ABSTRACT: BACKGROUND AND PURPOSE: Nitric oxide (NO) production and depression of neuromuscular transmission are closely related, but little is known about the role of L-citrulline, a co-product of NO biosynthesis, on neurotransmitter release. EXPERIMENTAL APPROACH: Muscle tension recordings and outflow experiments were performed on rat phrenic nerve-hemidiaphragm preparations stimulated electrically. KEY RESULTS: L-citrulline concentration-dependently inhibited evoked [(3)H]ACh release from motor nerve terminals and depressed nerve-evoked muscle contractions. The NO synthase (NOS) substrate, L-arginine, and the NO donor, 3-morpholinosydnonimine chloride (SIN-1), also inhibited [(3)H]ACh release with a potency order of SIN-1>L-arginine>L-citrulline. Co-application of L-citrulline and SIN-1 caused additive effects. NOS inactivation with N(omega)-nitro-L-arginine prevented L-arginine inhibition, but not that of L-citrulline. The NO scavenger, haemoglobin, abolished inhibition of [(3)H]ACh release caused by SIN-1, but not that caused by L-arginine. Inactivation of guanylyl cyclase with 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) fully blocked SIN-1 inhibition, but only partially attenuated the effects of L-arginine. Reduction of extracellular adenosine accumulation with adenosine deaminase or with the nucleoside transport inhibitor, S-(p-nitrobenzyl)-6-thioinosine, attenuated the effects of L-arginine and L-citrulline, while not affecting inhibition by SIN-1. Similar results were obtained with the selective adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. L-citrulline increased the resting extracellular concentration of adenosine, without changing that of the adenine nucleotides. CONCLUSIONS AND IMPLICATIONS: NOS catalyses the formation of two neuronally active products, NO and L-citrulline. While, NO may directly reduce transmitter release through stimulation of soluble guanylyl cyclase, the inhibitory action of L-citrulline may be indirect through increasing adenosine outflow and subsequently activating inhibitory A(1) receptors.
