Hypertension associated with decreased testosterone levels in natriuretic peptide receptor-A gene-knockout and gene-duplicated mutant mouse models

Abstract

Mice lacking the gene (Npr1) encoding the natriuretic peptide receptor A (NPRA) have hypertension with elevated blood pressure and cardiac hypertrophy. In particular, Npr1 gene-deficient male mice exhibit lethal vascular events similar to those seen in untreated human hypertensive patients. Serum testosterone levels tend to be lower in hypertensive male humans than in normal males without hypertension, but the genetic basis for this tendency remains unknown. To determine whether Npr1 gene function affects the testosterone level, we measured serum testosterone in male hypertensive mice lacking a functional Npr1 gene, wild-type animals with two copies, and the gene-duplicated littermates expressing four copies of the gene. In the Npr1 gene-knockout (zero-copy) mice, the serum testosterone level was 62% lower than that in the two-copy control mice (80+/-10 ts. 120+/-14 ng/ml, respectively; P < 0.005). Serum testosterone in the four-copy mice was 144% (P < 0.005) of that in the two-copy wild-type control mice. To investigate the role of NPRA in testicular steroidogenesis, we analyzed atrial natriuretic peptide (ANP)-dependent guanylyl cyclase activation, accumulation of intracellular cGMP, and testosterone production in purified primary Leydig cells from animals with zero, two, or four copies of the Npr1 gene. Leydig cells lacking the Npr1 gene did not show ANP-stimulated guanylyl cyclase activation or cGMP accumulation and had no ANP-dependent testosterone production. ANP stimulation of Leydig cells from the four-copy males elicited a 2-fold greater production of cGMP compared to that in the two-copy wild-type counterparts (260+/-12 vs. 126+/-7 pmol/l x 10(6) cells; P < 0.001). Similarly, ANP-dependent testosterone production in Leydig cells was nearly twice as high in four-copy mice as in two-copy wild-type controls (561+/-18 vs. 325+/-11 ng/l x 10(6) cells; P < 0.001). ANP-dependent guanylyl cyclase activation and production of cGMP in Leydig cells increased progressively with the number of Npr1 gene copies. Our results establish the existence of an alternate mechanism for testicular steroidogenesis that is stimulated by NPRA-dependent cGMP signaling, in addition to that mediated by gonadotropins, via a cAMP pathway. These findings demonstrate the role of Npr1 gene function in the maintenance of serum testosterone levels and testicular steroidogenesis and provide a genetic link between hypertension associated with decreased NPRA and low testosterone levels.