L-ARGININE DEFICIENCY CAUSES SUPPRESSION OF NONADRENERGIC NONCHOLINERGIC NERVE-MEDIATED SMOOTH-MUSCLE RELAXATION - ROLE OF L-CITRULLINE RECYCLING

Studies were performed on the internal anal sphincter (IAS) smooth mus cle strips obtained from opossums (Didelphis virginiana). Isometric te nsion and L-arginine levels of the tissues were measured under basal c onditions, in the presence of electrical field stimulation (EFS) and a fter treatment with different concentrations of arginase. For the nona drenergic noncholinergic nerve stimulation, short trains (4 sec) as we ll as continuous EFS were used. During continuous EFS, after the initi al IAS relaxation, the response began to fade within several min to si milar to 80% recovery of the basal tone. We also examined the influenc e of L-arginine and L-citrulline on these responses. For some studies, the tissues were pretreated with L-glutamine (an inhibitor of L-citru lline uptake), L-glutamate or N-G-hydroxy-L-arginine (an inhibitor of arginase). Interestingly, the basal levels of L-arginine were found to be significantly higher in the IAS (tonic smooth muscle) than in the rectal (phasic smooth muscle) smooth muscle. Arginase caused a concent ration-dependent attenuation of the IAS relaxation caused by EFS. L-Ci trulline and L-arginine were equipotent in reversing the attenuation. Both arginase (60 min pretreatment) and continuous EFS (tissues collec ted at the time of maximal recovery of the basal IAS tone after the in itial relaxation) caused significant decreases in L-arginine levels. T he decreases in the levels of L-arginine were restored by the exogenou s administration of either L-arginine or L-citrulline. The restoration of L-arginine levels by L-citrulline but not by L-arginine was select ively blocked by L-glutamine. Furthermore, the IAS relaxation, attenua ted by arginase was unaffected by L-glutamine but was restored by N-G- hydroxy-L-arginine pretreatment. The studies suggest that L-citrulline -L-arginine recycling may play a significant role in the maintenance o f IAS relaxation in response to nonadrenergic noncholinergic nerve sti mulation.