Epinephrine action on glucose uptake by rat diaphragm; effect of ionic composition

Abstract

The in vitro inhibitory effect of epinephrine on glucose uptake by the isolated rat diaphragm was reinvestigated under various experimental conditions. This effect of epinephrine was manifest only in the Krebs-Ringer phosphate incubating medium and did not occur in the Krebs-Henseleit bicarbonate medium or in the phosphate medium in which the concentration of Mg++ was increased twofold. A change in the concentration of Ca++ ions in the phosphate incubating medium did not affect the inhibition produced by epinephrine. An increase in H+ ion concentration in the phosphate medium diminished control glucose uptake as well as the inhibitory effect of epinephrine. It is suggested that the absence of in vitro inhibition of glucose uptake by epinephrine in the bicarbonate and high Mg phosphate media is due to the normal functioning of the phosphofructokinase enzyme system in these two media only. This in turn produces a faster rate of removal of glucose-6-phosphate, the product of glucose phosphorylation and glycogen breakdown. The accumulation of glucose-6-phosphate would inhibit the enzyme hexokinase, as is the case in the phosphate medium. An explanation for the conflicting reports in the literature regarding the in vitro effect of epinephrine on glucose uptake in muscle is suggested.