Molecular mechanism of KCl-induced relaxation of the esophagus

KCl (40 mM) caused reproducible relaxations in frog esophagus. NG-nitro-l-arginine (L-NOARG; 1–100 µM), a steriospecific inhibitor of nitric oxide synthase (NOS), completely inhibited the relaxations induced by KCl but not those induced by vasoactive intestinal polypeptide (VIP) antagonist. The inhibitory effect of L-NOARG was prevented by l-arginine (L-ARG; 0.1–1 mM), the precursor of nitric oxide (NO) biosynthesis, but not by d-arginine (D-ARG; 0.1–0.5 mM), the enantiomer of l-arginine. L-ARG or D-ARG alone did not significantly modify the effect of KCl. The relaxations to KCl were significantly inhibited by omega conotoxin (ω-conotoxin; 0.1 μM), a selective blocker of N-type calcium channels. Propranolol (0.1–1 µM), a nonselective blocker of β-adrenergic receptors, prazosine (0.01–0.1 µM), a selective blocker of α1-adrenergic receptors, phentolamine (0.1–1 µM), a nonselective blocker of adrenergic receptors, atropine, a selective blocker of muscarinic cholinergic receptors, and lidocaine (1–10 µM), a blocker of sodium channels, had no effect on KCl-evoked relaxations. Caffeine (500 µM), an intracellular calcium releasing agent, did not significantly modify the effect of KCl. In contrast, ruthenium red (100 µM), a selective blocker of ryanodine receptors (intracellular Ca2+ channels), significantly inhibited these relaxations. Similarly, potassium channel blockers such as 4-aminopyridine (4-AP; 100 µM) and tetraethylammonium (TEA; 100 µM) caused a significant inhibition on relaxations to KCl. In addition, ouabain (100 µM), a specific blocker of Na+–K+–ATPase, also caused a significant inhibition on these relaxations. The results suggest that NO, Na+–K+–ATPase and potassium channels may have a role on relaxations induced by 40 mM KCl in the frog esophagus.