The influx of Ca2+ induced by the administration of glucagon and Ca2+-mobilizing agents to the perfused rat liver could involve at least two separate pathways

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Abstract
The Ca2+-mobilizing actions of ADP, ATP and epidermal growth factor (EGF) and their interaction with glucagon were studied in a perfused liver system incorporating a Ca2+-selective electrode. ADP (1-100 microM), ATP (1-100 microM) and EGF (10-50 nM) all induced a net efflux, followed by a net uptake of Ca2+ in the intact liver. The co-administration of glucagon (or of cyclic AMP) with these agents resulted in a synergistic potentiation of the Ca2+ uptake response in a way which resembles the synergism observed when glucagon is administered with phenylephrine, vasopressin or angiotensin [Altin & Bygrave (1986) Biochem J. 238, 653-661]. The inability of diltiazem, verapamil and nifedipine to inhibit the Ca2+-influx response suggests that the stimulation of Ca2+ influx does not occur through voltage-sensitive Ca2+ channels. By contrast, the synergistic effects of glucagon in the stimulation of Ca2+ influx are inhibited by 10 mM-neomycin, and a lowering of the extracellular pH to 6.8. Simultaneous measurements of perfusate Ca2+ and pH changes suggest that the Ca2+ influx response is not mediated by a Ca2+/H+ exchange. The inability of neomycin and low extracellular pH to inhibit the refilling of the hormone-sensitive pool of Ca2+, after the administration of Ca2+-mobilizing agents alone, provides evidence for the existence in liver of at least two Ca2+-influx pathways, or mechanisms for regulating Ca2+ influx.