Discordance between growth hormone (GH) responses after GH-releasing hormone and insulin hypoglycemia in myotonic dystrophy.

Plasma GH responses to human GHRH, arginine, L-dopa, and insulin-induced hypoglycemia were determined in seven myotonic dystrophy (MD) patients. An iv bolus injection of GHRH-(1-44)-NH2 (1 microgram/kg BW) only slightly increased plasma GH concentrations in MD patients. The mean peak plasma GH level after GHRH injection [4.2 +/- 0.8 (+/- SE) micrograms/L] was significantly lower than that in 10 age-matched normal subjects (26.7 +/- 4.3 micrograms/L) or that in 6 patients with progressive muscular dystrophy (22.8 +/- 6.6 micrograms/L) whose nutritional status was similar to that of the MD patients. Even with a larger dose of GHRH (3 micrograms/kg BW), the plasma GH rises were minimal in the MD patients (mean peak, 5.9 +/- 1.8 micrograms/L). The plasma GH responses to a 30-min iv infusion of arginine (0.5 g/kg BW) and oral ingestion of L-dopa (0.5 g) were attenuated to a similar extent, whereas insulin-induced hypoglycemia caused a significant increase in plasma GH in all seven MD patients [mean peak, 17.4 +/- 4.1 (+/- SE) microgram/L]. The plasma TSH responses to TRH and plasma insulin-like growth factor I levels were similar in the MD patients and normal subjects. These findings suggest that 1) the impaired GH release after GHRH, arginine, and L-dopa administration in MD patients is not due to somatotroph deficiency, since the GH response to hypoglycemia is well preserved; and 2) insulin-induced hypoglycemia may stimulate GH release at least in part via inhibition of somatostatin release.