Myogenic nitric oxide synthase activity in canine lower oesophageal sphincter: morphological and functional evidence

Abstract

Studies on canine lower oesophageal sphincter (LOS) evaluated the existence and function of a myogenic, nitric oxide synthase (NOS) by use of immunocytochemistry for NOS isozymes, NADPH-d histochemistry, [³H]-L-arginine to [³H]-L-citrulline transformation. In addition, functional studies in the muscle bath were performed.
Smooth muscle bundles 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 membrane-enriched fractions from fresh and cultured LOS cells converted [³H]-L-arginine to [³H]-L-citrulline; activity was mostly Ca²⁺/calmodulin-dependent.
N-Nitro-L-arginine (L-NOARG) persistently increased tone (blocked by L-arginine) in muscle strips despite blockade of nerve function. Nifedipine prevented or abolished L-NOARG-induced, but not carbachol-induced, contraction showing that tone increase by L-NOARG required functional L-Ca channels.
Membrane-bound, myogenic NOS in canine LOS may release NO continuously when Ca²⁺ entry through L-Ca channels occurs under physiological conditions and thereby modulate tone in LOS.

Keywords: Calcium, constitutive nitric oxide synthase, oesophageal sphincter, smooth muscle, tone