type voltage-dependent Ca(2+) channels. 3. Pretreatment with octreotide or with L-Tyr(8)Cyanamid 154806, a sst(2) receptor antagonist, abolishes the SRIF-induced inhibition of [Ca(2+)](i). Octreotide is known to operate through agonist-induced desensitization, while the antagonist operates through receptor blockade. 4. sst(1) and sst(2) receptor-immunoreactivities (-IRs) are localized to cell membranes. sst(2), but not sst(1) receptor-IR, internalizes after cell exposure to octreotide. 5. SRIF-induced inhibition of basal [Ca(2+)](i) or FSK-induced Ca(2+) entry is blocked by pertussis toxin (PTX). 6. FSK-induced cyclic AMP accumulation is only partially decreased by SRIF or octreotide, indicating that sst(2) receptors are coupled to intracellular pathways other than adenylyl cyclase (AC) inhibition. 7. In the presence of H-89, an inhibitor of cyclic AMP-dependent protein kinase (PKA), SRIF-induced inhibition of basal [Ca(2+)](i) is still present, although reduced in amplitude. 8. SRIF inhibits [Ca(2+)](i) by activating sst(2) receptors. Inhibition of AC activity is only partly responsible for this effect, and other transduction pathways may be involved