MYOGENIC NITRIC-OXIDE SYNTHASE ACTIVITY IN CANINE LOWER ESOPHAGEAL SPHINCTER - MORPHOLOGICAL AND FUNCTIONAL EVIDENCE

1 Studies on canine lower oesophageal sphincter (LOS) evaluated the ex istence and function of a myogenic, nitric oxide synthase (NOS) by use of immunocytochemistry for NOS isozymes, NADPH-d histochemistry, [H-3 ]-L-arginine to [H-3]-L-citrulline transformation. In addition, functi onal studies in the muscle bath were performed. 2 Smooth muscle bundle s or freshly isolated smooth muscle cells of LOS were NADPH-d reactive but did not recognize some antibodies against neural, endothelial or inducible NOS. NADPH-d reactivity and immunoreactivity to a neural NOS antibody were colocalized in LOS enteric nerves. Muscle plasma membra ne-enriched fractions from fresh and cultured LOS cells converted [H-3 ]-L-arginine to [H-3]-L-citrulline; activity was mostly Ca2+/calmoduli n-dependent. 3 N-Nitro-L-arginine (L-NOARG) persistently increased ton e (blocked by L-arginine) in muscle strips despite blockade of nerve f unction. Nifedipine prevented or abolished L-NOARG-induced, but not ca rbachol-induced, contraction showing that tone increase by L-NOARG req uired functional L-Ca channels. 4 Membrane-bound, myogenic NOS in cani ne LOS may release NO continuously when Ca2(+) entry through L-Ca chan nels occurs under physiological conditions and thereby modulate tone i n LOS.