Free fatty acid metabolism in Chinese hamsters

In normal Chinese hamsters (Cricetulus griseus) the mean concentration of free fatty acids (FFA) in serum varied from group to group, but was (i) consistently 4 to 9 times greater than in rats, dogs, or man; (ii) slightly higher than in Syrian hamsters; (iii) two- to four-fold higher than in fasting or alloxan-diabetic rats. The epididymal adipose tissue of the Chinese hamster (i) had initial concentrations of FFA comparable to those in the rat and Syrian hamster; (ii) released, in the same time interval, 8- to 10-fold more FFA in vitro than this tissue of the rat; (iii) had higher concentrations of FFA after incubation than the incubated tissue of the rat. The retroperitoneal (perirenal) adipose tissue of the Chinese hamster was less active in release of fatty acids in vitro than the epididymal, but was, however, more active than the epididymal adipose tissue of the rat. These characteristics of FFA metabolism in the Chinese hamster were apparently attributable to species, not to age, diet, or sex. In the Chinese hamster, the weight of the epididymal adipose tissue per gram of body was relatively high. It appears that in this species the rate of release of fatty acids from adipose tissue is great, leading to high FFA concentrations in serum.In Chinese hamster and rat adipose tissues in vitro, glucose and insulin (separately) reduced the rate of release of FFA and the amount of FFA in the tissues, but glucose and insulin together produced the greatest reductions. The net reduction in FFA release by glucose and insulin in vitro was greater in tissue from the Chinese hamster. Insulin markedly increased glucose uptake by the adipose tissues of both species. The possible relation of the results to spontaneous diabetes in the Chinese hamster is discussed.

Growth hormone-induced diabetes and high levels of serum insulin in dogs

Single injection of growth hormone (3 mg. per kilogram of body) in fasting dogs did not appreciably alter serum insulin and glucose concentrations within five hours. However, serum free fatty acids (FFA) decreased initially, then rose to two- and fourfold the initial at five and twenty-four hours. Following intravenous injection of glucose (1 gm. per kilogram of body) in the control period, serum insulin rose fourfold in 15 min. then regressed exponentially to the initial. Glucose/insulin remained unchanged during the two-hour test period, despite great alterations in the concentrations of glucose and insulin. Serum FFA decreased, then returned to or above the initial at one to two hours. Single injection of growth hormone did not alter these responses to the infusion of glucose.

During the daily administration of 2 mg. of growth hormone per kilogram of body per day: (1) Mean serum insulin concentrations, in the postabsorptive periods, increased to two-, six- and eightfold the control value after one, two and four days, respectively, in six dogs, and thereafter the values remained high during seven to eleven days of observation in four dogs, while two developed resistance. (2) Serum insulin increased before an appreciable rise in serum glucose occurred. (3) Due to a greater rise in insulin than in glucose, the glucose/insulin ratio decreased. (4) After two days of injection the increase in serum insulin was most pronounced in those dogs that exhibited hyperglycemia and glycosuria. (5) Serum FFA increased greatly after one day of injection and remained high. During this growth hormone treatment, in one hour after injection of growth hormone with the ingestion of a meal, serum insulin rose, with lesser rise in glucose (further decrease in glucose/insulin), and fall in FFA. In the control period, however, no appreciable changes in serum insulin or glucose occurred following a meal.

During daily growth hormone treatment, following glucose infusion: (1) Mean peak concentrations of serum insulin increased sevenfold in four days. (2) The rate of regression of serum insulin decreased. (3) Areas under the serum insulin curves increased up to tenfold the control value. (4) The mean rate of regression of serum glucose decreased, from the control value k — 4.28 per cent to —1.14 per cent after seven days. (5) The serum glucose/insulin ratio was unaltered, although the initial (fasting) values decreased. (6) The decline in serum FFA in 15 to 30 min. was more marked, but the levels remained above control.

The changes in glucose/insulin indicate that growth hormone administration altered homeostatic control of pancreatic beta cells, such that at any given concentration of glucose in blood a greater rate of insulin secretion occurred. It is concluded that the “hyperinsulinemia” induced by the daily administration of growth hormone was associated with sustained increase in insulin secretion, and with augmentation of insulin secretion in response to a meal and to the infusion of glucose.