Mechanism of secretion of plasma insulin-like growth factor-I into milk of lactating goats.
125I-Labelled insulin-like growth factor-I (IGF-I) was infused as the free form directly into the pudic artery supplying one gland of lactating goats (n = 6). The infusion was for 60 min and 0.4 +/- 0.09% (S.E.M.) of the infusate was secreted into milk from the infused gland during its first passage through that gland. A large proportion of the 125I-labelled IGF-I escaped into the systematic circulation and was secreted into milk of both glands. A total of 5.2 +/- 0.4% of infused radioactivity was recovered in milk from both glands from 0 to 720 min. Radioactivity consisted of trichloroacetic acid (TCA)-precipitable and -soluble counts which were shown by gel filtration to be authentic IGF-I and degraded products of the peptide. The amount and time course of TCA-soluble radioactivity in milk from both glands was similar, suggesting degradation of 125I-labelled IGF-I at extramammary sites. Maximum specific activity for 125I-labelled IGF-I in milk from the infused gland was reached 80-120 min after the start of infusion and was 2.5-fold greater than milk from the non-infused gland. The time course of appearance of 125I-labelled IGF-I in milk suggests that transfer was via the transcellular pathway and this was further supported by comparing the pattern of transfer of [14C]sucrose and [14C]amino acids. When excess unlabelled IGF-I was included in the infusate, specific activity in milk from the infused gland was reduced to that of the non-infused gland, indicating a competitive and saturable mechanism of secretion for 125I-labelled IGF-I.(ABSTRACT TRUNCATED AT 250 WORDS)
Prosser
C G
CG
AFRC Institute of Animal Physiology and Genetics Research, Babraham, Cambridge.
Fleet
I R
IR
Davis
A J
AJ
Heap
R B
RB
D000818:Animals; D004730:Endothelium, Vascular / Q000378:metabolism; D004848:Epithelium / Q000378:metabolism; D005260:Female; D006041:Goats / Q000378:metabolism*; D007261:Infusions, Intra-Arterial; D007334:Insulin-Like Growth Factor I / Q000032:analysis / Q000493:pharmacokinetics*; D007457:Iodine Radioisotopes / Q000378:metabolism; D007774:Lactation / Q000097:blood / Q000378:metabolism*; D008321:Mammary Glands, Animal / Q000098:blood supply / Q000378:metabolism*; D008892:Milk / Q000378:metabolism*; D011247:Pregnancy; D013997:Time Factors
D007457:Iodine Radioisotopes; D007334:Insulin-Like Growth Factor I