Pharmacological evaluation of the role of cytochrome P450 in intracellular calcium signalling in rat pancreatic acinar cells

1 We have investigated whether the cytochrome P450 system is involved in Ca2+ signalling in rat pancreatic acinar cells. Intracellular free [Ca2+] ([Ca2+](i)) was measured in collagenase-isolated cells using fura-2 microspectrofluorimetry and imaging. 2 The imidazole P450 inhibitor ketoconazole (5-50 muM) inhibited [Ca2+](i) oscillations induced by cholecystokinin octapeptide (CCK). However, ketoconazole also raised baseline [Ca2+](i) when applied in the absence of CCK. These effects were mimicked by 5-50 muM SKF96365, an imidazole widely used as an inhibitor of Ca2+ entry. 3 The non-imidazole P450 inhibitor proadifen (SKF525A) inhibited CCK-induced [Ca2+](i) oscillations at a concentration of 10-50 muM. Proadifen alone caused intracellular Ca2+ release at 25 or 50 muM, but not at 10 muM. 4 Octadecynoic acid and 1-aminobenzotriazole, structurally-unrelated non-imidazole P450 inhibitors, did not alter baseline [Ca2+](i) or CCK-evoked oscillations. 5 We compared cumulative CCK dose-response relationship in control cells and in cells where P450 had been induced by prior injection of animals with beta -naptithoflavone. Only minor differences were apparent, with induced cells showing some decrease in responsiveness at moderate and higher concentration of CCK (30 pM-3 nM). 6 Direct assessment of depletion-activated Ca2+ entry showed no clear differences between control and induced cells. 7 In conclusion, we could find no compelling evidence for a role of P450 in controlling Ca2+ signalling generally, or Ca2+ entry in particular, in pancreatic acinar cells. Induction of P450 is therefore probably toxic to acinar cells via a Ca2+-independent mechanism.