Evidence for differential roles of nitric oxide (NO) and hyperpolarization in endothelium‐dependent relaxation of pig isolated coronary artery

<jats:p><jats:list list-type="explicit-label"> <jats:list-item><jats:p>The possible roles of endothelial and smooth muscle cell hyperpolarization and nitric oxide (NO) in endothelium‐dependent relaxation were examined in isolated rings of pig right coronary artery.</jats:p></jats:list-item> <jats:list-item><jats:p>The effects of hyperpolarization were prevented with high K<jats:sup>+</jats:sup> (30–125 m<jats:sc>m</jats:sc>), isotonic Krebs solutions. Functional antagonism due to high K<jats:sup>+</jats:sup>‐induced smooth muscle contraction was prevented with 0.3 μ<jats:sc>m</jats:sc> nifedipine (in all treatments, for consistency). All rings were contracted with the thromboxane‐mimetic U46619, (1 − 100 n<jats:sc>m</jats:sc>) to bring them to an initial active force of within 30–50% of maximum contraction.</jats:p></jats:list-item> <jats:list-item><jats:p>High K<jats:sup>+</jats:sup> had no effects on the sensitivity (EC<jats:sub>50</jats:sub>) or time course of endothelium‐dependent (substance P, SP; bradykinin, BK; calcimycin, A23187) and ‐independent (sodium nitroprusside, SNP) agents. Maximum relaxations (<jats:italic>R</jats:italic><jats:sub>max</jats:sub>) to SP, BK and A23187 were reduced significantly by approximately 20% but only with 125 m<jats:sc>m</jats:sc> K<jats:sup>+</jats:sup>.</jats:p></jats:list-item> <jats:list-item><jats:p>In normal K<jats:sup>+</jats:sup> Krebs solution (5.9 m<jats:sc>m</jats:sc>), N<jats:sup>G</jats:sup>‐nitro‐<jats:sc>l</jats:sc>‐arginine (<jats:sc>l</jats:sc>‐NOARG; 100 μ<jats:sc>m</jats:sc>) caused 40%, 20% and no reduction in <jats:italic>R</jats:italic><jats:sub>max</jats:sub> for SP, BK and SNP respectively. EC<jats:sub>50</jats:sub>s for SP and BK were decreased significantly by approximately two fold whereas that for SNP was increased significantly by approximately ten fold. At all high K<jats:sup>+</jats:sup> concentrations (30–125 m<jats:sc>m</jats:sc>), <jats:sc>l</jats:sc>‐NOARG (100 μ<jats:sc>m</jats:sc>) caused complete inhibition of relaxations to SP and BK but those to SNP were unaffected.</jats:p></jats:list-item> <jats:list-item><jats:p>High K<jats:sup>+</jats:sup> (30 m<jats:sc>m</jats:sc>) unmasked potent and concentration‐dependent inhibition of relaxations of SP by <jats:sc>l</jats:sc>‐NOARG. At 10 μ<jats:sc>m l</jats:sc>‐NOARG, all relaxation responses to SP were abolished and at the higher concentrations of SP (1 − 10 n<jats:sc>m</jats:sc>) small but significant contractions were observed.</jats:p></jats:list-item> <jats:list-item><jats:p>N<jats:sup>G</jats:sup>‐monomethyl‐<jats:sc>l</jats:sc>‐arginine (<jats:sc>l</jats:sc>‐NMMA) had similar effects on relaxations to SP in the presence of 30 m<jats:sc>m</jats:sc> K<jats:sup>+</jats:sup> except that maximum inhibition (40%) of <jats:italic>R</jats:italic><jats:sub>max</jats:sub> was achieved at 10 μ<jats:sc>m l</jats:sc>‐NMMA and this was not increased with either 100 or 1000 μ<jats:sc>m l</jats:sc>‐NMMA. In normal K<jats:sup>+</jats:sup>, <jats:sc>l</jats:sc>‐NMMA (1000 μ<jats:sc>m</jats:sc>) only decreased the EC<jats:sub>50</jats:sub> by approximately two fold, without affecting <jats:italic>R</jats:italic><jats:sub>max</jats:sub>.</jats:p></jats:list-item> <jats:list-item><jats:p>High choline<jats:sup>+</jats:sup> (25, 75 and 125 m<jats:sc>m</jats:sc>) isotonic Krebs also had no direct effect on the relaxations to SP, but like high K<jats:sup>+</jats:sup>, enabled <jats:sc>l</jats:sc>‐NOARG (100 μ<jats:sc>m</jats:sc>) to inhibit these responses completely. Neither charyb‐dotoxin (30 n<jats:sc>m</jats:sc>) nor substitution of 25 m<jats:sc>m</jats:sc> NaCl with 50 m<jats:sc>m</jats:sc> sucrose had any direct effect on relaxations to SP or on the block of relaxations to SP by <jats:sc>l</jats:sc>‐NOARG (100 μ<jats:sc>m</jats:sc>).</jats:p></jats:list-item> <jats:list-item><jats:p>In conclusion, most if not all of the endothelium‐dependent relaxation in the pig coronary artery <jats:italic>in vitro</jats:italic> is due to NO, but hyperpolarization can supplement 60%–80% of this response if NO synthesis is blocked. Multiple endothelium‐derived factors could not only explain heterogeneity of the degree of block of endothelium‐dependent relaxation responses by <jats:sc>l</jats:sc>‐arginine analogues, but also constitute important ‘back‐up’ mechanisms for control of arterial diameter.</jats:p></jats:list-item> </jats:list></jats:p>