Gamete Biology

Adenosine and Catecholamine Agonists Speed the Flagellar Beat of Mammalian Sperm by a Non-Receptor-Mediated Mechanism

Abstract
Activation of rapid motility apparently is one of the first steps of sperm capacitation and can be studied in vitro. Our past work found that 2-chloro-2'-deoxyadenosine or the catecholamine isoproterenol activate mouse sperm motility in vitro via a pathway mediated by cAMP that requires extracellular Ca²⁺, the atypical sperm adenylyl cyclase, and sperm-specific protein kinase A. We now show that several other adenosine analogs and catecholamines accelerate the flagellar beat of mouse and human sperm. Unexpectedly, the potent adenosine receptor agonist CGS21680 does not accelerate the beat, and the adenosine receptor antagonist DPCPX does not diminish the accelerating action of 2-chloro-2'-deoxyadenosine. The pharmacological profile for activation by catecholamines also is unusual. Both agonists and antagonists of beta-adrenergic receptors elevate the beat frequency. Moreover, both l- (-) and d-(+) isomers of epinephrine, norepinephrine, and isoproterenol produce similar acceleration of the beat. In contrast, inhibitors of equilibrative nucleoside transporters effectively slow the onset of the accelerating action of adenosine analogs. Replacement of external Na⁺ with Li⁺ also diminishes the accumulation of cAMP and slows the resultant accelerating action of 2-chloro-2'-deoxyadenosine, suggesting the involvement of a Na⁺-dependent concentrative nucleoside transporter. Our results show that adenosine and catecholamine agonists act in a novel signaling pathway that does not involve G protein-coupled cell-surface receptors that link to conventional adenylyl cyclases. Instead, adenosine and analogs may be transported into sperm via equilibrative and concentrative nucleoside transporters to act on unknown intracellular targets.

Key words: Catecholamines • Cyclic adenosine monophosphate • Sperm capacitation • Sperm motility and transport • nucleoside transporter