Abstract
We have investigated the possible roles of cyclic GMP (cGMP) in initiating or regulating capacitiative Ca2+ entry in rat pancreatic acinar cells. In medium containing 1.8 mM external Ca2+, thapsigargin activated Ca2+ entry and slightly but significantly increased intracellular cGMP concentration. This rise in cGMP levels was prevented by pretreating the cells with the guanylate cyclase inhibitor, LY-83583, or by omitting Ca2+ during stimulation by thapsigargin or methacholine. LY-83583 and NG-nitro-L-arginine (L-NA, an inhibitor of NO synthase) both had a small inhibitory effect on Ca2+ entry when they were added after thapsigargin in Ca2(+)-containing medium, and they reduced by 32 and 48% respectively the thapsigargin-induced capacitative Ca2+ entry when added to the cells during a 20 min preincubation period. However, neither dibutyryl cGMP (Bt2cGMP) nor sodium nitroprusside, an NO mimic, affected either basal intracellular Ca2+ concentration [Ca2+]i or thapsigargin-induced capacitative Ca2+ entry. Further, the inhibitory effects observed after preincubation with LY-83583 or L-NA could not be prevented by preincubation with Bt2cGMP, nor could they be reversed by adding Bt2cGMP, 8-bromo-cGMP or sodium nitroprusside acutely after activation of capacitative Ca2+ entry by thapsigargin. Finally, pretreatment of cells with LY-83583 or L-NA did not affect Ca2+ signalling in response to 1 microM methacholine, including the pattern of [Ca2+]i oscillations. In conclusion, in pancreatic acinar cells, the rise in cellular cGMP levels appears to depend on, rather than cause, the increase in [Ca2+]i with agonist stimulation.