Plasma and tissue insulin concentrations in foetal and postnatal rabbits

Tissue and plasma insulin concentrations were measured in foetal and postnatal rabbits. Insulin was detected in 18-day foetuses and pancreatic insulin concentrations from the 22nd day of foetal life onwards were similar to one another. Insulin was detected in foetal plasma from the earliest time that foetal blood could be collected: 20 days. Foetal plasma insulin levels did not change significantly with the length of gestation but maternal levels were lower on day 30 than on day 20. There was a positive maternofoetal plasma insulin gradient on day 20 and a negative gradient on day 30.

The effect of a high-fat diet on glucose, insulin sensitivity and plasma insulin in rats

The influence of the diet on the levels of insulin was studied in rats on a high-fat diet. Plasma and glucose insulin concentrations of a control group and of rats on a high-fat diet were compared, and so was the insulin concentration in the pancreas of the two groups.

The mean plasma insulin concentration in the control group was 40 μ-u./ml. and that of insulin extracted from the pancreas was 2·5 μg./100 mg. tissue; plasma glucose was 156 mg./100 ml.

The animals fed on a high-fat diet showed diabetic features. The mean plasma insulin level was 9 μ-u./ml., and plasma glucose increased to 210 mg./100 ml. The insulin concentration in the pancreas was not significantly different from that in the controls. In vitro the epididymal fat and the diaphragm of the high-fat-diet group were less sensitive to insulin than the same tissues in the control group.

Insulin secretion in rats with elevated levels of circulating growth hormone due to MtT-W15 tumor

Circulating levels of growth hormone (GH) in Wistar-Furth rats bearing the MtT-W15 tumor are twenty to thirty times higher than normal. The effect of constantly elevated levels of GH on the pancreatic islets was studied. Tumor-bearing rats (TR) and normal controls (C) were pair-fed for seven weeks. In TR, fasting serum levels of GH and insulin were 599 ± 157 ng./ml. and.142 ± 43 μU./ml., respectively, and only 18.2 ± 1.9 ng./ml. and 26.9 ± 7.9 μU./ml., respectively, in the controls. The islets of TR were enlarged with an islet/acinar ratio of 3.38 per cent (C: 0.82 per cent). The extracted pancreatic insulin was 9.84 ± 0.9 mU./mg. (C: 5.36 ± 0.4 mU./mg.). Incubated slices or isolated islets of TR pancreas released greater amounts of insulin when submitted to glucose stimulation. Incorporation of labeled leucine into insulin was increased also in TR. An exaggerated insulin response to a glucose load was found in vivo (peak value at 30 min., TR: 183 dt 42.5 μU./ml; C: 71 ± 17.9 μU/ml.). None of the TR became diabetic, however, and two to six weeks after removal of the tumors serum GH levels had returned to normal (17.2 μg./ml.) as had insulin levels both fasting (19.0 μU/ml.) and following a glucose load (56.4 μU./mg.). Insulin output by incubated pancreas decreased 50 per cent (26 μU./mg.). The islet/acinar ratio returned to normal but the insulin content remained high. These results indicate that in animals exposed to hjgh levels of endogenous GH there is an increased responsiveness of the islets to glueose stimulation.

Potassium ions and the secretion of insulin by islets of Langerhans incubated in vitro

1. A method was devised for the isolation of islets of Langerhans from rabbit pancreas by collagenase digestion in order to study the influx and efflux of K(+) in islets during insulin secretion. 2. Glucose-induced insulin release was accompanied by an increased rate of uptake of (42)K(+) by the islets of Langerhans, though this was not the case for secretion in response to tolbutamide. Ouabain significantly inhibited the uptake of (42)K(+) by islet tissue. 3. No significant increase in the rate of efflux of (42)K(+) was demonstrated during active insulin secretion. 4. Slices of rabbit pancreas were incubated in media of different K(+) content, and rates of insulin release were determined. Alteration of the K(+) concentration of the medium between 3 and 8mm had no effect on the rate of insulin release by pancreas slices. However, decrease of the K(+) concentration to 1mm resulted in inhibition of secretion in response to both glucose and to tolbutamide. Conversely, an increase in K(+) concentration increased rates of insulin release in response to both these stimuli. 5. It is concluded that, though unphysiological concentrations of K(+) may influence the secretion of insulin, fluxes of K(+) in the islets do not appear to be important in the initiation of insulin secretion.