Dose-response studies on male reproductive parameters in dogs with nafarelin acetate, a potent LHRH agonist.

Adult male beagle dogs were administered daily subcutaneous injections of either 0.5 or 2.0 micrograms/kg of a potent LHRH agonist, nafarelin acetate, for 44 days. Although there was a rise in the circulating levels of the gonadotropins and of testosterone following the early injections of agonist, continued treatment caused a marked decline in acute response and basal levels of both LH and testosterone and smaller decreases in the acute FSH response. The decline in LH and testosterone was accompanied by decreases in testicular volume, ejaculated sperm count, sperm motility, ejaculate volume, and duration of ejaculation. The decline in these parameters was more rapid at 2.0 micrograms/kg than at 0.5 micrograms/kg. The profile of responses to 2.0 micrograms/kg could be superimposed on that previously shown for the injection of 10.0 micrograms/kg. At the end of treatment, prostate weights were 36% and 68% of vehicle-treated controls for high- and low-dose animals, respectively. Spermatogenesis was absent in the testes of all agonist-treated animals. Over the dose range tested, the dose-response on all parameters was characterized by a slower evolution to the same maximal effect, rather than by a partial effect. If these data can be extrapolated to man, they would suggest that administration of higher dose levels of LHRH agonists than presently reported should be explored.

Effects of an LHRH agonist analog upon sexual function in male dogs. Suppression, reversibility, and effect of testosterone replacement.

Male beagle dogs were injected once daily with 10 micrograms/kg of [6-D-(2-naphthyl)alanine]-LHRH (D-Nal(2)6-LHRH), a potent LHRH agonist, for periods up to 42 days, with recovery periods up to 172 days. Blood samples collected at regular intervals were assayed for LH, FSH, and testosterone; total ejaculates were collected and analyzed weekly, and animals were sacrificed at various intervals for sex organ weights and histology. The first injection of D-Nal(2)6-LHRH caused an acute elevation in plasma levels of LH, FSH, and testosterone, measured at 2 and 4 hours after the injection. This acute response to injection was attenuated with each successive injection and by two weeks no elevation was seen, suggesting a down-regulation of pituitary response. Basal levels of LH and testosterone were maximally depressed by four days of treatment. Testis volume, duration of erection, ejaculate volume, sperm count, sperm motility and testis volume all declined during treatment, with sperm count significantly lowered by two weeks and ejaculation volume becoming zero by five weeks of treatment. Spermatogenesis, assessed histologically, was partially suppressed at ten days and completely suppressed by 38 days of treatment. All parameters returned to normal following cessation of treatment. Recovery time was longer for the dogs treated for 42 days than for those treated for ten days. When testosterone was supplemented during 42 days of agonist treatment, basal plasma testosterone levels were maintained at the low end of the normal range. Testosterone supplementation did not prevent pituitary down-regulation, suppression of spermatogenesis, or the decrease in testis and epididymis weights, but prevented the decline in duration of erection. Ejaculate volume and sperm count declined more slowly with combination treatment than with agonist alone. During the decline in sperm count sperm motility was maintained with combination treatment. Injection of hCG into control and agonist treated dogs resulted in similar percentage increases in plasma levels of testosterone, although peak levels were greater in control than in treated animals. The data suggest a pituitary desensitization with this LHRH agonist in the dog but only a minor role for testicular desensitization.

Inability of continuous long-term administration of D-Nal(2)6-LHRH to abolish fertility in male rats.

A highly potent agonist of LHRH, [6-D-(2-naphthyl)-alanine]-LHRH, was administered chronically for 12 weeks to adult male rats by repetitive implantation of pellets, and its effects upon mating, fertility, and reproductive organ weights have been evaluated. Although significant declines in testicular (P less than 0.001) and epididymidal (P less than 0.001) weights were achieved, no effects on seminal vesicles, prostate, or pituitary weights were observed. After 12 weeks of continuous treatment, three of six agonist-treated rats were still successfully impregnating females. The decline in successful impregnation appeared to be related to the observed reduction in testicular spermatogenesis and in numbers of epididymal spermatozoa. The drug effects appeared reversible, as all six of the agonist-treated rats were fertile by the fifth week after cessation of treatment. Plasma levels of testosterone were markedly elevated immediately after implantation of each pellet and consistently, but not significantly, lowered during the inter-implantation periods. These observations, and the lack of effect on accessory organ weights, are consistent with the maintenance of libido in these treated rats. This is the second demonstration of a selective inhibition of spermatogenesis in the absence of a marked decline in gonadal steroidogenesis with this agent. As in the first demonstration using twice weekly injections, the degree of inhibition of spermatogenesis was insufficient to abolish fertility in the treated male rats.

Opposite and mutually antagonistic effects on uterine contractility of two epimeric forms of a synthetic prostaglandin analog.

Comparisons were made of the ability of intravenous infusions of prostalene and PGF2 alpha to stimulate increases in uterine contractility in vivo in the 14 day pregnant hamster. Whereas PGF2 alpha gave a linear dose response over the range 0.25 to 6.25 microgram/min., to achieve 5 fold increase in uterine activity, prostalene induced a multiphasic dose response curve. Inhibition was observed at 0.01 microgram per min. followed by a shallow stimulatory dose response to 0.25 microgram/min. plateauing at 1.8 fold stimulation to at least 1.25 microgram per min. When the epimeric mixture was separated and the two 15 position epimers studied separately the reason for this atypical dose response curve became clear. The 15 alpha OH epimer induced a linear stimulatory dose response from 0.05 to 6.25 microgram per min. infusion rates to a 5 fold increase in uterine contractility. The 15 beta OH epimer, however, induced a linear inhibitory dose response over the range 0.4 to 50 ng/min. It was, therefore, apparent that the atypical dose response curve observed from the mixture was the resultant of two opposite and mutually antagonistic activities. The data suggests also that the inhibitory epimer is capable of antagonizing both endogenous and exogenous prostaglandin, probably by a mechanism of competitive inhibition.