Effects of continuous and repetitive administration of a potent analog of GH-RH(1-30)NH2 on the GH release in rats.

We examined the desensitization and/or sensitization phenomenon in the pituitary GH responsiveness induced by continuous infusion and multiple pulses at different frequencies of a potent GH-RH analog [D-Ala2, Leu15, Nle27, GABA30-GH-RH(1-30)amide]. Further, we investigated the correlation between doses and GH responses, as well as between pulse frequency and GH responses in male rats in vivo and in vitro. Long-term, continuous administration was attained by osmotic minipumps releasing low and high doses of the analog for 14 days. The effects of repetitive administration of the GH-RH analog on the pituitary GH release was investigated by injecting 4-6 pulses of the analog at different doses and pulse frequencies. The in vitro experiments were performed in the superfused rat anterior pituitary cell system. Pituitary cells were challenged with continuous, repetitive and simultaneous continuous and repetitive perfusion of the analog. Continuous infusion with low doses of the GH-RH analog in vivo induced sensitization of the pituitary GH-secretory responsiveness and resulted in moderately increased GH releases (129% of the control) to additional bolus injections of the same analog, whereas continuous stimulation of the pituitary with high doses of the GH-RH analog evoked desensitization and resulted in blunted GH responses (29% of the control). Despite the desensitization of the pituitary GH-secretory responsiveness, high doses of the analog elevated the serum GH concentration to 310% and induced acceleration of body weight gain (160% of the control). Repetitive pulsatile administration of the GH-RH analog evoked both sensitization and desensitization of the pituitary GH-secretory responsiveness, depending on the dose and pulse frequency administered.(ABSTRACT TRUNCATED AT 250 WORDS)