Effect of receptor blockers (methysergide, propranolol, phentolamine, yohimbine and prazosin) on desimipramine-induced pituitary hormone stimulation in humans--I. Growth hormone.

In this report the effects of various receptor blockers on desimipramine (DMI)-induced growth hormone (GH) secretion in healthy male subjects are presented. Each trial consisted of two administrations: one of DMI i.v. alone and one of DMI i.v. in combination with the respective receptor blocker: methysergide (serotonin (5-HT) receptor blocker), propranolol (beta receptor blocker), phentolamine (alpha-1/alpha-2 receptor blocker), yohimbine (alpha-2 greater than alpha-1 receptor blocker), and prazosin (alpha-1 receptor blocker). DMI-induced GH stimulation was not significantly different after DMI i.v. alone (n = 12) than after three days' pretreatment with 12 mg methysergide p.o. in another group of subjects (n = 12). Following combined administration of DMI and propranolol (15 mg i.v.), GH secretion was significantly increased by 25 mg DMI (p less than 0.05) and 50 mg DMI (incomplete block design, n = 18). GH secretion was significantly lower (p less than 0.01) after DMI in combination with 60 mg phentolamine i.v. compared to that after administration of DMI alone in the same group (n = 12). Following 10 mg yohimbine i.v. in combination with DMI (n = 6), the DMI-induced GH increase was also significantly less (p less than 0.05) than that after DMI alone. The DMI-induced GH increase following DMI plus 1 mg prazosin p.o. (n = 12) was comparable to that after DMI alone. The results indicate that the GH-stimulating effect of DMI is primarily related to the ability of DMI to inhibit noradrenaline (NA) reuptake. Should serotonergic receptors be involved in the DMI-induced GH secretion at all, they transmit a positive stimulus. The alpha-1 receptors are most likely not (or not essentially) involved, whereas alpha-2 receptors affect the DMI-induced secretion positively, and beta receptors have an inhibitory effect.