HETEROGENEOUS RESPONSE OF PORCINE GONADOTROPE SUBPOPULATIONS TO GONADOTROPIN-RELEASING-HORMONE (GNRH) DURING POSTNATAL-DEVELOPMENT

In previous papers, we showed the porcine gonadotrope population to be composed of three GtH subpopulations that can be separated by density using a continous Percoll density gradient. We also demonstrated that these subpopulations exhibited different hormonal storage patterns an d morphological features during porcine postnatal development at three representative ages: neonates (30 days), prepubers (5-6 months) and m atures (16-18 months). In this work, we investigated whether these mor phologically heterogeneous subpopulations are also functionally differ ent. Thus, the effect of the hypothalamic gonadotropic hormone-releasi ng factor (GnRH) on these subpopulations was assessed in order to asce rtain whether a mutual relationship between the reported morphological features, hormonal storage patterns and physiological response to the stimulation can be established. For this purpose, gonadotropin secret ion was measured by cell immunoblot assay and hormonal content by scan ning cytophotometry. Low-density gonadotropes (1.049 g/cm(3)), present in the three age groups studied, were mainly composed of bihormonal L H/FSH cells in neonates and monohormonal LH cells in prepubers and mat ures. GnRH stimulation was found to increase both LH and FSH secretion , as well as the intracellular content. These results indicate that Gn RH can stimulate both the synthesis and release of both gonadotropins in this subpopulation. Middle-density gonadotropes (1.062 g/cm(3)), pr esent in prepubers and matures only, were composed of bihormonal cells . GnRH stimulated the secretion of LH and FSH in prepubers and matures , but decreased hormonal contents except that of LH in prepubers. Howe ver, GnRH stimulation increased the proportion of immunoreactive gonad otropes (particularly monohormonal cells). Finally, high-density cells (1.087 g/cm(3)), present in neonates and prepubers only, were mostly composed of bihormonal LH/FSH gonadotropes, and exhibited low (neonate s) or no response (prepubers) in terms of LH release and content when treated with GnRH. In conclusion, these results indicate that porcine gonadotrope subpopulations are morphologically and physiologically het erogeneous. The heterogeneity remained through porcine postnatal devel opment, thus suggesting that all the subpopulations are physiologicall y relevant. However, the different hormonal storage patterns between s ubsets of the same density suggest age-related differences within each subpopulation due, at least in part, to the different physiological c ondition of the animals during development.