A comparative study on the electrical and mechanical properties of human and guinea pig atrial muscles in vitro

: The transmembrane action potential and contractile force of isolated human atrial muscles which were obtained during cardiac surgery by using cardio-pulmonary by-pass technique here studied, compared with those of guinea pig left atrium. Force-requency curves showed a characteristic triphasic pattern, just as other mammalian atria. When extracellular calcium concentration was raised, percent change in contractile force was strengthened at high frequency of stimulation. When extracellular potassium concentration was reduced, contractile force became stronger and showed a monophasic pattern in response to increased frequency of stimulation. In a solution of lower ouabain concentration, action potential leaved unchanged or showed larger amplitude and longer durati-n. In a solution of higher or "therapeutic" concentration of ouabain, the duration of action potential was shortended, and in its "toxic" concentration, marked decreases in amplitude and duration were induced. The positive inotropic effect of ouabain was attributable to an increase in intensity of active state, because the rate of development of tension was increased without any significant change in the time to peak tension. When extracellular potassium concentration was reduced, contractile force became maximum at louer conce,tration of ouabain, and further increase of concentration induced ectopic impulse formation. In other words, atrail muscles cecame more sensitive to ouabain in a solution with decreased extracellular potassium concentration.