INSULIN-LIKE GROWTH-FACTOR (IGF-I) MESSENGER-RNA AND IGF-I RECEPTOR IN TROUT TESTIS AND IN ISOLATED SPERMATOGENIC AND SERTOLI CELLS

Few data exist concerning the occurrence and potential role of an insu lin-like growth factor (IGF) system in fish gonads. Using Northern and slot blot hybridization with a specific salmon IGF-I cDNA, we confirm ed that IGF-I transcription occurs in trout testis. Testicular IGF-I m RNA abundance may be increased by long-term GH treatment in juvenile f ish, while shorter treatment with growth hormone (GH) or a gonadotropi n (GTH-II) in maturing males had no statistically significant effect. Radiolabelled recombinant human IGF-I binds with high affinity to crud e trout testis preparation, to cultured isolated testicular cells, and to a membrane fraction of these cells (Ka = 0.2 to 0.7 x 10(10) M(-1) ; Bmax = 10 to 20 fmol/10(7) cells, and 68 fmol/mg protein of membrane ). The binding site was identified as type 1 IGF receptor by its bindi ng specificity (IGF-I > IGF-II >>> insulin) and the molecular size of its alpha-subunit labelled with I-125-IGF-I (M(r)125 - 140 kDa). I-125 -IGF-II also bound to the type 1 receptor whereas IGF-II/mannose 6 pho sphate receptors could not be detected. Separation of isolated testicu lar cells by Percoll gradient and centrifugal elutriation provided pop ulations enriched in different types of intratubular cells. IGF-I mRNA (detected by reverse transcription + polymerase chain reaction [PCR]) and IGF-I receptors (measured by competitive binding) were observed t o a greater extent in Sertoli cell-enriched populations and in spermat ogonia with primary spermatocytes. Therefore, IGF-I is a potential par acrine/autocrine regulator inside the spermatogenic compartment and ap pears as a possible mediator of GH action at the gonadal level in fish . (C) 1996 Wiley-Liss, Inc.