Male rats secrete luteinizing hormone and testosterone episodically.

We studied the temporal aspects of endocrine signaling between the pituitary gland and testes by measuring moment to moment changes in blood LH and testosterone levels in individual male rats. Each rat was fitted with an indwelling vascular cannula, and blood was withdrawn every 5 min for 8-12 h. Rats were maintained throughout the intensive blood-sampling period with an isotonic blood replacement mixture containing rat red blood cells and a human plasma protein preparation. LH and testosterone measurements were made in plasma volumes of 50 and 60 microliters. Most rats released LH in well defined pulses, characterized by a rapid increase in plasma LH within 5-10 min and a gradual decline lasting for the next 50-70 min. LH pulses occurred singly or in trains of two to four. Episodes of testosterone secretion spanned 3-6 h and were marked by a slowly graded rise and fall of plasma testosterone. In several instances, testosterone episodes were preceded (1-2 h) by a train of closely coupled LH pulses. Within a particular animal on different days, hormone episodes varied in number, amplitude, and timing. A particular hormone profile did not serve as a reliable hormone signature for an individual rat. Many rats displayed a characteristic sequence of 1) multiple LH pulses, 2) a sustained testosterone episode, and 3) a period of no LH pulses. This tripartite sequence of events is viewed as the essence of pituitary-testicular stimulation, and testicular negative feedback. Intermittent, short term fluctuations in peripheral levels of LH and testosterone represent the blood-borne, gland to gland signals controlling hypothalamic-pituitary-testicular function in the normal rat.