Actomyosin from mammary myoepithelial cells and phosphorylation by myosin light chain kinase

The oxytocin-sensitive myoepithelial cells of the mammary gland form a system with characteristics of a potentially useful model for studying the mechanism of action of oxytocin and coupling phenomena of excitation-contraction. Our objectives were to develop a method for isolating mammary actomyosin, to determine the amount of actomyosin in the glands of lactating and nonlactating animals, and to investigate control of contractile protein interaction. Actomyosin in mammary glands represented a substantial portion of the soluble protein in the gland ranging from 9% of the total in lactating to 17% in weaned rats. The isolated actomyosin had a molecular composition like that of actomyosin of smooth muscle and the isolated actomyosin contained a light chain kinase that phosphorylated the 20,000 dalton light chain of myosin (L20). The kinase isolated as a component of actomyosin preparations did not show calcium control, but it did when isolated from mammary cytosol. Strips of involuted mammary tissue from rats developed tension when oxytocin was added to the bathing medium; thus, the myoepithelial cells appeared to retain their sensitivity to oxytocin even in nonlactating animals and may be a useful model for studying the action of oxytocin. We suggest that one of the final steps in the milk-ejection reflex is phosphorylation of myosin causing a contraction of the myoepithelial cells of the mammary gland.