Response of plasma insulin to small doses of tolbutamide in obesity and acromegaly.

The early response of plasma insulin (IRI) to successive intravenous doses of 50, 100 and 200 mg of tolbutamide was studied in nondiabetic obese subjects, in acromegalic patients, and in healthy controls. The smallest dose raised the plasma IRI level within two minutes in all subjects. The insulin response was correlated with the basal IRI in controls and in obese subjects but not in acromegalic patients. At each tolbutamide dose level the mean IRI response of obese subjects was about three times greater than in controls. In acromegalic subjects an increased response was observed only after the 200 mg dose of tolbutamide. The results suggest that in obesity the stimulus-secretion coupling of insulin output is normal but the number of secretory units is increased. In acromegaly the insulin release pattern is compatible with an increased number of secretory units which, however, are relatively insensitive to tolbutamide, probably due to an elevated serum growth hormone level.