Cleavage of Rat Prolactin Occurs Within Rat Mammary Tissue,and the Cleaved Form is Present in Serum but Not in Milk

Abstract

The present study was undertaken to determine whether cleavage of rat prolactin (rPRL) occurs within rat mammary tissue and to investigate whether the cleaved form and its 16-kDa amino terminal fragment are present in rat serum and milk. The relative abundance of the different forms of rPRL was determined by SDS-PAGE and immunoblot analysis using an antiserum to the 16-kDa fragment. Explants of mammary glands from lactating rats were incubated with intact rPRL. The tissue was then washed extensively to remove external PRL forms. The ratios of cleaved to intact rPRL in mammary tissue homogenate, in mammary-conditioned medium, and in serum from lactating rats were 4.2, 0.44, and 0.16, respectively. Although intact rPRL was found in milk, the cleaved form was not detectable. The 16-kDa fragment of rPRL was not detected in concentrated extracts of either serum or milk. These results indicate that lactating rat mammary tissue can internalize and cleave intact rPRL, but the cleaved form does not appear in milk. The cleaved rPRL is present in rat serum, but no evidence was obtained for the presence of the 16-kDa fragment of rPRL in either serum or milk.