Paradoxical effect of trifluoperazine, a calmodulin antagonist, on pepsinogen secretion

Abstract

Pepsinogen secretion (PS) is modulated at the intracellular level by both cAMP and calcium ion. Cholecystokinin octapeptide (CCK-8), a potent stimulus for PS, is believed to act through calcium. The most extensively studied pathway for calcium-mediated modulation involves the formation of calcium/calmodulin complexes, leading to activation of calmodulin. We have therefore examined the hypothesis that an inhibitor of calmodulin might inhibit PS stimulated by CCK-8. The phenothiazine derivative trifluoperazine (TFP) was chosen as a calmodulin antagonist. We measured in vitro secretion of pepsinogen by isolated gastric glands as a function of TFP concentration (10 −6 M-5 × 10 −4 M), in the presence and absence of a maximal concentration of CCK-8 (10 −7 M). Cellular viability was determined by measurement of release of the enzyme lactate dehydrogenase (LDH) into the medium. TFP did not significantly inhibit PS stimulation by CCK-8 at any concentration ( P>0.05). At 10 −4 M, TFP actually augmented PS stimulation by CCK-8 ( P < 0.05). TFP alone significantly stimulated PS ( P < 0.05) at 5 × 10 −5 M and above. TFP did not raise cAMP levels at any concentration tested ( P>0.05), in contrast to the adenylate cyclase activator forskolin, 10 −5 M, which caused a 6- to 37-fold increase ( P < 0.05). TFP, 2 × 10 −4 did not increase LDH levels significantly ( P>0.05). Thus a calmodulin inhibitor, TFP, paradoxically stimulates PS. This stimulatory effect of TFP is not cAMP-dependent and is not accompanied by a nonspecific release of LDH into the medium. The explanation of these observations is unclear. Since TFP is an amphipathic molecule and is intercalated into the cell membrane, it is possible that the increase in pepsinogen secretion may be due to as yet undefined actions of TFP on other intracellular calcium-activated kinases or phosphatases. The results suggest a hitherto undescribed alternative mechanism for the modulation of PS. TFP is unlikely to be of use as a blocker of pepsinogen secretion.