NEUROENDOCRINE CONTROL OF FOLLICLE-STIMULATING-HORMONE (FSH) SECRETION .1. DIRECT EVIDENCE FOR SEPARATE EPISODIC AND BASAL COMPONENTS OF FSH-SECRETION

Continuous sampling of hypophyseal portal blood from unrestrained shee p is providing an unprecedented means for measuring and defining the c haracteristics of the secretory profile of GnRH. With this method, GnR H has been shown to be released in discrete pulses lasting 5-8 min, wi th the amplitude of some pulses exceeding 50-fold. Although the relati onship between these pulses and the accompanying pulses of LH measured in the jugular vein are unambiguous, the relationship of GnRH pulses to the release of FSH has not been well defined due to the longer clea rance of FSH. In previous studies we have shown that hypophyscal porta l blood, in addition to serving as a source material for hypothalamic secretions, provides a means to define secretory patterns of pituitary hormones. Because of this we hypothesized that the GnRH-FSH secretory relationships would be easier to define in hypophyseal portal than in jugular vein blood before the secretory products are subjected to dis persion and clearance in circulation. To test this possibility, we mon itored hormonal patterns in blood collected at 5-min intervals for 6-1 2 h from the peripheral and hypophyseal portal circulation of six ovar iectomized ewes from a previous study. In contrast to the nonpulsatile pattern of FSH in the peripheral blood, 93% of GnRH pulses were assoc iated with essentially coincident, discrete pulses of FSH in the porta l plasma. Of potentially even greater interest, additional episodes of FSH release were clearly discernible between the GnRH-associated puls es of FSH. As concentrations of peripheral plasma FSH did not reach th ose in hypophyseal portal plasma, the inter-GnRH episodes of FSH secre tion could not result from contaminating peripheral blood. In addition to the episodic mode of secretion, substantial amounts of FSH were fo und between FSH pulses. This basal component of FSH appeared to be the dominant mode of secretion rather than pulses. The results of this st udy not only confirm that GnRH pulses lead to pulsatile release of FSH , they also suggest that some other mechanism or factor may be control ling the non-GnRH-associated episodes as well as the basal components of FSH secretion.