EFFECTS OF FASTING, MEDIUM GLUCOSE, AND AMINO-ACID-CONCENTRATIONS ON PROLACTIN AND GROWTH-HORMONE RELEASE, INVITRO, FROM THE PITUITARY OF THE TILAPIA OREOCHROMIS-MOSSAMBICUS

Previous investigations have shown that the release of PRL and GH from the tilapia pituitary is directly sensitive to osmotic pressure and a variety of endocrine and neuroendocrine factors. The present studies were aimed at determining whether the spontaneous release of PRL and GH, in vitro, is: (1) sensitive to the nutritional status of the fish, and (2) responsive to variations in the d-glucose and total amino acid content of the incubation medium. In the first series of experiments, male fish (50 to 60 g) were divided into two groups. One group was fed twice daily for 2 weeks while the second received no food. A nearly homogeneous mass of PRL-secreting cells was dissected from the rostral pars distalis (RPD) and incubated for 18 to 20 hr in either hyposmotic (300 mOsmolal) or hyperosmotic (355 mOsmolal) medium. Similarly, a mass of GH-secreting cells was dissected from the proximal pars distalis (PPD) and incubated for 18 to 20 hr in isosmotic (320 mOsmolal) medium. Fasting was found to alter the total amount of PRL and GH in the culture well (tissue + medium) at the end of the incubations, decreasing PRL and increasing GH. Fasting was also found to both reduce spontaneous PRL release in vitro and suppress its stimulation by reduced osmotic pressure (P < 0.01). By contrast, fasting resulted in a substantial increase in spontaneous GH release from the PPD in vitro (P < 0.01). In the second series of experiments, GH release was found to increase as the d-glucose concentration of the medium decreased (P < 0.01), while prolactin release was unresponsive. The spontaneous release of both PRL and GH in vitro was inversely related to the quantity of essential amino acids (Eagle's minimum essential medium; MEM) added to the incubation medium (P < 0.01). These findings suggest that the activity and regulation of PRL and GH cells in the tilapia pituitary may be sensitive to the nutritional status of the fish. It would also appear that GH cells, but not PRL cells, may respond to changes in circulating d-glucose levels and that both may respond to variations in circulating amino acids. Additional studies will be required to determine whether the latter response is directed by specific amino acids. © 1992.