Pretreatment with muscarinic receptor agonist activates a calcium-inhibitable adenylate cyclase in GH3 pituitary cells

We have demonstrated previously that pretreatment of GH3 pituitary cells with muscarinic agonists may induce a higher cAMP formation in response to vasoactive intestinal peptide (VIP) or forskolin. In the present study, we further examined the adenylate cyclase (AC) that may be involved. We found that carbachol-pretreatment enhanced both VIP- and forskolin-activated AC activities. The addition of calcium ions to the incubation buffer diminished this enhancing effect. Carbachol was found to induce a decrease in intracellular calcium concentration [Ca2+]i by inhibiting calcium influx through L-type Ca2+ channels. However, the incubation of cells in Ca2+-free buffer or in the presence of L-type Ca2+ channel blockers had no influence on forskolin-stimulated cAMP formation, although both treatments induced decreases in [Ca2+]i as carbachol did. On the other hand, incubation in the presence of LaCl3 at a low concentration not being able to enter cells, forskolin-stimulated cAMP formation as well as the enhancing effect of carbachol-pretreatment on this response, were both suppressed. Similar phenomena were observed when membrane-bound AC activities were measured in the presence of LaCl3. Taken together, these results seem to suggest that pretreatment of GH3 cells with muscarinic receptor agonist may activate a Ca2+-inhibitable AC for a higher stimulated response. Low intracellular calcium concentrations are essential but not sufficient for this effect.