Promiscuous coupling and involvement of protein kinase C and extracellular signal-regulated kinase 1/2 in the adenosine A1 receptor signalling in mammalian spermatozoa

Mammalian spermatozoa require a maturational event after ejaculation that allows them to acquire the capacity for fertilisation. This process occurs spontaneously during the transit through the female reproductive tract where spermatozoa are in contact with micromolar concentrations of adenosine that might act as a capacitative effector. This study shows that the adenosine A1 receptor agonist, 2-chloro-N6-cyclopentyladenosine, can induce capacitation, i.e., the ability to undergo the acrosome reaction and to become fertile. This receptor, already known to be bound to Galpha(i2), is also bound to G(q/11). These G proteins are functional in the signalling pathway elicited by the A1 receptor and correlate with the multiple intracellular events that follow its activation. The use of protein kinase C isoform inhibitors and MEK inhibitors, resulting in the abolition of the biological response to the selective agonist, indicates the involvement of protein kinase C and MEK in its signalling. In agonist-treated spermatozoa an extracellular calcium influx, involvement of alpha and gamma PKC isoforms and transient phosphorylation of ERK1/2 have been observed. Our results, besides showing that adenosine A1 receptor prompts mammalian spermatozoa to undergo the acrosome reaction hence supporting a role for adenosine as agent for fertilisation, show that 2-chloro-N6-cyclopentyladenosine triggers signalling mechanisms that involve both Galpha(i2) and G(q/11), extracellular calcium influx, modulation of classical Ca2+-dependent PCK isoforms and up-regulation of the ERK1/2 phosphorylation.