Abstract

Calcium-mobilizing agonists induce intracellular Ca²⁺ concentration ([Ca²⁺]_i) changes thought to trigger cellular responses. In connected cells, rises in [Ca²⁺]_i can propagate from cell to cell as intercellular Ca²⁺ waves, the mechanisms of which are not elucidated. Using fura2-loaded rat hepatocytes, we studied the mechanisms controlling coordination and intercellular propagation of noradrenaline-induced Ca²⁺ signals. Gap junction blockade with 18 -glycyrrhetinic acid resulted in a loss of coordination between connected cells. We found that second messengers and [Ca²⁺]_i rises in one hepatocyte cannot trigger Ca²⁺ responses in connected cells, suggesting that diffusion across gap junctions, while required for coordination, is not sufficient by itself for the propagation of intercellular Ca²⁺ waves. In addition, our experiments revealed functional differences between noradrenaline-induced Ca²⁺ signals in connected hepatocytes. These results demonstrate that intercellular Ca²⁺ signals in multicellular systems of rat hepatocytes are propagated and highly organized through complex mechanisms involving at least three factors. First, gap junction coupling ensures coordination of [Ca²⁺]_i oscillations between the different cells; second, the presence of hormone at each hepatocyte is required for cell–cell Ca²⁺ signal propagation; and third, functional differences between adjacent connected hepatocytes could allow a 'pacemaker-like' intercellular spread of Ca²⁺ waves.