Evidence for a direct action of exogenous insulin on the pancreatic islets of diabetic mice: II. Prolonged insulin therapy before islet isolation and perifusion.

24 week old diabetic mice of the C57Bl/Ks (db/db) strain were treated with subcutaneous injections of Ultratard M.C. insulin at a dose of 40 mU/animal rising to 100 mU/animal after 12 weeks. The mean plasma insulin value was 146 +/- 20 microU/ml (n = 6). During this time, the mean body weight of the animals rose significantly (p less than 0.05) and the mean blood glucose concentration fell significantly (p less than 0.005) until a value 2-3 times normal was reached, the insulin dose was then adjusted empirically to maintain hyperglycaemia at this level. Pancreatic islets from these animals, untreated diabetic mice and normal (+/+) mice were isolated by collagenase digest and perifused with a modified Krebs-Ringer medium containing either 3 mmol/L or 15 mmol/L glucose. During glucose challenge, normal islets demonstrated the usual biphasic insulin response, but high glucose levels elicited no response from islets of untreated diabetic mice. Previous treatment of diabetic mice with insulin led to a restoration of the first phase of glucose mediated insulin secretion on perifusion and a significant increase (p less than 0.05) in the second phase. Such an improvement in beta-cell function in the face of prevailing hyperglycaemia, even allowing for the possible mediating effect of a 30% fall in blood glucose, was taken as further evidence for a direct action of exogenous insulin on the islets of diabetic mice.