Pituitary response to growth hormone-releasing factor in diabetes. Failure of glucose-mediated suppression

To evaluate the mechanism underlying raised growth hormone levels in diabetes, we compared the response to growth hormone-releasing factor (GRF) in type I diabetic and healthy control subjects. In 12 poorly controlled diabetic subjects (fasting plasma glucose 276 +/- 27 mg/dl) basal serum growth hormone levels were elevated by 200-300% (P less than 0.02), yet the incremental increase in growth hormone after GRF injection was no greater than in control subjects. Furthermore, five additional diabetic subjects with normal growth hormone levels after long-term insulin pump treatment also showed an identical response to GRF. Thus, raised basal growth hormone levels in diabetes and the fall that follows intensive insulin treatment may reflect changes in hypothalamic regulation of, rather than in pituitary responsiveness to, GRF. However, when five normal subjects were restudied during glucose infusion, even quite modest hyperglycemia (plasma glucose approximately 150 mg/dl) caused marked suppression of the response to GRF (P less than 0.005). Thus, the "normal" response to GRF in poorly controlled diabetes is actually inappropriate. Failure of the pituitary to suppress in response to hyperglycemia in diabetes implies a second abnormality that may further aggravate disordered growth hormone secretion.