The effects of exogenous amino acids on the relaxant responses of pig urethral smooth muscle evoked by stimulation of the inhibitory nitrergic nerves.

Inhibitory innervation of urethral smooth muscle is mediated partly through release of NO. We investigated the mechanisms involved in the supply of the substrate L: -arginine to NO synthase by examining the relaxant response of the muscle to electrical field stimulation (EFS) and the effects of addition of amino acids to the bathing medium. Relaxant responses persisted during hours of repetitive stimulation but were enhanced rapidly by addition of L: -arginine (the "arginine paradox"). Addition of L: -lysine (competes with L: -arginine for transport on the y(+) carrier) and L: -glutamine (competing on the y(+)L carrier) attenuated the enhancement. Enhancement persisted after washing but was reversed by application of L: -lysine, suggesting that exogenous L: -arginine fills an intracellular pool and that L: -lysine can trans-stimulate its efflux from the pool. After prolonged depolarization in high-K(+), Na(+)-free solution the relaxant response became purely nitrergic. Addition of L: -arginine during the exposure continued to enhance the subsequent responses but L: -glutamine added with L: -arginine, could no longer reduce this enhancement. The results show the arginine paradox in inhibitory nerves and suggest the involvement of y(+) and y(+)L carriers in the transport of L: -arginine.