An increase from twice daily milking to thrice daily (Henderson et al., 1983) or, in experimental situations, hourly milking (Blatchford and Peaker, 1982), increased the rate of milk secretion. The acute response occurred not through additional milking-stimulated release of galactopoietic hormones, because it could be elicited unilaterally if more frequent milking was applied to just one gland of the udder. It was also not due to alteration of the pattern of physical distension of the gland; if milk removed by an extra third milking was immediately replaced by an equal volume of isosmotic sucrose - thereby maintaining the pattern of gland distension and relaxation while still removing milk more often - milk secretory rate was stimulated nevertheless (Henderson and Peaker, 1984). Different fractions of milk were tested, and the physiological negative feedback mechanism was found to be present in the milk whey fraction containing proteins of 10 to 30 kD of molecular weight (Wilde and Knight, 1989). When introduced into the gland of lactating goats via the teat duct, as represented by the arrow, this whey fraction caused a temporary reduction in the rate of milk secretion; the other gland, which received an equal volume of isosmotic sucrose (20 to 25 ml) showed no significant effect of intraductal injection.
Last year, Wilde and collaborators finally took the time to further subfractionate the 10-30 kD whey fraction, using anion exchange chromatography and producing 7 subfractions, shown in the graphic, which were tested on rabbit mammary explants for their ability to inhibit synthesis of milk constituents. All the inhibitory ability was found within subfraction 3; further screening of this subfraction eventually led to the isolation of a single protein. After partial sequencing of this protein, it was observed that it had not been described before.
Wilde et al. in 1995 denominated this new protein FIL, for Feedback Inhibitor of Lactation, a protein of 7.6 Mr. Synthesis of FIL by the mammary secretory cells on which it acts further suggests that this feedback inhibition of milk secretion is an autocrine mechanism. Although autocrine regulation is not a typical process on exocrine glands, this type of control is also not without precedent: removal of secretion from viper venom glands increases expression of the genes encoding venom products (cited by Wilde et al., 1995).
FIL, which was identified in vitro by tissue-culture bioassay, also decreased the rate of milk secretion in vivo, in lactating goats. The injection of FIL unilaterally caused the same type of inhibition that the injection of the 10-30 Kd whey fraction used to elicit in earlier experiments (Wilde et al., 1995).
Reversible concentration-dependent inhibition by FIL suggests that it is a physiological regulator of the rate of milk secretion. Studies in other species, invariably with the participation of Dr. Wilde, including man and a macropod marsupial, suggest strongly that autocrine control by FIL, or a homologous protein, is ubiquitous in mammals.
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