Free Insulin-Like Growth Factor I (IGF-I) and IGF-II in Human Saliva*

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Abstract
We found that human saliva contains both insulin-like growth factor I (IGF-I) and IGF-II but no significant binding proteins, and that salivary IGF-I levels correlated with plasma GH levels. Mixed saliva had globular proteins precipitated by freezing/thawing. After centrifugation the clear supernatant was used directly in the IGF-I RIA (Van Wyk and Underwood antibody) and in a human placental membrane RRA for IGF-II. The lower limits of detection for IGF-I and IGF-II were 0.7 ng/mL (μg/L) and 1.2 ng/mL (μg/L), respectively. Iodinated IGF added to saliva was not degraded, as assessed by trichloroacetic acid precipitability and placental membrane binding. In saliva from 14 normal subjects, IGF-I was measurable in all. IGF-II was detectable only in 8 of 14 subjects; the mean value in these 8 subjects was 2.6 ± 0.6 (±se) ng/mL (μg/L). The mol wt of salivary IGF was similar to that of free plasma IGF after acid or neutral pH gel chromatography. Human saliva contained no significant IGF-binding protein. Eluates from neutral gel chromatography of concentrated (20-fold) normal saliva did not inhibit IGF-II binding to placental membrane receptors. Eluted proteins from saliva samples subjected to prior acid gel chromatography failed to bind radiolabeled IGF after neutralization. Saliva samples assayed for binding protein using an amniotic fluid binding protein RIA had values at or below the lower limit of detection [μgeq/mh (mgeq/L)]. Salivary IGF-I concentrations did not change with increasing salivary flow rates above normal, with time of day, or with storage at room temperature for up to 24 h before freezing. The mean IGF-I concentration in mixed saliva from 14 normal young adults (8 men) was 2.3 ± 0.3 (±se) ng/mL (μg/L), and their mean plasma IGF-I level was 315 ± 27 ng/mL (μg/L). Mean salivary IGF-I was significantly lower in 15 patients with GH deficiency [1.3 ± 0.2 ng/mL (μg/L); P < 0.01] and 8-fold higher in 5 acromegalic patients [17.2 ± 6.3 ng/mL (Mg/L); P < 0.01]. Removal of their GH adenomas led to a fall in salivary IGF-I to 5.6 ± 1.3 ng/mL (μg/L); P < 0.05). In summary, saliva contains free IGFs but no significant quantities of specific binding proteins. Salivary IGF-I levels reflect the GH status of the donor.