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The acute and late phases of insulin secretion were studied in mongrel dogs before and after the induction of mild alloxan diabetes. Fasting glucose and insulin levels were unchanged from pretreatment values. The alloxan-diabetic dogs had significantly decreased early-phase insulin responses to glucose pulses (0.5 gm./kg.) and slower plasma glucose disappearance rates. In contrast, these mildly diabetic dogs achieved comparable insulin levels and higher glucose levels during a four-hour 40 mg./min. glucose infusion than pre-alloxan control values. Similar findings in human congenital mild diabetes have been interpreted as beta cell insensitivity or impedance to efficiency of plasma glucose uptake. The present observations in alloxan-induced mild diabetes in dogs suggest that reduced early-phase secretion and intact later phase of insulin secretion are not dependent on genetic determinants and may be induced in a model of acquired diabetes.