Studies on the Physiologic Role of Glucagon with Special Reference to the Cooperation of Glucagon and Insulin in the Control of Carbohydrate Metabolism

Abstract

The role of glucagon in carbohydrate metabolism seems to be better understood nowadays. But since there is no reliable bioassay method for the estimation of glucagon-like activity (Abbr. as glucagon activity) in blood, no detailed studies on the role of glucagon in the carbohydrate metabolism could be carried out.
In our clinic, a new bioassay method of blood glucagon activity was developed by Shinko, basing the glucagon activity on the liver slices.
By using this method, the author tried to analyse the physiologic role of blood glucagon.
The results are summarized as follows :
(A) Studies with in vitro technique.
Wistar strain rats were used, weighing 100-120gm. They were fed for 3 to 5 days with a carbo-hydrate rich diet and then made to fast for 48 hours. This experimental condition was necessary to standardize the glycogen content of the rat liver. Liver slices were taken and then incubated in Warburg's apparatus. As incubation medium, modified Buchanan-Hasting's Solution with 1000mg./dl. glucose, was used. Novo-semilente insulin or Glucagon-Lilly was added to the medium in various concentrations. Δ-glucose in the medium and Δ-glycogen of the liver were examined and these values were observed.
The values are summarized as follows ;
(1) Increasing doses of insulin added to incubation medium usually increase Δ-glycogen and glucose uptake
(2) When glucagon was added to the Insulin free (incubation) medium, in which liver slices were incubated, no particular change of glucose uptake was observed with low concentration of glucagon. If the doses of glucagon is elevated to 1μg./ml. or more it causes only a slight decrease of glucose uptake and Δ-glycogen.
(3) When insulin was added to the medium, which contained 10^-4μg./ml. of glucagon, glucose uptake from the medium increased but this increase of glucose uptake was lower than that in the experiment, in which no glucagon was added to the medium. Δ-glycogen of the liver slices decreases under insulin addition. These results indicate the possible co-action of glucagon and insulin in some instances.
(4) When insulin was added to an incubation medium, of which glucagon concentration was 10^-2μg./ml., there was an increase of glucose uptake under insulin action. On the other hand, action of glucagon is potentiated by insulin and this leads to increased glycogenolysis. These two reactions act antagonistically to each other, the result of which is expressed as a mild fluctuation of glucose uptake.
(5) When insulin is added to a medium, which contains glucagon to 1μg/.ml. or 10μg./ml. concentration, a progressive glycogenolytic action of glucagon is observed with the addition of increasing doses of insulin.
(6) Glycogenolytic effect of added glucagon on the liver glycogen was detectable for about 30 minutes in case of simple glucagon action. In 120 minutes, glycogenolitic effect of added glucagon is over and there is an enhanced glucose uptake from the medium as compared with liver slices incubated in simple Buchanan-glucose medium.
Thus glucagon effect is suppressed by coexisting insulin in the first phase of time, but on the whole, this glucagon effect has a tendency to remain active for a long time.
Glycogenolytic effect of epinephrine and glucagon on the liver slices was compared in a medium, to which 1 u./ml. insulin was added. In the first phase of epinephrine action, slight glycogenolysis was seen but in the course of 120 minutes, the glycogenolytic effect of epinephrine was over and even the glucose uptake from the medium was observed. In the same kind of experiment where glucagon was used instead of epinephrine, there was an enhancement of glycogenolysis. These results point to quite a different mode of action of glucagon and epinephrine.
(B) Experiments in vivo