Lack of endogenous adenosine receptor activation explains the insensitivity of neonatal rat Leydig cells to treatment with pertussis toxin.

We have previously shown that the Gi protein-mediated negative modulation of FSH-stimulated cAMP production in neonatal rat Sertoli cells can be blocked by pretreatment with pertussis toxin (PT), an inhibitor of the Gi protein function. In contrast, hCG-stimulated testosterone (T) production by neonatal Leydig cells was insensitive to PT action, despite the demonstration of Gi protein in these cells by immunohistochemistry. We now tested the hypothesis that the missing Gi protein function in neonatal Leydig cells is due to absence of its endogenous activator, adenosine. It was first reconfirmed in dispersed cells of 7-day-old rats that PT preincubation enhanced FSH-stimulated cAMP production (i.e. Sertoli cell function), but had no effect on hCG-stimulated T production (Leydig cell function). The adenosine agonist n-phenylisopropyladenosine (PIA 1 mumol/l), acting through the Gi-protein, attenuated FSH-stimulated cAMP production, and this effect was abolished when the Gi protein was inactivated by preincubation with PT. PIA (1 mumol/l) also inhibited (P < 0.05-0.01) the basal and hCG-stimulated rates of Leydig cell T production, and the PIA effect was abolished by PT preincubation. In conclusion, the present data show that the apparent absence of a functional Gi protein function of neonatal Leydig cells is due to insufficient endogenous levels of the stimulating ligand, adenosine. When the Gi protein function is activated by exogenous adenosine agonist (PIA), the enhancement of hCG-stimulated T production by PT can be demonstrated. At the same time, the endogenous level of adenosine is sufficient for activation of the Gi protein function in Sertoli cells.