Objective: To determine the bioavailability and bioactivity of subcutaneously injected insulin.
Research design and methods: A randomized block design with six male mongrel dogs as subjects. In protocol 1, purified pork insulin was infused intravenously to simulate the pattern of appearance in the blood that would have been expected from subcutaneous injection. Three intravenous doses (0.05, 0.10, and 0.15 U/kg) were infused on separate days in a pattern (0-300 min) designed to approximately simulate the absorption rate of subcutaneously injected insulin. In protocol 2, interscapular subcutaneous injections of pork insulin at 0.10 U/kg were made.
Results: Integrated insulin, decrement in plasma glucose, and maximal glucose clearance for subcutaneous injection experiments were similar to intravenous infusion of equal dose (P greater than 0.10) but significantly different from low-dose infusions (P less than 0.025). Similar results were observed for hepatic glucose output and glucose uptake. Hypoglycemia elicited counterregulatory responses that appeared to be under a threshold differentiated at a plasma glucose of approximately 3 mM. Integrated insulin was plotted against insulin dose to create dose-response curves for intravenous data. The curve was then used to predict the actual appearance rate of insulin in plasma for subcutaneous injection. The estimated bioavailability of subcutaneous insulin was 103.0 +/- 10.5% of the injected dose.
Conclusions: We concluded that, in dogs, insulin delivered subcutaneously in the interscapular area is not significantly degraded before absorption, resulting in metabolic effects equal to intravenous insulin infusion of equivalent dose.