Effect of ouabain on the lactogenic action of prolactin and on the level of mammary prolactin receptors

Ouabain added to the culture medium of rabbit mammary gland inhibits prolactin action on the initiation of lactose and casein synthesis. The degree of inhibition is a function of the ouabain concentration in the medium. Likewise, ouabain blocks the accumulation of casein mRNA supported by prolactin. In addition, ouabain provokes a rapid disappearance of prolactin receptors. Conversely prolactin keeps its capacity to enhance the concentration of casein mRNA and the parallel casein synthesis when K+ ions are totally absent from the culture medium. These results suggest that although prolactin induces a modification of the K+/Na+ ratio in the mammary cell and ouabain prevents this effect of prolactin, the inhibitory action of ouabain on lactogenesis can be explained essentially by its effect on the hormone receptors.