Age-related differences in the growth hormone secretory response to hGHRH 1-44 in male rats from infancy through puberty

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Abstract
The GH secretory response to varying doses (15, 30, 60 .mu.g/kg) of sc administered hGHRH 1-44 (or normal saline) was measured in vivo in 10, 20, 30, 40, 50, 60 and 130 days old pentobarbital-anaesthetized, male rats. The 10-min GH level and .DELTA.GH were in general significantly greater in older rats (50, 60, 130 days old) than in younger rats (10; 20 days old) following all doses of hGHRH. Ten-day-old animals had no significant GH response to any dose of hGHRH tested. .DELTA.GH correlated significantly with age (r = 0.36; P < 0.0001) and SM-C level (r = 0.29; P < 0.01) but not with serum testosterone concentrations. Monolayer pituitary cell cultures were established in rats aged 10 to 130 days and were incubated with varying concentrations of hGHRH 1-44 (0.05, 0.5, 5.0, 50 nmol/l or incubation medium). Cultures from 10- and 20-day-old animals had a greater percentage increase over basal GH secretion than other groups at all concentrations of hGHRH tested (P < 0.05). Age-related differences in the GH secretory response to hGHRH are present in male rats from 10 to 130 days. The in vitro results reported here suggest that the increase in magnitude and sensitivity of the GH response to hGHRH observed in pubertal animals in vivo under pentobarbital anaesthesia is likely due to influences above the level of the somatotrope receptor.