ONTOGENY OF THE INHIBITORY GUANINE-NUCLEOTIDE-BINDING REGULATORY PROTEIN IN THE RAT TESTIS - MESSENGER-RNA EXPRESSION AND MODULATION OF LH AND FSH ACTION

The ontogeny of function and mRNA expression of the inhibitory guanine nucleotide-binding regulatory protein (G(i)) was studied in the rat testis. Dispersed testis cells of animals aged 8, 15, 20 and 30 days were cultured with or without 100 mu g/l pertussis toxin (PT) for 24 h. The cells were then cultured for another 24 h with medium only, cholera toxin (CT), PT, or their combination, and the amount of testosterone and cAMP production was measured. PT preincubation increased CT-stimulated cAMP production at all ages, thus indicating the presence of a functional G(i)-protein in the postnatal testis. However, when testosterone production was measured, the enhancing effect of PT was absent at the age of 8 days only, indicating that Leydig cells at this age did not have functional G(i)-protein. We then cultured 2-day-old and 8-11-day-old testis cells, after 24 h pretreatment with PT, in the presence of ovine follicle-stimulating hormone (FSH) (1 mg/l). The FSH-stimulated cAMP production was enhanced at both ages, thus indicating the presence of a functional G(i)-protein in neonatal Sertoli cells. In Northern blot analyses, fetal and postnatal testis tissue had very similar levels of G(alpha i2) and G (alpha i3) mRNAs; the mRNA level of G(il) in Northern blots remained low compared to those of G(i alpha 2) and G(i alpha 3). In conclusion, the G(i) protein appears in the developing rat testis in utero but the activity first seems to be confined to non-leydig cells including the Sertoli cell. In Leydig cells, the functional G(i)-protein appears between days 8-15 post partum. This finding may be related to the fact that the fetal-neonatal population of Leydig cells possesses a high steroidogenic capacity and an apparent lack of the ability to respond to high gonadotropic stimulation with LH receptor down-regulation and steroidogenic enzyme desensitization.