Studies on heart. XIX. Isolation of an atrial peptide that improves the rhythmicity of cultured myocardial cell clusters.

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Abstract
A new peptide that improves the rhythmicity of cultured myocardial cell clusters, designated as AAP (antiarrhythmic peptide), has been discovered and isolated in a pure form from bovine atria by extraction with boiling water at pH 4.5, followed by column chromatography procedures, two-dimensional paper chromatography and electrophoresis. The yield was 200 μg per 1.0 kg of wet tissues. The homogeneity of this peptide was confirmed by the chromatographic and/or electrophoretic behavior of the native peptide and its dansyl derivative. It was found that AAP is a hexapeptide composed of hydroxyproline (1), proline (1), glycine (3) and alanine (1), from the results of amino acid analysis and apparent molecular weight determination on a Sephadex G-15 column. The NH2-terminal amino acid was glycine as determined by dansylation, and the COOH-terminus was also glycine (carboxypeptidase A digestion and dansylation after hydrazinolysis of AAP). This peptide changed the rhythmic beatings with continuous cellular fibrillation of myocardial cell clusters induced by 0.7 mM potassium, 3 mM calcium or 0.05 mM ouabain in modified Eagle's minimum essential medium supplemented with 2.5% bovine serum to normal rhythmic beatings, and changed irregular beatings with cellular fibrillation at 0.5 mM potassium or 5 mM calcium to rhythmic beatings with cellular fibrillation, followed by normal beatings. This peptide also restored the arrhythmic movements of isolated rat atrium induced by low potassium concentration and addition of acetylcholine to a normal rhythm, in the same way as quinidine. The activities of this peptide on myocardial cells and isolated atrium were stronger than those of quinidine. No antiarrhythmic activity was found in amino acids, peptides and proteins chemically related to AAP, such as hydroxyproline, proline, glycine, alanine, Gly-Pro, Gly-Pro-Leu, Gly-Pro-Leu-Gly-Pro, gelatin and heat-treated gelatin.
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