The effects of insulin- and proinsulin-induced hypoglycemia on pituitary hormone and catecholamine secretion were compared in normal men to search for possible hypothalamic or pituitary inhibitory effects of proinsulin on glucocounterregulatory responses. When subjects received 0.1 U/kg i. v. human insulin and 25–38 μg/kg i.v. human proinsulin on separate occasions, plasma glucose decreased more rapidly after insulin, and the nadir was slightly lower, but integrated hypoglycemic responses were similar. Cortisol, growth hormone (GH), prolactin, epinephrine, and norepinephrine responses occurred more rapidly after insulin than after proinsulin. Peak and integrated cortisol, GH, and catecholamine responses to insulin and proinsulin were similar, but those of prolactin were reduced after proinsulin when compared with insulin by 42% (P < .01) and 34% (P < .05), respectively. When euglycemia was maintained by a variable glucose infusion rate after the injection of insulin and proinsulin, no differences were observed in plasma levels of any of the hormones. The intravenous injection of a dose of proinsulin (6 μg/kg), which did not produce hypoglycemia but was the molar equivalent of insulin used in the first protocol, failed to modify the GH or prolactin responses to a combined injection of GH-releasing hormone (1 μg/kg) and thyrotropin-releasing hormone (500 (xg). Our results indicate that the onset of pituitary hormone and catecholamine responses to hypoglycemia are related to the rate of plasma glucose decline, with the slower responses to proinsulin reflecting a more gradual onset of hypoglycemia. The magnitude of the cortisol, GH, and catecholamine responses, however, was comparable with proinsulin- and insulin-induced hypoglycemia. In contrast, the reduced peak and integrated prolactin responses to proinsulin imply that proinsulin-induced hypoglycemia is a submaximal stimulus for prolactin secretion or that proinsulin inhibits prolactin secretion at the hypothalamic level.

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