Pulse amplitude and frequency are often used to describe measurements of LH in blood. Such analyses are compatible with models of LH being released from the pituitary in episodes that are controlled by pulses of hypothalamic gonadotrophin-releasing hormone. The amplitudes of these secretory episodes as seen in blood are usually defined as the net heights of peaks above a baseline. As a measure of each pituitary secretory episode, this is valid only if peaks are regularly and widely spaced, making overlap negligible. When episodes are erratic and frequent so that only fractions of peaks have been cleared from the circulation before others follow, nadirs between peaks include output from previous episodes and do not define a physiologically meaningful baseline. Applied to overlapping peaks, such measures of amplitude usually underestimate pituitary secretory episodes and imply a tonic mode of LH secretion in addition to pulsatile release.
Using the additional information of fitted LH clearance coefficients to define the shapes of LH peaks, a simple method based on an episodic mode of release alone is described, for estimating more accurately the relative sizes of secretory episodes as observed in blood, free of the effects of overlapping peaks. Using this analysis we have described the variation in amplitude, interval and clearance rates of LH secretory episodes within and between four normal menstrual cycles of a single individual. Thirteen, 3–6 h blood sampling sessions were performed during early follicular growth at the transition from luteal to follicular phases when the frequencies of LH peaks, LH/FSH ratios and progesterone concentrations were changing markedly.
Our secretory episode model described all data well without the need to introduce a tonic mode of release. When frequent pulses overlapped we found that amplitudes of episodes were usually higher than peaks estimated by conventional methods, but a decrease in both amplitude and pulse interval occurred after the start of menstruation. Highly variable patterns of LH release were demonstrated in the late luteal phase of this normal individual while FSH levels rose consistently.