The presence of GH-R in Sertoli cells is also strongly suggested in the present work. In preliminary studies with different populations of isolated testicular cells, high-affin-ity GH binding sites could be detected in cultures enriched in Sertoli cells but not in round germ cells, in spermatozoa, or in mixed testicular cell populations. Furthermore, the marked reduction of GH-R concentration during the reproductive cycle was compatible with the decreased proportion of Sertoli cells during fish spermatogenesis (by a factor 10 ). asthma inhalers
Therefore, GH could act directly on the testis, not only by interfering with the interstitial steroidogenic cells as previously proposed, but also at the level of the germinal epithelium. In previous studies we found that trout Sertoli cells express IGF-I mRNA and that IGFs can directly stimulate spermatogonial multiplication in vitro. Furthermore, rtGH has a stimulatory effect on in vitro spermatogonia and spermatocyte multiplication when testicular somatic cells (Sertoli cells plus interstitial cells) are present (unpublished results), while it has no effect in the absence of these cells. GH-R on trout Sertoli cells could therefore have an important physiological role in the regulation of spermatogenesis. Our data support the recent hypothesis by Swanlund et al. that GH promotes tubular and Sertoli cell maturation, and may provide new information for the interpretation of the beneficial effect of GH in the treatment of male infertility.