MECHANISM OF EPINEPHRINE INHIBITION OF THE MILK EJECTING RESPONSE TO OXYTOCIN’

Under stress, nursing is either unsatisfactory or impossible. It has been shown that stress inhibits release of oxytocin from the neuro-hypophysis (Cross, 1955b; Fitzpatrick, 1961). Oxytocin in addition to its well-known uterine stimulating activity also has a specific action on the lactating mammary gland. It stimulates the myoepithelial cells in the mammary gland (Lin-zell, 1955). Contraction of these cells causes milk ejection or milk let-down from the lactating gland. In the absence of oxytocin milk ejection cannot take place and the suckling young are unable to remove milk from the mammary gland. This appears to be true in all mammals (Whittle-stone et at., 1952) including lactating women (Newton and Newton, 1948). The inability to nurse under stress may also be due to inhibition of milk ejection by the activation of the sympathoadrenal system. Cross (1955a) showed that electrical stimulation of the sympathetic nerves supplying the mammary gland or the adrenal gland inhibited the milk ejecting response to oxytocin in the rabbit. Intravenous injection of epinephrine or norepi-nephrine also produced the same inhibitory ef-fect. Pickford (1960) also reported the inhibi-tion by epinephrine of milk ejection in the dog. The mechanism of inhibition of milk ejection by these sympathetic amines has not been fully elucidated. The limited available information (Cross, 1955a; Linzell, 1955; Kullander, 1963) suggests that the inhibitory activity of epineph-rime and norepinephrine results from their vaso-constrictor property preventing oxytocin from reaching the mammary myoepithelial cells to provoke milk ejection. In this paper, we de-scribe experimental data in which we attempted to find out whether the inhibitory activity of epinephrine is, indeed, secondary to its vaso-constrictor property. We also attempted to find out whether other sympathomimetic amines and Received for publication August 21, 1964. ‘This investigation was supported in part b