Pyridostigmine-mediated growth hormone release: evidence for somatostatin involvement.

Pyridostigmine (PD), a cholinesterase inhibitor, has been shown to elicit GH release when given alone and to potentiate the GH response to GH-releasing hormone (GHRH) in man. Numerous experiments have indirectly indicated that somatostatin (SS) inhibition is its likely mechanism of action. This study sought to establish the ability of PD to induce GH release in the rat, determine the dose-response relationship, and test the hypothesis that SS inhibition is the method of action. Three experiments were performed to monitor the GH response to PD. I) Five groups of male rats were food deprived for 72 h. The groups were then treated iv with saline, SS antibody (SS-ab), and 10, 100, and 1000 micrograms/kg PD, respectively. Blood samples were drawn before and after treatment. II) Two groups of male rats were pretreated iv with GHRH antibody (GHRH-ab) and either SS-ab or normal sheep serum (NSS). Blood samples were drawn every 30 min for 8.5 h, during which time each animal was injected with PD (10 micrograms/kg) in the third hour and again in the sixth hour. III) Male rats received a PD injection (10 micrograms/kg, iv) during a spontaneous GH trough period and a second PD injection during a spontaneous GH peak period. Blood samples were drawn at regular intervals preceding and following treatments. In Exp I, PD induced a clear 4- to 5-fold increase in GH concentrations in food-deprived rats. The maximal GH responses occurred after the 10 and 100 micrograms/kg doses, although the pattern and duration were different with these two doses. In Exp II, PD induced an approximately 2-fold increase in GH values in animals pretreated with GHRH-ab and NSS, but failed to induce a change in GH in the animals treated with GHRH-ab and SS-ab. In Exp III, PD failed to induce any change in GH concentration when administered during spontaneous GH peaks or troughs. The first two experiments suggest that PD increases GH secretion in the rat via inhibition of SS. The failure of PD to alter GH during a spontaneous peak is consistent with the current hypothesis that the level of SS is low at this time. Its failure to alter GH during trough periods may be related to very high SS tone. In conclusion, our results support the hypothesis that PD acts via inhibition of SS secretion.