EFFECT OF UNDERNUTRITION ON PULSATILE LUTEINIZING-HORMONE (LH) SECRETION IN CASTRATE AND INTACT MALE-RATS USING AN ULTRASENSITIVE IMMUNOFLUOROMETRIC LH ASSAY

The recent development of an ultrasensitive immunofluorometric rat LH assay makes possible evaluation of pulsatile LH secretion in intact ma le rats under physiological conditions of minimal volume blood samplin g without requiring orchidectomy. Specifically, we applied this assay to determine the effect of macronutrient restriction on pulsatile LH s ecretion in the presence or absence of testes. In testes-intact rats, halving of food intake for 7 days while maintaining micronutrient supp ly caused a reduction of mean, maximal, and basal LH levels and LH pul se amplitude (all P < 0.05) compared with those in ad libitum fed cont rols. The loss of body weight was positively correlated with decreases in mean LH level, pulse amplitude, and area under the curve (all P < 0.009). In contrast, the same food restriction in castrated rats cause d an increase in pulse length and area under the curve and a decrease in pulse frequency, but did not change mean, maximal, and basal LH lev els or LH pulse amplitude compared to castrated ad libitum fed control s (all P < 0.02). The observed positive correlations between body weig ht and the LH secretion parameters in intact rats were absent or rever sed in castrated rats. This study demonstrates qualitatively different effects of macronutrient restriction on pulsatile LH secretion in cas trated and intact rats, indicating that it is not necessary valid to e xtrapolate consequences of undernutrition on LH secretion from castrat e to intact male rats. We conclude that undernutrition-induced inhibit ion of LH secretion involves both an indirect suppression of LH secret ion via amplification of endogenous testicular negative feedback as we ll as more direct suppression of GnRH release.