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Glucose Response to Exogenous Insulin and Kinetics of Insulin Metabolism in Obese and Lean Heifers

Cornell University², Ithaca, NY 14853

Abstract

Changes in serum concentrations of glucose and insulin after iv injection of a low (20 mU/kg) and high (200 mU/kg) dose of bovine insulin were used to quantify insulin resistance and calculate kinetic variables of injected insulin, respectively, in four obese and four lean heifers. Serum samples from jugular venous blood were collected 60, 45, 30, 15 and 1 min before and 2.5, 5, 10, 20, 30, 40, 60, 80, 100, 120, 150, 180, 210 and 240 min after each treatment. Mean (± SE) pretreatment concentration of insulin (µU/ml) was higher (P<.01) in obese (50 ± 6.6) than lean (20 ± 1.8) heifers, even though glucose concentrations were similar in both groups (71 ± 2.9 mg/100 ml). Concentrations of insulin after each treatment were similar in both groups and returned to pretreatment values by 60 and 120 min after injection of the low and high doses, respectively. Glucose concentrations during the first 40 min after treatment with the low dose were lower (P<.05) in lean than obese heifers, but were similar in both groups during the first 40 to 60 min after the high dose of insulin. The high insulin dose decreased (P<.05) glucose concentrations below those of the low dose in each group, but the difference was greater (P<.01) in obese than lean heifers. These results indicated that obese heifers were insensitive to the glucoregulatory effects of exogenous insulin, although the maximum responses to insulin were similar. Fractional removal rates (k, min^–1) and biological half-lives (t, min) for injected insulin were affected (P<.01) only by insulin dose. For all heifers, mean (± SE) values of k decreased from .124 ± .77 to .057 ± .004 and values of t increased from 5.7 ± .4 to 12.4 ± .8 after the high compared with the low dose of insulin. Mean (± SE) secretion rates of insulin (µU·kg^–1·min^–1) were greater (P<.01) in obese (218 ± 33) than lean (96 ± 11) heifers.

¹ Address correspondence to Dr. Joseph P. McCann, Dept. of Physiol., New York State College of Vet. Med.

² Diagnostic Endocrinol. Lab., New York State College of Vet. Med.