Regulatory properties of adenylyl and guanylyl cyclase in human spermatozoa

Completion of maturation of spermatozoa (capacitation) occurs in the female genital tract. As a result, spermatozoa acquire the high motility and the capability for acrosomal reaction, which determines their fertility. There are evidences that adenylyl cyclase and guanylyl cyclase signaling systems detected in human and mammalian spermatozoa are involved in these processes. The goal of the present study was characterization of these systems in human ejaculate spermatozoa (ES) and in human fertile spermatozoa (FS) isolated by a density gradient centrifugation. In FS homogenate the basal activity of the adenylyl cyclase (AC) was significantly higher as compared with ES (47 ± 5 vs. 28 ± 3 pmol cAMP/min per mg of protein). At the same time, the AC stimulatory effects of non-hormonal activators of soluble and membrane-bound forms of AC (NaHCO₃, Mn²⁺, forskolin, and non-hydrolyzable GTP analogue—GppNHp) in FS were lower as compared with ES. Isoproterenol, serotonin, PACAP-38, and, to the lesser extent, noradrenalin and adenosine stimulated the AC activity in ES. Among hormones inhibiting AC, only adenosine decreased the enzyme activity. At the same time, in FS the inhibitory AC effects of adenosine, noradrenalin, and serotonin were markedly expressed, and the stimulatory effects of these hormones were decreased or absent. The basal activity of guanylyl cyclase (GC) in ES and FS homogenates was 27 ± 3 and 21 ± 2 pmol cGMP for 1 min per 1 mg protein, respectively, and was significantly increased in the presence of 10 mM Mn²⁺. The stimulatory GC effects of natriuretic peptides—ANP and CNP, activators of receptor forms of GC, was significantly higher in ES than in FS, and the effect of ANP was more pronounced as compared with CNP. The data indicate the multiplicity of cAMP- and cGMP-dependent signaling cascades regulating fertility of human spermatozoa. We found that the sensitivity of AC and GC to hormones in the common pool of ES and in the fraction of highly motile FS isolated by centrifugation was essentially different, which is to be considered when using FS for accessory reproductive technology.