Glucagon-stimulated calcium efflux in the isolated perfused rat liver is dependent on cellular redox potential

In the absence of any exogenous substrates, glucagon (1 X 10(-9) M) stimulated 45Ca2+ efflux from perfused livers derived from fed rats but not in livers of 24-h-fasted animals. In livers of 24-h-fasted animals perfused under conditions which would decrease cellular NAD(P)H/NAD(P)+ ratio (pyruvate (2.0 mM) or acetoacetate (10.0 mM], glucagon (1 X 10(-9) M) did not stimulate 45Ca2+ efflux. Similarly, in livers of 24-h-fasted animals perfused with substrates which increase cellular NAD(P)H content (lactate (2.0 mM) or beta-hydroxybutyrate (10.0 mM], glucagon (1 X 10(-9) M) did not increase 45Ca2+ efflux. Glucagon (1 X 10(-9) M) elicited an increase in 45Ca2+ efflux from livers of 24-h-fasted animals, only when the livers were perfused with lactate]/pyruvate] and beta-hydroxybutyrate]/acetoacetate] ratios similar to those reported for livers of fed rats. Stimulation of 45Ca2+ efflux elicited by either 8-CPT-cAMP, a cAMP analog, or high glucagon concentrations (1 X 10(-8) M) was not affected whether livers were perfused with pyruvate (2.0 mM) or lactate (2.0 mM). Administration of isobutylmethylxanthine (50 microM) alone, or glucagon (1 X 10(-9) M) in the presence of isobutylmethylxanthine (50 microM) stimulated 45Ca2+ efflux from livers of 24-h-fasted animals perfused with pyruvate (2.0 mM) but not from livers perfused with lactate (2.0 mM). The ability of glucagon (1 X 10(-9) M) to elevate tissue cAMP levels was also regulated by the oxidation-reduction state of the livers. The data indicate that glucagon-stimulated 45Ca2+ efflux from perfused livers is mediated via cAMP and is dependent on the oxidation-reduction state of the livers.