Receptor-oriented intercellular calcium waves evoked by vasopressin in rat hepatocytes.

Agonist-induced intracellular calcium signals may propagate as intercellular Ca²⁺ waves in multicellular systems as well as in intact organs. The mechanisms initiating intercellular Ca²⁺ waves in one cell and determining their direction are unknown. We investigated these mechanisms directly on fura2-loaded multicellular systems of rat hepatocytes and on cell populations issued from peripheral (periportal) and central (perivenous) parts of the hepatic lobule. There was a gradient in vasopressin sensitivity along connected cells as demonstrated by low vasopressin concentration challenge. Interestingly, the intercellular sensitivity gradient was abolished either when D-myoinositol 1,4,5-trisphosphate (InsP₃) receptor was directly stimulated after flash photolysis of caged InsP₃ or when G proteins were directly stimulated with AlF^4­. The gradient in vasopressin sensitivity in multiplets was correlated with a heterogeneity of vasopressin sensitivity in the hepatic lobule. There were more vasopressin-binding sites, vasopressin-induced InsP₃ production and V1a vasopressin receptor mRNAs in perivenous than in periportal cells. Therefore, we propose that hormone receptor density determines the cellular sensitivity gradient from the peripheral to the central zones of the liver cell plate, thus the starting cell and the direction of intercellular Ca²⁺ waves, leading to directional activation of Ca²⁺-dependent processes. [ABSTRACT FROM AUTHOR]