Interaction between signal transduction pathways contributing to gallbladder tonic contraction

Muscle strips were used to study the mechanisms that generate cat gallbladder tone. Strontium substitution for calcium and the protein kinase C (PKC) inhibitor H-7 abolished the tone, whereas the calmodulin antagonist W-7 had no effect, suggesting that tone depends on intracellular calcium release and the PKC pathway. Basal levels of diacylglycerol (DAG) and inositol-1,4,5-trisphosphate (IP3) were higher in gallbladder muscle than in esophageal muscle, which does not maintain tone. These data suggest that IP3 might interact with DAG to activate PKC during tonic contraction. This interaction was demonstrated in single cells in which a low dose of IP3 potentiated DAG and the potentiation was blocked by H-7. Furthermore, low doses of IP3 induced contraction, which was blocked by H-7 and unaffected by the calmodulin antagonist CGS-9343B; high doses of IP3 were unaffected by H-7 but were blocked by CGS-9343B; DAG-induced contraction was blocked by activated calmodulin. We conclude that 1) the synergistic action of DAG and IP3-calcium release, which further activates PKC, might be responsible for gallbladder tone and 2) activated calmodulin appears to inhibit the effect of PKC.