Effect of Fasting and Insulin on the Glucagon-lnduced Orthophosphate Incorporation to the Isolated Perfused Rat Liver

Abstract

In previous experiments (1) it was shown that glucagon, epinephrine , CAMP, and dB-cAMP were all able to increase the incorporation of orthophosphate to the isolated perfused fed rat liver. This phenomenon may explain, in part, the well-known hypophosphatemic effect exhibited by these glycogenolytic factors (2-6). The functional meaning of the increased orthophosphate incorporation to the liver has not been clarified yet. It has been postulated that such incorporation may be related to an increment of phosphate requirements during glycogenolysis (1); on the other hand, in the last few years several investigators have pointed out that glucagon and cAMP may activate protein phosphorylations in the liver (7-12), and this effect could also have some relationship with the above mentioned phenomenon.Our previous experiments (1) were performed in fed rat livers in which a clear increase in spontaneous or glucagon-induced glycogenolysis was evidenced; therefore, it was considered of interest to study the effect of glucagon in two experimental conditions which inhibit glycogenolysis through different mechanisms: fasting and insulin administration.Material and methods. Male Sprague-Dawley rats (300-350 g body weight) bred in our Institute were used in the experiments. Livers were perfused by means of a previously described technique (1 , 13) using 70 ml of perfusate which consisted of Krebs-Ringer-bicarbonate containing 3 g/100 ml of bovine serum albumin (Armour Fraction V), 80 mg/lOO ml of glucose, 1.7 mg/lOO ml of buffered sodium orthophosphate, and 20-22 vol % of washed rat erythrocytes. Heparin (500 U) was added at the begin-'