Carbohydrate metabolism and insulin / by John James Rickard Macleod.

  • John James Rickard Macleod
Date:
1926
Catalogue details

Licence: In copyright

Credit: Carbohydrate metabolism and insulin / by John James Rickard Macleod. Source: Wellcome Collection.

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    or one blood sugar estimation as the method for carrying out the assay. As a matter of fact, it is impossible that the exact value of a unit can be laid down in absolute terms, because of the variability with which different animals react towards insulin. For practical purposes the important thing is to have some standard with which the assay arrived at by any dependable method can be compared. A dry preparation of insulin hydro- chloride has been prepared (by the Medical Research Council) for this purpose, and its assay has been found to be such that I mg. equals 8 units. This result, it may be mentioned, is based mainly on the degree of hypoglycaemia which the powder causes in rabbits, but it has also been obtained by comparing its convulsion-producing tendency (on mice) with that of another standard preparation.1 The international unit of insulin may therefore be defined as the quantity which produces an effect on carbohydrate metabolism equal to that of one-eighth of a milligram of the standard preparation of insulin hydrochloride. The introduc- tion of this standard will not materially alter the assays of the various preparations of insulin in present use, for it has been carefully compared with these, so as to avoid such confusion, and it will fix a definite value to the unit for all time. It leaves open the exact method to be used in conducting the bio-assay, for it would be hazardous at the present time, when the physio- logical action of insulin is so little understood, to adopt any particular method for this purpose. We will briefly outline the principles of the methods most commonly employed at present. The Methods of Assay.—It may be explained that in all methods a preliminary assay is made to determine the approxi- mate strength of the preparation. i. The Toronto Method.—Each of three properly selected and pre- pared rabbits is injected subcutaneously with i c.c. of a solution, con- taining, as nearly as possible, 2*5 units of insulin (just short of a convulsive dose), three other rabbits with o-8 c.c. of the same solution (equalling 2 units), and a third group of three with o-6 c.c. (equalling i*5 units). The concentrations of insulin being the same in all cases, 1 This international standard will be kept under the auspices of the Health Committee of the League of Nations, and from time to time testing laboratories will have the opportunity of having their own standards compared with it,