Stimulation by α-adrenergic agonists of Ca2+ fluxes, mitochondrial oxidation and gluconeogenesis in perfused rat liver

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Abstract
Glucose output from perfused livers of 48 h-starved rats was stimulated by phenylephrine (2 microM) when lactate, pyruvate, alanine, glycerol, sorbitol, dihydroxyacetone or fructose were used as gluconeogenic precursors. Phenylephrine-induced increases in glucose output were immediately preceded by a transient efflux of Ca2+ and a sustained increase in oxygen uptake. Phenylephrine decreased the perfusate [lactate]/[pyruvate] ratio when sorbitol or glycerol was present, but increased the ratio when alanine, dihydroxyacetone or fructose was present. Phenylephrine induced a rapid increase in the perfusate [beta-hydroxybutyrate]/[acetoacetate] ratio and increased total ketone-body output by 40-50% with all substrates. The oxidation of [1-14C]octanoate or 2-oxo[1-14C]glutarate to 14CO2 was increased by up to 200% by phenylephrine. All responses to phenylephrine infusion were diminished after depletion of the hepatic alpha-agonist-sensitive pool of Ca2+ and returned toward maximal responses after Ca2+ re-addition. Phenylephrine-induced increases in glucose output from lactate, sorbitol and glycerol were inhibited by the transaminase inhibitor amino-oxyacetate by 95%, 75% and 66% respectively. Data presented suggest that the mobilization of an intracellular pool of Ca2+ is involved in the activation of gluconeogenesis by alpha-adrenergic agonists in perfused rat liver. alpha-Adrenergic activation of gluconeogenesis is apparently accompanied by increases in fatty acid oxidation and tricarboxylic acid-cycle flux. An enhanced transfer of reducing equivalents from the cytoplasmic to the mitochondrial compartment may also be involved in the stimulation of glucose output from the relatively reduced substrates glycerol and sorbitol and may arise principally from an increased flux through the malate-aspartate shuttle.

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