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Investigation of the roles of Ca²⁺ and InsP₃ diffusion in the coordination of Ca²⁺ signals between connected hepatocytes

¹ INSERM U442, Université de Paris-Sud, bât 443, 91405 Orsay, France
² INSERM U356, IFR 58, Institut des Cordeliers, 15 rue de l’Ecole de Médecine, 75270 Paris, France
³ Université Libre de Bruxelles, IRIBHN, Faculté de Médecine, Campus Erasme, Route de Lennik 808, B-1070 Brussels, Belgium
⁴ Université Libre de Bruxelles, Faculté des Sciences CP231, Boulevard du Triomphe, B-1050 Brussels, Belgium

Author for correspondence (e-mail: laurent.combettes{at}ibaic.u-psud.fr)

Accepted February 27, 2001

Glycogenolytic agonists induce coordinated Ca²⁺ oscillations in multicellular rat hepatocyte systems as well as in the intact liver. The coordination of intercellular Ca²⁺ signals requires functional gap-junction coupling. The mechanisms ensuring this coordination are not precisely known. We investigated possible roles of Ca²⁺ or inositol 1,4,5-trisphosphate (InsP₃) as a coordinating messengers for Ca²⁺ spiking among connected hepatocytes. Application of ionomycin or of supra-maximal concentrations of agonists show that Ca²⁺ does not significantly diffuse between connected hepatocytes, although gap junctions ensure the passage of small signaling molecules, as demonstrated by FRAP experiments. By contrast, coordination of Ca²⁺ spiking among connected hepatocytes can be favored by a rise in the level of InsP₃, via the increase of agonist concentrations, or by a shift in the affinity of InsP₃ receptor for InsP₃. In the same line, coordination cannot be achieved if the InsP₃ is rapidly metabolized by InsP₃-phosphatase in one cell of the multiplet. These results demonstrate that even if small amounts of Ca²⁺ diffuse across gap junctions, they most probably do not play a significant role in inducing a coordinated Ca²⁺ signal among connected hepatocytes. By contrast, coordination of Ca²⁺ oscillations is fully dependent on the diffusion of InsP₃ between neighboring cells.

Key words: Calcium oscillations, Waves, Liver, Gap junctions

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