Suppression of Ca2+ oscillations induced by cholecystokinin (CCK) and its analog OPE in rat pancreatic acinar cells by low-level protein kinase C activation without transition of the CCK receptor from a high- to low-affinity state

Abstract

Cholecystokinin (CCK) analogs, JMV-180 and OPE, release Ca2+ from intracellular stores and induce oscillations in the concentration of cytosolic Ca2+ ([Ca2+]i), but do not generate a detectable rise in inositol 1,4,5-trisphosphate (InsP3) levels. In contrast, high concentrations of CCK elevate InsP3, as well [Ca2+]i, to a peak which decreases to near basal levels without oscillations. The mechanisms which underlie inhibition of [Ca2+]i oscillations observed with high CCK concentrations are unclear, but are believed to involve a low-affinity CCK receptor state. Alternately, CCK analogs may be weak partial agonists of the phospholipase C pathway, whereas native CCK, as a full agonist of this pathway, stimulates low levels of protein kinase C (PKC) activity. Preincubation of acini with 1 nM 12 O-tetradecanoyl-phorbol 13-acetate (TPA) for 15 min at 37 degrees C did not affect OPE binding to acini, but abolished OPE-induced (at 1 microM) [Ca2+]i oscillations without affecting the initial [Ca2+]i spike. These transformed OPE-induced [Ca2+]i responses mimicked those induced by supramaximal CCK octapeptide (CCK-8) concentrations. Inhibition of [Ca2+]i oscillations by 1 nM TPA was reversed by the PKC inhibitor staurosporine (0.2 microM). After [Ca2+]i oscillations were induced with OPE or low concentrations of CCK-8 (20 pM), 1 nM TPA caused a gradual slowing of oscillation frequency over 15-20 min without affecting [Ca2+]i spike amplitude. In contrast, 1 microM TPA inhibited OPE binding and caused a more generalized inhibition of OPE- and CCK-evoked Ca2+ signals.(ABSTRACT TRUNCATED AT 250 WORDS)