| Abstract: | Most experimental information regarding the mechanism of action of prolactin in its diverse array of target tissues has been discovered using mammary tissues. Evidence has recently been presented that suggests that prolactin may be "internalized" into its target cells and have intracellular actions. Accordingly, it has been reported that prolactin stimulates RNA synthesis in isolated nuclei from mammary tissues; and by immunoflorescent studies, prolactin has been located within its target cells. It has been further suggested from additional experimental studies that the primary action of prolactin may involve its initial interaction with fixed plasma membrane receptor sites. Subsequent actions of prolactin may involve the following: a) an increased intracellular concentration of potassium and a reduced level of sodium, b) an increased level of cGMP and a reduced level of cAMP, c) an enhanced rate of prostaglandin biosyntheesis mediated by a stimulation of phospholipase A2 activity, and d) a stimulation of polyamine synthesis. It has also been shown that the actions of prolactin require calcium ions in the extracellular environment. Laboratory studies have thus indicated that the actions of prolactin may be carried out by a number of processes; but a single, primary action of this hormone that accounts for all of its actions has not yet been proven. |
