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Lactation was suppressed in rats using a combined treatment of bromocriptine (to reduce prolactin concentrations) and a specific antiserum to rat GH administered twice daily for 2 days. When milk production had ceased, as determined by litter weight loss and the absence of milk in the stomachs of pups, attempts were made to reinitiate lactation using prolactin, GH, insulin-like growth factor-I (IGF-I) precomplexed to recombinant human IGF-binding protein-3 (hIGFBP-3) or IGF-I plus IGF-II precomplexed to hIGFBP-3. Despite the fact that all treatments except prolactin led to increases in serum IGFs and IGFBP-3, only prolactin and GH provoked the reinitiation of milk production as determined by increased litter weight gain, milk in the stomach of pups and a significant increase in the weight of the mammary glands.

Since the mammary gland has been shown to produce IGFBPs which may inhibit IGF action we also tested three IGF-I analogues, R₃-IGF-I, Long-IGF-I and Long-R₃-IGF-I. R₃-IGF-I has a single amino acid substitution (Glu to Arg) at position 3 whereas Long-IGF-I has a 13 amino acid N-terminal extension. These modifications dramatically reduce the ability of these analogues to bind to IGFBPs although they remain active at the IGF-I receptor. Such IGF analogues would therefore be expected to be active irrespective of the production of inhibitory IGFBPs. However, none was effective in reinitiating lactation, even at doses which have been shown to be biologically effective in terms of nitrogen retention.

We therefore conclude that, despite the fact that GH induces increases in serum IGF-I, IGF-II and IGFBP-3 when administered to lactating rats, the combination of all of these factors fails to reinitiate lactation. The biological significance of these changes and the mechanism by which GH stimulates milk secretion, when there appear to be no GH receptors on mammary epithelial cells, remain unclear, although the fact that both GH and prolactin were able to prevent reductions in DNA content of the gland suggest that regulation of apoptosis may be involved.

Journal of Endocrinology (1994) 140, 211–216

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