SUMMARYThe plasma clearance of intravenously injected '25I-labelled insulin-like growth factor I (IGF-I, n = 13) and IGF-II (n = 12) and their transfer into lymph draining the foreleg of 3 5-to 8-week-old British Saanen goats was studied. Both peptides were initially distributed into a volume equivalent to the plasma volume, while the final distribution spaces for IGF-I and IGF-II were 90 + 9-8 and 125 + 12 ml/kg live weight respectively. There were two phases to the plasma clearance of both IGF-I and IGF-II, with the half-lives of both phases for IGF-I (9-6 + 0-9 and 287 + 23 min, first and second phase respectively) being significantly (P < 0 001) longer than those of IGF-II (4-2 + 0-6 and 172 + 18 min, respectively). The radioactivity transferred into lymph originated from intact IGF-I and IGF-II as well as degraded products of these compounds, as assessed by precipitation with trichloroacetic acid and gel filtration. Levels of undegraded IGF-I in lymph were 50 % greater than IGF-II. While more than 90 % of either peptide was bound to specific IGF-binding proteins in plasma, in lymph 34 + 2 % of IGF-I and 23 + 3 % of IGF-II remained in the free form 60-80 min after injection. The plasma: lymph ratio for free IGF-I was 2: 1 and for bound IGF-I, 8:1. The corresponding values for IGF-II were 3:2 and 8:1 respectively. These results provide direct experimental evidence for transfer of undegraded IGF-I and IGF-II from blood into lymph of the foreleg, consistent with the ability of these factors to act in an endocrine role in growing tissues.
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