Hypophysectomy does not alter the effects of somatostatin or growth hormone on canine splanchnic biogenic amine release

We have demonstrated previously that growth hormone (GH) and somatostatin (somatotropin release inhibitory factor, SRIF) exert comparable effects on the release of splanchnic biogenic amines. The purpose of the present investigation was to study further the response of the two hormones and see whether the similarity persists in dogs completely deprived of endogenous GH. Experiments were conducted in seven hypophysectomized dogs fitted with an indwelling portal catheter. Two to 4 weeks after surgery the responsiveness of their catecholaminergic neurons was tested by an injection of human β-endorphin (20 μg/kg); it caused a rise in portal catecholamine levels equivalent to that seen in intact dogs. Then the effect of a spike concentration of SRIF or GH on hepatic portal and peripheral levels of free serotonin and catecholamines was studied, all by radioenzymatic methods. The intravenous injection of ovine GH (100 μg/kg) or equimolar amounts of SRIF (7.5 μg/kg) produced in the hepatic portal circulation a transient but statistically significant rise of serotonin and a concomitant reduction in the concentration of dopamine, norepinephrine, and epinephrine. No changes were found in the peripheral circulation. The response patterns to SRIF or GH were virtually identical, which is in keeping with our other data, suggesting that the effect of GH on splanchnic biogenic amine secretion is SRIF-dependent and mediated by SRIF-containing neurons. The results were nearly identical with those obtained by us previously in normal intact dogs, indicating that (1) the action of SRIF does not depend on the presence of pituitary hormones, including GH and (2) in terms of splanchnic biogenic amines, hypophysectomized dogs do not display enhanced sensitivity to administration of GH. We postulate that the SRIF-mediated GH effect remained unaltered because the enteric neural plexuses, specifically the intrinsic serotonergic and SRIF-containing interneurons, and the extrinsic adrenergic neurons, retained their functional integrity after hypophysectomy.