Mechanisms of impaired growth hormone secretion in genetically obese Zucker rats: roles of growth hormone-releasing factor and somatostatin.

GH secretion is markedly blunted in obesity; however, the mechanism(s) mediating this response remains to be elucidated. In the present study we examined the involvement of the two hypothalamic GH-regulatory hormones, GH-releasing factor (GRF) and somatostatin (SRIF), using the genetically obese male Zucker rat. Spontaneous GH, insulin, and glucose secretory profiles obtained from free moving, chronically cannulated rats revealed a marked suppression in amplitude and duration of GH pulses in obese Zucker rats compared to their lean littermates (mean 6-h plasma GH level, 3.9 +/- 0.4 vs. 21.5 +/- 3.8 ng/ml; P less than 0.001). Obese rats also exhibited significant hyperinsulinemia in the presence of normoglycemia. The plasma GH response to an iv bolus of 1 microgram rat GRF-(1-29)NH2, administered during peak and trough periods of the GH rhythm, was significantly attenuated in obese rats at peak (137.4 +/- 26.1 vs. 266.9 +/- 40.7 ng/ml; P less than 0.02), although not at trough, times. Passive immunization of obese rats with a specific antiserum to SRIF failed to restore the amplitude of GH pulses to normal values; the mean 6-h plasma GH level of obese rats given SRIF antiserum was not significantly different from that of obese rats administered normal sheep serum. Both pituitary wet weight and pituitary GH content and concentration were reduced in the obese group. Measurement of hypothalamic GRF immunoreactivity revealed a significant (P less than 0.05) reduction in the mediobasal hypothalamic GRF content in obese rats (503.2 +/- 60.1 pg/fragment) compared to that in lean controls (678.1 +/- 50.2 pg/fragment), although no significant difference was observed in hypothalamic SRIF concentration. Peripheral SRIF immunoreactive levels were significantly (P less than 0.01) elevated in both the pancreas and stomach of obese rats. These results demonstrate that the genetically obese Zucker rat exhibits 1) marked impairment in both spontaneous and GRF-induced GH release, which cannot be reversed by SRIF immunoneutralization, 2) significant reduction in pituitary GH concentration, 3) depressed hypothalamic GRF content, and 4) elevated gastric and pancreatic, but not hypothalamic, SRIF levels. The findings suggest that the defect in pituitary GH secretion observed in the genetically obese Zucker rat is due, at least partially, to insufficient stimulation by hypothalamic GRF, and that SRIF does not play a significant role.