A model for the effect of daily injections of gonadal hormones on LH levels in recently-castrated adult rats and its comparison with experiment

Abstract

A model is presented to correlate data on LH levels obtained in recently-castrated rats which receive daily injections of a single gonadal hormone. This experimental animal can permit a controlled evaluation of the feedback effects of individual gonadal hormones on LH behavior. First, the data on decrease in both pituitary and serum LH, ΔLH, with increased hormonal injection rate, I H , is examined and postulated to be approximated by a linear variation of ΔLH with I H , rather than a threshold effect. The slope ΔLH/I H for each experiment is then determined from the data. The feedback effects on LH production and release rates are taken to be linearly proportional to the hormonal injection rate, with constants of proportionality λ and μ , respectively. These constants of proportionality, or feedback coefficients, are then determined by comparing model results with experimental values of the slopes ΔLH/I H for various lengths of time of injections. A first estimate for μ seems to be zero, implying that the direct effect of gonadal hormones on LH release is not crucial in modelling daily-injection experiments. However, these hormones do influence LH release through the production-rate feedback constant, λ . In the model, this occurs because release is taken to be proportional to pituitary LH levels and these levels are determined by the production rate. The feedback constants for the negative effect of gonadal hormones on LH production rate are found to be λ TP = 9·0 ± 0·3 × 10 −4 μg(LH)/mg (pit.)/day 2 / μg(TP)/100 g (body wt.)/day λ Eb = 0·4 ± 0·15 μg(LH)/mg (pit.)/day 2 / μg(Eb)/100 g (body wt.)/day