Dose and Time Relationships of Intravenously Injected Rat Recombinant Luteinizing Hormone and Testicular Testosterone Secretion in the Male Rat¹

^c Department of Physiology, University of Turku, 20520 Turku, Finland ^d Division of Biology of Growth and Reproduction, Department of Pediatrics, University of Geneva Medical School, 1211 Geneva 14, Switzerland

The ability of hCG and LH to induce testosterone (T) secretion by Leydig cells is well documented. However, the influence of the pulsatile nature of LH secretion, with varying frequency and amplitude, on T production in vivo is less clear. In our earlier studies on the relationship between pulsatile LH release and T secretion in adult male rats, no simple causality was observed. The recent availability of rat recombinant (rec) LH prompted us to study the effects of one and of three i.v. pulses of different doses of rat recLH on T secretion in adult male rats rendered hypogonadotropic by treatment with the GnRH antagonist cetrorelix. One or three supraphysiological pulses of 1.0 µg of rat recLH produced similar maximal T responses. In contrast, high physiological LH pulses (0.1 µg) produced discrete T-response peaks, whereas multiple low pulses of LH (0.03 µg) were needed before a T response was achieved. The T stimulation was greatly diminished after an LH pulse of 0.1 µg if rats had been treated on the previous day with pulses of 0.03 vs. 0.1 µg rat recLH, apparently because of prolonged LH deprivation and lack of Leydig cell priming due to the GnRH antagonist treatment. The novel preparation of rat recLH provides a physiologically relevant tool for studying the complex relationship between pulsatile LH release and T secretion in male rats.

¹ This work was supported by grants from the Sigrid Jusélius Foundation and from the Swiss National Science Foundation No 31-39729-93.

² Correspondence: Ilpo Huhtaniemi, Department of Physiology, University of Turku, Kiinamyllynkatu 10, FIN-20520 Turku, Finland. FAX: 358-2-2502610; ilpo.huhtaniemi{at}utu.fi

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