Growth hormone-binding protein (GH-BP) levels in follicular fluid from human preovulatory follicles: correlation with serum GH-BP levels.

Serum GH-binding protein (GH-BP), which is identical with the extracellular domain of the GH-receptor, has important implications for the distribution and physiological activity of GH and may enable evaluation of GH-receptor function. Recent studies suggest that GH plays an important role in the modulation of ovarian function and GH-receptors/BPs are found in the female reproductive system. The purpose of the present study was to investigate the presence of GH-BP in human follicular fluid (FF) and compare the levels of FF GH-BP with those detectable in serum, in 46 women undergoing in vitro fertilization. Levels of GH-BP were determined by incubation of [125I]hGH with 50 microL FF or serum, in the absence or presence of excess hGH and specific binding was expressed as a percentage of the total counts per min incubated. The mean GH-BP level in patient FF was 12.39 +/- 0.63% (mean +/- SE) and this correlated significantly with serum GH-BP levels (r = 0.82; P < 0.001). The binding of hGH to FF was dose dependent, highly specific, and of high affinity and low capacity. The affinity constants (Ka) obtained by Scatchard analysis for hGH binding to patient's FFs and sera were not significantly different. Furthermore, we have analyzed the sodium dodecyl sulfate gel electrophoretic pattern of GH-BP in FF and serum by both the ligand-blot technique and after cross-linking with [125I]hGH. Our results show that there is close similarity between serum and FF GH-BPs. Significantly lower FF GH-BP levels were measured in patients older than 36 yr compared to younger women (P < 0.05), whereas increased values were obtained both in patients with elevated E2 concentrations in serum (> 7000 pmol/L) and in FF (> 2200 nmol/L), (P < 0.02 and P < 0.05, respectively). This first demonstration of GH-BP in FF is expected to increase our understanding of the possible direct effect of GH on ovarian steroidogenesis, and suggests a possible regulatory role for GH-BP in folliculogenesis.