Stable enhancement of calcium retention in mitochondria isolated from rat liver after the administration of glucagon to the intact animal

Abstract

Mitochondria isolated from rat liver by centrifugation of the homogenate in buffered iso-osmotic sucrose at between 4000 and 8000 g-min, 1 h after the administration in vivo of 30 .mu.g of glucagon/100 g body wt, retain Ca2+ for over 45 min after its addition at 100 nmol/mg of mitochondrial protein in the presence of 2 mM Pi. In similar experiments, but after the administration of saline (.09% NaCl) in place of glucagon, Ca2+ is retained for 6-8 min. The ability of glucagon to enhance Ca2+ retention is completely prevented by co-adminstration of 4.2 mg of puromycin/100 g body wt. The resting rate of respiration after Ca2+ accumulation by mitochondria from glucagon-treated rats remains low by contrast with that from saline-treated rats. Respiration in the latter mitochondria increased markedly after the Ca2+ accumulation, reflecting the uncoupling action of the ion. Concomitant with the enhanced retention of Ca2+ and low rates of resting respiration by mitochondria from glucagon-treated rats was an increased ability to retain endogenous adenine nucleotides. An investigation of properties of mitochondria known to influence Ca2+ transport revealed a significantly higher concentration of adenine nucleotides, but not of Pi, in those from glucagon-treated rats. The membrane potential remained unchanged, but the transmembrane pH gradient increased by approximately 10mV, indicating increased alkalinity of the matrix space. Depletion of endogenous adenine nucleotides by Pi treatment in mitochondria from both glucagon-treated and saline-treated rats led to a marked diminution in ability to retain Ca2+. Adenine nucleotide translocase activity was unaffected by glucagon treatment of rats in vivo. Although the data are consistent with the argument that the Ca2+-translocation cycle in rat liver mitochondria is a traget for glucagon action in vivo, they do not permit conclusions to be drawn about the molecular mechanisms involved in the glucagon-induced alteration to this cycle.