Effects of an inhibitor of adenylate cyclase on acrosome reaction induced by protein kinase C activators

The diagnostic significance of the induced acrosome reaction

In trying to discern potential causes of male infertility, little investigation has been done to determine if there is predictive value in assessing acrosome reaction inducibility of human spermatozoa for clinical interpretation and application. The purpose of this review is to outline what is currently known in terms of the diagnostic significance of the induced acrosome reaction and to discuss what might be anticipated for the future. In constructing this review, every effort has been made to provide a comprehensive literature review for the reader and a number of review articles have been cited to serve as sources for additional related material. Regarding the aforementioned, it is essential to qualify that, with the exception of the Section entitled: ‘General characteristics of capacitation and the acrosome reaction’, only characteristics, properties and areas relevant to human spermatozoa will be addressed. Thus, in total, this review will: (1) provide a general description of capacitation and the acrosome reaction; (2) briefly characterize the acrosome and its properties; (3) address the signalling mechanism(s) by which the acrosome reaction occurs; (4) outline various methods for detecting the acrosome reaction; (5) definein vitroculture conditions that facilitate the acrosome reaction; (6) define agents, i.e., biological and chemical, that induce the reaction; (7) discuss the association of the acrosome reaction with fertilization; (8) discuss the clinical implications of the induced acrosome reaction; (9) discuss the relevance of sperm function assays in general; and lastly, (10) discuss future application of acrosome reaction assessment for the clinical diagnosis of male infertility.

Oral phosphodiesterase-5 inhibitors and sperm functions

This review aims to elucidate the possible effects of phosphodiesterase-5 (PDE5) inhibitors on sperm functions. PDEs hydrolyze cyclic nucleotides, and together with adenylyl and guanylyl cyclase, which catalyze the formation of cAMP and cGMP, regulate the levels of these second messengers in cells. cGMP-specific PDE5 is one of the PDEs that have been intensively studied because of its fundamental pharmacological relevance, as oral PDE5 inhibitors are used successfully in treating erectile dysfunction. In addition, they have shown diverse beneficial actions in different disease categories. Specific relevance of the cGMP system in reproductive functions has been recently proposed. Its use was shown to be devoid of effects on semen volume, concentration, sperm membrane integrity or sperm penetration assay. Most available studies demonstrated a significant increase in sperm motility and viability both in vivo and in vitro, which seems to be enhanced at low doses and reduced at high concentrations. Also, these molecules showed a role in capacitation and a debated one concerning acrosome reaction. However, due to the relative short period since the launching of oral PDE5 inhibitors, more investigations should be carried out in wider scales to assess their effect(s) on variant sperm function that could be beneficial as potential therapeutic approaches.

Effects of phosphodiesterase-5 inhibitors on sperm parameters and fertilizing capacity

The aim of this review study is to elucidate the effects that phosphodiesterase 5 (PDE5) inhibitors exert on spermatozoa motility, capacitation process and on their ability to fertilize the oocyte. Second messenger systems such as the cAMP/adenylate cyclase (AC) system and the cGMP/guanylate cyclase (GC) system appear to regulate sperm functions. Increased levels of intracytosolic cAMP result in an enhancement of sperm motility and viability. The stimulation of GC by low doses of nitric oxide (NO) leads to an improvement or maintenance of sperm motility, whereas higher concentrations have an adverse effect on sperm parameters. Several in vivo and in vitro studies have been carried out in order to examine whether PDE5 inhibitors affect positively or negatively sperm parameters and sperm fertilizing capacity. The results of these studies are controversial. Some of these studies demonstrate no significant effects of PDE5 inhibitors on the motility, viability, and morphology of spermatozoa collected from men that have been treated with PDE5 inhibitors. On the other hand, several studies demonstrate a positive effect of PDE5 inhibitors on sperm motility both in vivo and in vitro. In vitro studies of sildenafil citrate demonstrate a stimulatory effect on sperm motility with an increase in intracellular cAMP suggesting an inhibitory action of sildenafil citrate on a PDE isoform other than the PDE5. On the other hand, tadalafil's actions appear to be associated with the inhibitory effect of this compound on PDE11. In vivo studies in men treated with vardenafil in a daily basis demonstrated a significantly larger total number of spermatozoa per ejaculate, quantitative sperm motility, and qualitative sperm motility; it has been suggested that vardenafil administration enhances the secretory function of the prostate and subsequently increases the qualitative and quantitative motility of spermatozoa. The effect that PDE5 inhibitors exert on sperm parameters may lead to the improvement of the outcome of assisted reproductive technology (ART) programs. In the future PDE5 inhibitors might serve as adjunct therapeutical agents for the alleviation of male infertility.

Alterations in sperm protein phosphorylation in male infertility

Protein phosphorylation is involved in sperm capacitation, so the effect of protein phosphatase inhibitors on the capacitation of spermatozoa of males with unexplained infertility was investigated. d-mannose ligand specific receptor expression in fresh, living spermatozoa, capacitated or treated with calyculin A (an inhibitor of protein phosphatases 1 and 2A), was studied in three groups of men: pre-vasectomy (fertile) males, males in couples with male infertility, and males in couples with infertility of unknown aetiology. Flow cytometry showed significant differences between infertile couples with a male factor and fertile couples (P < 0.05), both after capacitation and after treatment with calyculin A. In the group of couples with infertility of unknown aetiology (n = 15), d-mannose receptor expression was diminished in six cases after classical capacitation. However, when the spermatozoa of these six men were treated with calyculin A, five showed an increased specific d-mannose receptor expression. From these results it is suggested that in vitro treatment of spermatozoa with inhibitors of protein phosphatases may be of great value in some cases of unexplained infertility.