Identification and characterization of arginine vasopressin receptors in the rat testis

We have previously shown that arginine vasopressin (AVP) directly inhibits testicular steroidogenesis in vitro. In the present study, binding of neurohypophysial peptides to interstitial cells of the rat testis was studied using [³H]AVP as the ligand. Interstitial cells were obtained from adult rat testis after collagenase dispersion and were incubated with [³H]AVP in the presence or absence of unlabeled AVP. Binding equilibrium was reached by 60 min at 4 C, while incubation at higher temperatures (23 and 37 C) resulted in an apparent decrease in binding. Scatchard plot analysis of equilibrium binding data revealed the existence of one class of high affinity, low capacity binding sites (K_d = 1.0 ± 0.3 nM; maximal binding = 8.5 fmol/10⁶ cells). In addition, the rate constants of association and dissociation were calculated to be 0.024 nM^-1 min^-1 and 0.009 min^-1, respectively. Addition of naturally occurring neurohypophysial hormones as well as their synthetic analogs inhibited [³H]AVP binding to testis cells, resulting in parallel displacement curves. The order of potencies for the native peptides was: AVP = lysine vasopressin = arginine vasotocin (IC₅₀, 5 × 10^-10 M) > oxytocin = mesotocin (IC₅₀, 4 × 10^-7 M) > isotocin = glumitocin (IC₅₀ > 10^-6 M). Furthermore, two potent vasopressor antagonists, d(CH₂)₅Tyr(Me)AVP ([1-(β-mercapto-β,β-cyclopentamethylenepropionic acid), 2-(O-metfiyl)tyrosine]AVP) and dPTyr(Me)AVP ([1-deaminopenicillamine-2-(0-methyl)tyrosine] AVP) competed for [³H]AVP binding with a higher affinity (IC₅₀, -10^-11 M) than native AVP. In contrast, a selective antidiuretic agonist, dDAVP (1-deamino-8-D-AVP), only competed weakly for receptor binding, while a specific oxytocic agonist, (Thr⁴, Gly⁷)oxytocin, did not affect AVP binding. These results suggested that the testis may contain the V₁ receptor subtype. Studies on the intratesticular distribution of AVP receptors indicated minimal binding to cells derived from the seminiferous tubule, while most of the AVP-binding sites sediment with enriched fractions of Leydig cells after Metrizamide density gradient centrifugation. AVP-binding sites were also found in rat liver, kidney, and anterior pituitary (10.7, 2.6, and 1.7 fmol/mg protein), whereas adrenal, cerebellum, prostate, and hypothalamus were devoid of AVP-binding sites. Thus, we have demonstrated the presence of high affinity, stereospecific receptors for AVP in the interstitial cell compartment of the rat testis. These V₁ receptors may mediate the direct inhibitory action of neurohypophysial hormones on testicular Leydig cell steroidogenesis.