THE FORMATION OF GLYCOGEN FROM CARBOHYDRATES.
What now further happens with the carbohydrates? The work of Claude Bernard furnishes an answer to this question. His in vestigations have been frequently repeated and in the main confirmed. We are indebted to E. and C. von Volt, however, for some amplifications of his teachings.
As long as the quantity of carbohydrates passing from the intestine to the liver remains moderate, it is a matter of no importance as re gards their further destiny under what form they enter the liver; the result is the same whether it be grape sugar, fruit sugar, milk sugar, or other kinds, that the portal blood conveys from the intestine to the liver. The liver takes hold of the carbohydrate and converts it into glycogen, which is deposited in the hepatic cells in the form of flakes, and probably in loose combination with albumin (glycogen fattening of the liver).
The liver is a reservoir for carbohydrates, and is so capacious that it may contain as much as 14 per cent. of its own weight of glycogen.
The existence of this storage-place permits of a regulation of the amount of sugar contained in the blood of the hepatic veins and of the arteries. An examination for sugar of the blood of the hepatic veins and of the arteries in the most various parts of the body has been repeatedly made in healthy animals (and also in healthy men) after the ingestion of carlmhydrats and of alumni in, and when fasting, and the proportion has always been found to be constant or to vary between very narrow limits, viz., from 0.12 to 0.18 per cent. The in terposition of this glycogen reservoir also acts in such a way that the outflow remains the same, whatever fluctuations there may be in the inflow, and that there is constantly circulating in the arterial blood a saccharine solution of a concentration best adapted to favor the normal course of all the functions. We may say, in short, that the liver watches over and regulates the proportion of sugar contained in the blood leaving it.
We have now several times spoken of the percentage of sugar in the blood of the hepatic veins and of the arteries. What kind of sugar is it? It has been clearly proven that we have here to deal with grape sugar. Of other carbohydrates glycogen alone enters into consideration, but this is met with only in almost infinitesimal quan tities. The maximum percentage found by Huppert (in the dog) was only 0.0025.
Glycogen is found, however, in other organs than the liver, espe cially in the muscles. The quantity contained in the latter varies, being greater after the ingestion of large amounts of carbohydrates and after a long period of rest, than after abstinence from food and after exhausting labor. It has been found that in general the
amounts of glycogen contained in the liver and in the muscles in crease and decrease along parallel lines, that as a rule the total quan tity of glycogen in the liver is about equal to that contained in all the muscles of the body, and finally that, in the case of glycogen impov erishment of the body (see below), the muscles are more tenacious of their store of glycogen than is the liver. The muscles are therefore likewise a glycogen depot.
How does the glycogen reach the muscles? It would appear that the latter elaborate their own store of glycogen from grape sugar. This grape sugar is always at hand for the use of the muscles, for the liver looks out for the maintenance of a certain percentage of grape sugar in the blood. The liver does this while giving up its glycogen. It excretes at the most only traces of this substance un changed, the greater part of it being first converted into grape sugar. Thus we see that the liver is more than a simple carbohydrate reser voir, it is also endowed with recoiuing powers. It collects the super fluous carbohydrates from the portal vein, fixes them in a form which is not readily diffusible but which is well adapted for fixation in the cells (glycogen), and gives them up again in a soluble form which is well suited for transportation and for conversion in the tissue cells (grape sugar).
But the subject of carbohydrate, and particularly of glycogen, formation in the body is not vet exhausted.
It is an established fact that glycogen may be produced from the albuminates, for it has been found that, in animals which have through certain processes (see below) been made glycogen-free, a new formation of glycogen takes place when they are fed exclusively upon albuminoids. Many other facts also, not the least of which are those gleaned in a study of diabetes mellitus in man and of experimental diabetes in animals, bear testimony to the same effect. We are driven, through investigations made up to the present, to the con clusion that a part of the non-nitrogenous atom-groups of the large albumin molecule is always first converted into carbohydrate or car bohydrate-like combinations before it is further burned up into CO, and 11,0. In this manner there is formed at least 45 grams of car bohydrate out of every 100 grams of albumin decomposed in the body (Minkowski).
In what part of the body this formation of carbohydrate from albumin takes place is not clear. Many facts point to the liver, many others again speak in favor of the facultative participation of the muscles in this process.
Possibly, as we shall see later, the question of carbohydrate for mation (glycogenesis) is not yet exhausted.