Endocrine effects of a new histamine H2-receptor antagonist, nizatidine (LY139037), in the male rat.

A new orally active histamine H2-receptor antagonist, nizatidine (LY139037), was evaluated in male rats for effects on mechanisms regulating accessory sex organ growth and function. Cimetidine antagonized androgen binding to cytosolic receptors in vitro while nizatidine had no effect. Nizatidine and cimetidine were administered at the ED50, 5 X ED50, or 10 X ED50 doses for inhibition of gastric acid secretion previously determined using in vivo dog and rat models. The relative potencies of both agents to antagonize histamine H2-receptor-mediated gastric acid secretory responses have been confirmed in human clinical trials. Neither nizatidine nor cimetidine antagonized the in vivo uptake or nuclear translocation of radiolabeled androgen into the hypothalamic-preoptic-amygdala, pituitary, or ventral prostate. Nizatidine, given at doses equal to and 10 X the ED50 gastric acid secretion inhibitory values, and cimetidine (10 X ED50 value) had no effect on the response of male accessory sex organs to a submaximally stimulating dose of androgen in castrated rats. High doses of dietary nizatidine (greater than 500 mg/kg-day) administered for 6 months did not alter intact rat male accessory sex organ weights or circulating androgen levels relative to untreated controls. Acute administration of either nizatidine or cimetidine produced transient elevations in plasma prolactin (PRL) levels. Cimetidine was more potent and consistent than nizatidine in producing these increases in circulating PRL. The data described herein support the contention that unlike cimetidine, nizatidine is not a pharmacological antagonist of androgen action and has less of a stimulatory effect upon plasma prolactin. Taken together, these studies indicate that in the male rat, nizatidine exhibits a large therapeutic index between its gastric antisecretory activity and potential endocrinological effects.

A new inhibitor of norepinephrine uptake devoid of affinity for receptors in rat brain.

LY135252, (+/-)-N-methyl-gamma-(2-methylphenoxy) phenylpropylamine hydrochloride, is a competitive inhibitor of norepinephrine uptake in synaptosomes of rat hypothalamus. The resolved optical (-)-isomer, LY139603, is 2 and 9 times more effective than the racemate and the (+)-isomer, LY139602, with inhibitor constants (Ki) of 1.9, 3.4 and 16.8 nM, respectively. All three compounds are relatively weak in the inhibition of dopamine and serotonin uptake, with Ki values at least two orders of magnitude greater. The racemate and the two optical isomers in vivo are potent inhibitors of norepinephrine uptake with relative effectiveness being parallel with their K1 values. The most potent and long-acting compound was the (-)-isomer, LY139603, which inhibited norepinephrine uptake ex vivo with an ED50 value of 2.2 mg/kg i.p, and a half-life of 6.4 hr. In comparison with the tricyclic antidepressants desipramine and imipramine, LY139603 is a relatively weak ligand for alpha-1, alpha-2, and beta adrenergic receptors, acetylcholine-muscarinic receptors, histaminergic H1 receptors and the receptors of gamma-aminobutyric acid and benzodiazepines. Thus, LY139603 is a remarkably specific inhibitor of norepinephrine uptake. Its potential as an antidepressant is discussed.