Differential Storage and Release of Luteinizing Hormone and Follicle-Releasing Hormone from Individual Gonadotropes Separated by Centrifugal Elutriation*

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Abstract
Two subtypes of pituitary gonadotropes are known to exist: monohormonal and multihormonal. When separated by centrifugal elutriation monohormonal cells are found in the small fractions whereas multihormonal cells predominate in the large gonadotrope-enriched fractions. Since GnRH is known to shift the proportion of subtypes to cells that are mainly multihormonal, we tested its effect on the small monohormonal gonadotropes. Dispersed cells from cycling female rats were plated on glass coverslips overnight. Storage was evaluated by application of dual immunocytochemical stains with anti-LH.beta. and anti-FSH.beta. to cells stimulated with 0.5 nM [D-Lys6]GnRH. Secretion was detected by the reverse hemolytic plaque assay for LH and a newly developed RHPA for FSH. After 4 h of GnRH stimulation, the percentage of total gonadotropes was increased only in fraction (Fr.) 3 [13.9 .+-. 0.4% (.+-. SE) to 25.2 .+-. 2%]. However, cells in Fr. 2 responded with an increase in multihormonal cells from 18.2 .+-. 7% to 46.2 .+-. 5%, a proportion not different from that in gonadotrope-enriched fractions. GnRH had less striking effects on changing the size or number of LH and FSH plaques. It seems, therefore, that these small gonadotropes are multipotential and respond initially to stimulation by synthesizing and storing the other gonadotropin. In contrast, the large gonadotropes (Frs. 6 and 7) showed no significant changes in storage patterns. Gonadotropes in both fractions were predominantly multihormonal. The LH and FSH reverse hemolytic plaques run with cells from Fr. 7 showed striking increases in both the number of plaque-forming cells and the area of the plaques in response to GnRH. However, in the multihormonal cells in Fr. 6, which were 53.5 .+-. 9% of the total cell population, a heterogeneous secretory pattern was observed. In the basal state, there were 36.6 .+-. 9% LH and only 2.7 .+-. 2% FSH plaques, whereas after GnRH stimulation there were 43.1 .+-. 10% LH and 10.5 .+-. 4% FSH plaques. This finding is the first direct evidence for nonparallel release of gonadotropins from cells known to store both hormones. These studies show that gonadotrope subpopulations exhibit differences in their storage and secretory responses to GnRH. It is suggested that other factors may be required for the release of FSH from unresponsive gonadotropes in Fr. 6.