The effect of the intravenous infusion of insulin plus glucose on plasma glucagon levels was studied in hyperglycemic fasting adult type and juvenile type diabetics and compared with fasting nondiabetics. Adult type diabetics were given insulin for 2 h at a rate of 0.03 U/kgxmin, raising their mean insulin to between 25 and 35 μU/ml; glucagon declined from a base line value of 71 ± 2 (SEM) to 56 ± 1 pg/ml at 120 min (P<0.001). In juvenile type diabetics given the same insulin glucose infusion, glucagon declined from a base line level of 74 ± 8 to 55 ± 5 pg/ml at 120 min (P<0.05). The absolute glucagon values in the diabetic groups did not differ significantly at any point from the mean glucagon levels in nondiabetics given insulin at the same rate plus enough glucose to maintain normoglycemia. When glucagon was expressed as percent of baseline, however, the normoglycemic nondiabetics exhibited significantly lower values than adult type diabetics at 90 and 120 min and juvenile type diabetics at 60 min. In nondiabetics given insulin plus glucose at a rate that caused hyperglycemia averaging between 134 and 160 mg/dl, glucagon fell to 41± 7 pg/ml at 120 min, significantly below the adult diabetics at 90 and 120 min (P<0.01 and <0.05) and the juvenile group at 60 min (P<0.01). The mean minimal level of 39 ± 2 pg/ml was significantly below the adult (P<0.001) and juvenile groups (P<0.05). When insulin was infused in the diabetic groups at a rate of 0.4 U/kgxmin together with glucose, raising mean plasma insulin to between 300 and 600 μU/ml, differences from the hyperglycemic nondiabetics were no longer statistically significant. It is concluded that, contrary to the previously reported lack of insulin effect in diabetics during carbohydrate meals, intravenous administration for 2 h of physiologic amounts of insulin plus glucose is accompanied in unfed diabetics by a substantial decline in plasma glucagon. These levels are significantly above hyperglycemic nondiabetics at certain points but differ from normoglycemic nondiabetics only when expressed as percent of the baseline. At a supraphysiologic rate of insulin infusion in diabetics, these differences disappear.
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