Human growth hormone but not ovine or bovine growth hormones exhibits galactopoietic prolactin-like activity in organ culture from bovine lactating mammary gland

Explants from the mammary gland of 6 lactating cows were cultured in M-199 medium containing insulin (1.0 μg/ml) and hydrocortisone (0.5 μg/ml) and supplemented with bPRL (0.2-1.0 μg/ml) or oGH (0.1, 0.5 and 2.5 μg/ml) or bGH or hGH (0.2 and 1.0 μg/ml). It was found that neither oGH nor bGH significantly increased (P < 0.05) the casein and fat synthesis and α-lactalbumin secretion above the control level. A significant and almost equal increase was, however, observed in the presence of bPRL and hGH, but was not influenced by the simultaneous addition of oGH to bPRL-containing medium (both at 0.5 μg/ml). Various hormonal treatments did not affect casein secretion and glucose uptake. High correlations (r = 0.89-0.97) were found between the mean values for casein and fat synthesis and α-lactalbumin secretion. Our results suggest that (a) the in vivo galactopoietic activity of bGH does not result from a direct effect of synthetic capacities on the mammary gland, and (b) hGH is a potent lactogen in the organ culture of bovine lactating mammary tissue.