Growth hormone secretion in the guinea-pig

The guinea-pig is unusual in that it continues to grow at a normal rate after hypophysectomy. Although its pituitary gland appears to contain a GH, this has not been isolated or characterized, and nothing is known about its secretion or physiological control. We have identified guinea-pig GH, established a sensitive heterologous radioimmunoassay and adapted our automatic blood microsampling method to study spontaneous GH secretion in this species. In male guinea-pigs, GH is released in an episodic pattern, reminiscent of the rat. Large multicomponent pulses of GH secretion occur every 3-4 h between periods of low or undetectable GH release, whereas most females showed a more uniform pulsatile pattern with pulses every 1-2 h. GH was released in response to GH-releasing factor (GRF) injections (2, 10 or 20 micrograms [Nle27]-GRF(1-29)NH2) in a dose-dependent fashion, and i.v. infusion of somatostatin (50 micrograms/h) blocked spontaneous GH pulses, eliciting a rebound release (from 2.0 +/- 0.8 (S.E.M.) to 36 +/- 17 micrograms/l 30 min after stopping the infusion). Infusions of a GH-releasing hexapeptide (100 or 400 micrograms/h for 4 h) also released GH. These results provide the first description of the pattern of GH release in the guinea-pig, and suggest that the striking episodic pattern is controlled by the same hypothalamic peptides that regulate GH in other species. Since the guinea-pig grows well in the absence of GH, this species may use GH for its metabolic, rather than growth-promoting actions. The guinea-pig may well prove a useful model, now that methods are available for studying its endogenous GH secretion.