P2X Receptor Currents in Smooth Muscle Cells Contribute to Nerve Mediated Contractions of Rabbit Urethral Smooth Muscle

Purpose: Adenosine triphosphate is capable of relaxing and contracting urethral smooth muscle. The mechanisms responsible for the relaxing effects of adenosine triphosphate have been well studied but those involved in the contractile response are still unclear. We investigated the contributions of interstitial cells of Cajal and smooth muscle cells to nerve mediated, adenosine triphosphate dependent contractions of urethral smooth muscle. Materials and Methods: Tension recordings were made from strips of rabbit urethral smooth muscle. Recordings were made of membrane potential and ionic currents from freshly isolated smooth muscle cells and interstitial cells of Cajal using the patch clamp technique. Results: Stimulating intramural nerves in urethral smooth muscle yielded contractions that were inhibited by the broad spectrum P2 receptor inhibitor pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and the P2X receptor agonist alpha,beta methylene adenosine triphosphate but not by the P2Y receptor antagonist MRS2500. When studied under voltage clamp at a holding potential of -60 mV, interstitial cells of Cajal showed spontaneous transient inward currents that were increased in frequency by adenosine triphosphate but not by alpha,beta-methylene adenosine triphosphate. In contrast, smooth muscle cells were quiescent but responded to adenosine triphosphate and alpha,beta-methylene adenosine triphosphate by producing a single transient inward current. Currents evoked by adenosine triphosphate in smooth muscle cells were inhibited by alpha,beta-methylene adenosine triphosphate, pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate and suramin, and by a decrease in extracellular Na+ from 130 to 13 mM. Conclusions: Stimulating purinergic nerves in rabbit urethral smooth muscle induces contractions via the activation of P2X receptors on smooth muscle cells.