The effect of glucagon on the kinetics of hepatic mitochondrial calcium uptake

Abstract

Previous work by this and other laboratories has shown that glucagon administration stimulates calcium uptake by subsequently isolated hepatic mitochondria. This stimulation of hepatic mitochondrial Ca2+ uptake by in vivo administration of glucagon was further characterized in the present report. Maximal stimulation of mitochondrial Ca2+ accumulation was achieved between 6-10 min after the intravenous injection of glucagon into intact rats. Under control conditions, Ca2+ uptake was inhibited by the presence of Mg2+ in the incubation medium. Glucagon treatment, however, appeared to obliterate the observed inhibition by Mg2+ of mitochondrial Ca2+ uptake. Kinetic experiments revealed the usual sigmoidicity associated with initial velocity curves for mitochondrial calcium uptake. Glucagon treatment did not alter this sigmoidal relationship. Glucagon treatment significantly increased the V max for Ca2+ uptake from 292 +/- 22 to 377 +/- 34 nmoles Ca2+/min per mg protein (n = 8) but did not affect the K 0.5, (6.5-8.6 microM). Since the major kinetic change in mitochondrial Ca2+ uptake evoked by glucagon is an increase in V max, the enhancement mechanism is likely to be an increase either in the number of active transport sites available to Ca2+ or in the rate of Ca2+ carrier movement across the mitochondrial membranes.

Synergistic stimulation of Ca+ uptake by glucagon and Ca2+-mobilizing hormones in the perfused rat liver. A role for mitochondria in long-term Ca2+ homoeostasis

“…The correlation of Ca2+_flux changes with an increase in the calcium content of fractions enriched in mitochondria obtained by Percoll-density-gradient subfractionation of the liver after hormonal challenge strongly indicates that the Ca2+ taken up by the liver is accumulated by the mitochondria, a conclusion consistent with that of Morgan et al (1983a). The data also are consistent with previous observations from our and other laboratories that the administration of glucagon in vivo to rats, or to the perfused rat liver, greatly enhances the ability of the subsequently isolated mitochondria to retain Ca2+ (Hughes & Barritt, 1978;Prpic et al, 1978;Andia-Waltenbaugh et al, 1978, 1981Prpic & Bygrave, 1980;Taylor et al, 1980). Mauger et al (1985), who studied the stimulation of 45Ca2+ uptake induced by the combined addition of glucagon and Ca2+-mobilizing hormones using isolated hepatocytes, concluded that plasma-membrane Ca2+ channels are the principal sites affected.…”

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

“…A large body of work showed, for instance, that administration of glucagon to intact rats will enhance the ability of the subsequently isolated microsomes to accumulate Ca21 in vitro (e.g. Bygrave and Tranter, 1978;Reinhart and Bygrave, 1981;Andia-Waltenbaugh et al, 1981). With mitochondria too, it was established that prior glucagon administration to the intact rat led to an increased ability of the subsequently isolated organelle to retain Ca2+ (e.g.…”

Glucagon stimulation of ruthenium red-insensitive calcium ion transport in developing rat liver

“…siological role of this transport system has yet to be assessed. One established point relevant to this is that insulin is able to offset the stimulatory effects of glucagon (Waltenbaugh & Friedmann, 1978;Taylor et al, 1979;Waltenbaugh et al, 1980). Previously we have argued that considerable information can be gained about the physiological role of metabolite transport by organelles by studying changes in their activity during development, or as a result of some specific physiological perturbation (Bygrave, 1977).…”

“…The very high rates of glycogenolysis that occur immediately after birth are believed to be induced by the high concentration of plasma glucagon (Girard et al, 1973(Girard et al, , 1974Blazquez et al, 1974;Bashan et al, 1979); injection of the hormone into foetuses is known to deplete stores of liver glycogen rapidly (Greengard & Dewey, 1970;Leskes et al, 1971). Since the regulation by glucagon of glycogenolysis in adult rat liver (Hales et al, 1977;Siess et al, 1977) is dependent on the redistribution of intracellular Ca2+ (see, e.g., Rylatt et al, 1979), it is of some interest from the metabolic viewpoint that a target of glucagon action in the liver is the Ruthenium Red-insensitive Ca2+-transport system (Bygrave & Tranter, 1978;Waltenbaugh & Friedmann, 1978;Taylor et al, 1979Taylor et al, , 1980b; the present work). One can envisage that the glucagon-induced rapid mobilization of Ca2+ from stores such as the mitochondria and the endoplasmic reticulum would stimulate the activity of phosphorylase kinase, the Ca2+-dependent enzyme that catalyses the conversion of phosphorylase b into phosphorylase a, and hence glycogen breakdown (see, e.g., Blackmore et al, 1978Blackmore et al, , 1979.…”