The levels of IGF-1 have been simultaneously measured by radioimmunoassay in samples of the toad Bttfo arenanun and of normal male Wistar rats. In addition, the different fractions of IGF-1 binding proteins (IGFBP) and their binding properties have been identified by lignnd blot and Scatchard analysis in the serum of both species. In the toad, we have measured levels of IGF-1 (2.78 ± 0.48 ng/ml) similar lo those previously reported in amphibians but far below those found in rats. IGFBP levels \Vere estimated at 129 ::!:: 23 and 4249 ± 321 pg/ml in toad and rat serum samples. Two main IGFBP fractions of 30-34 kDa, acco1npanied by a n1inor component of 24 kDa and seldom by another of 40 kDa, were identified in toad serum. In rat serum-as already reported-three bands of 40, 30, and 24 kDa were identified, the first being the main component and the last the minor one. The Scatchard analysis of a competitive binding assay showed two types of binding sites in toad serum: one of high affinity-low capacity (Ka 1 "" I .6 x 10io M-1 ; R 1 = I .2 x 10-11 M) and another with low affinity-high capacity (Ka 2 = 1.9 X 10 8 M-1 ; R 2 = 1.9 x 10-10 M). The percentage fraction of these binding sites occupied by lGF-l was 13.5%. The figures for K 1 and K 2 were lower and those for R 1 and R 2 were higher in rat than in load serum. The percentage fraction of occupied rat lOF binding sites was 3.6%. The lGF carrier levels (IGFBP,.) estin1ated in our laboratory in samples of rat and toad serum gave figures that were almost 33 times lower in the latter than in the former. Hence, the fraction of free and bound IUF-1 in toad and rat blood might be different. Our results provide new evidence of the presence and the properties of IGFBP in an1phibians, confirming the wide distribution of this carrier among different species and its possible role as modulator of IGF-1 biological effects. C
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