Relationship between testicular blood flow, testosterone secretion and spermatogenic activity in young and adult wild red foxes (Vulpes vulpes).

Testicular capillary blood flow (TCBF) was measured by the radioactive inert gas clearance technique throughout the reproductive life of young adult foxes and was related to the spermatogenic and androgenic activities of the testis. Mean (+/-S.E.M.) blood flow (ml min-1 g-1) was maximal in January in adults during the mating period (0-65 +/- 0-03), and in pubertal animals (0-62 +/- 0-04). At this time spermatozoa were observed in the testes of all animals, but testicular weight and circulating testosterone levels were lower in the pubescent foxes than in the adults. TCBF was minimal during immaturity (0-29 +/- 0-03) and during the resting period of the adult (0-12 +/- 0-01). These values were associated with a low testosterone level and with the multiplication of gonocytes in the young or with the seasonal very low spermatogenic activity in the adult. During the prepubertal period, TCBF slowly increased and was accompanied by testicular growth. In the adult, in September, TCBF rapidly increased without changes of testicular size and then slowly increased as the testes enlarged. High plasma testosterone concentrations occurred later. During the period of testicular regression, TCBF, testicular size, spermatogenic and androgenic activities decreased together.