Investigation of the mechanism of action of growth hormone in stimulating lactation in the rat

Abstract

The role of GH was examined using an antiserum to rat GH (anti-rGH). When administered to lactating rats on day 2 of lactation it was without effect, whereas bromocriptine markedly suppressed milk production, with no additional effect of combined treatment. On day 6 of lactation, treatment with anti\x=req-\ rGH was also without effect, whilst bromocriptine again suppressed milk production. Combined treat-ment, however, suppressed milk synthesis completely, suggesting that GH was capable of maintaining about 50 % of normal milk yield in the absence of prolactin at day 6 of lactation. By day 14 of lactation, anti-rGH treatment alone was capable of decreasing milk yield by about 20%, and again milk secretion only stopped completely when GH and prolactin were suppressed. These data suggest that the role of GH in supporting lactation increases as lactation progresses. The effects of GH in stimulating growth and in increasing milk yield in ruminants have been proposed to be mediated via insulin-like growth factor-I (IGF-I). In rats treated with anti-rGH, both IGF-I and IGF-II were decreased in serum. The concentra-tion of the major IGF-binding protein (IGFBP-3) was not, however, affected by inhibition of GH or pro-lactin individually, but was decreased in animals treated with bromocriptine and anti-rGH. In animals given both bromocriptine and anti-rGH, concurrent treatment with recombinant bovine GH maintained milk yield at 50 % of control values and normalized serum IGF-I, IGF-II and IGFBP-3 concentrations. By contrast, concurrent treatment with IGF-I or IGF-II, despite normalizing their respective concentrations in serum, failed to affect milk yield. These results suggest that neither IGF-I nor IGF\x=req-\ II is capable of mediating the effects of GH alone. It is, however, possible that they play a part in a co-ordinated series of responses to GH involving IGF-I