Effect of Calf Thymus Protein On Prolactin-release and 2nd Messenger Systems In Rat Pituitary-cells

Recent studies suggest that some thymic hormones can affect hormone secretion at the pituitary level. Recently we isolated and characterized a protein, named thymus protein hypocholesterolemic factor (TphF), from calf thymus and showed this protein produced significant effects on lipid metabolism by increasing the catabolism of low density lipoproteins (LDL) via the stimulation of the LDL receptor pathway. This action was mediated by a reduction of cellular cholesterol synthesis following the inhibition of 3-hydroxy-3-methylglutaryl-CoA reductase activity. In this study we investigated the effect of TphF on prolactin release, in both basal and stimulated conditions, in primary cultures of rat anterior pituitary cells. Our results show that TphF, at very low concentrations, stimulates prolactin secretion after a priming period of time. This effect does not seem to be mediated either by an increase in cAMP production or by an activation of phosphoinositide metabolism. Conversely, TphF has been able to enhance a TRH-induced increase in cytosolic calcium concentration, suggesting the possibility that TphF may have an indirect effect, acting via an increase in the response of pituitary cells to calcium mobilizing agents