ERYTHROCYTES FROM PATIENTS WITH LOW SERUM CONCENTRATIONS OF IGF-I HAVE AN INCREASE IN RECEPTOR-SITES FOR IGF-I

The binding characteristics of insulin-like growth factor I on erythrocytes were studied in 11 patients with long-term IGF-I deprivation and low serum IGF-I levels. Six patients had Laron type dwarfism and 5 idiopathic isolated growth hormone deficiency, with a mean (+/- SEM) serum IGF-I level of 6.01 +/- 1.01 nmol/l as compared with that in 25 normal controls of 26.35 +/- 2.73 nmol/l (p = 0.00001). The mean (+/- SEM) [I-125]IGF-I specific binding at a concentration of 4 x 10(12) cell/l was 12.11 +/- 1.29% for the patient group compared with 8.75 +/- 0.62% for the controls (p = 0.005). Scatchard analysis showed a curvilinear plot. Using a non-linear curve fit, the mean (+/- SEM) number of high-affinity receptor sites per cell was found to be 7.34 +/- 1.80 in the IGF-I-deprived patients and 2.84 +/- 0.29 in the controls (p = 0.0005). The mean +/- SEM dissociation constant was found to be 0.33 +/- 0.10 nmol/l for the patients and 0.26 +/- 0.08 nmol/l for the controls (NS). This study has demonstrated that the low serum concentration of IGF-I in Laron type dwarfism and isolated growth hormone deficiency is associated with an increase in receptor sites for IGF-I on the erythrocytes. The application of this property as a diagnostic aid remains to be established.