Effect of relaxin on mammary gland growth and lactation in the rat.

Mammary DNA, RNA and RNA/DNA were measured in groups of 10 ovariectomized rats treated for 20 days with relaxin (R) in doses of 20 and 90 guinea pig units (GPU) alone or in combination with 1 mug estradiol benzoate (EB) or EB plus 3 mg progsterone (P). No treatment, EB only, and EB plus P served as control groups. Twenty GPU R alone significantly increased DNA over the no treatment group while 90 GPU did not increase DNA any higher. Although DNA values for the EB group were higher than no treatment and EB plus P higher than EB, R was not effective in increasing DNA in combination with either EB or EB plus P.R alone at either level did not change RNA from the no treatment group, but 90 GPU R in association with EB and both levels of R in association with EB plus P significantly reduced RNA in relation to the approiate control groups. Relaxin acts synergistically with estrogen and progesterone to develop the mammary apparatus and at the same time to suppress lactation. Removing the inhibitory effects of relaxin at parturition may be an important prelude to lactogenesis.