The action of 2,3‐butanedionemonoxime on the inotropic state in guinea‐pig myocardium

Isolated papillary muscles from guinea-pig right ventricles were used (temperature 33 degrees C, stimulation frequency 0.5 Hz). Isometric twitch and action potentials were recorded. Upon addition of 2,3-butane-dionemonoxime (BDM) (2 mM) the peak twitch force was reduced from 4.17 +/- 0.4 mN/mm2 to 1.68 +/- 0.3 mN/mm2 (n = 9, P less than 0.001). The time course of the isometric twitch was slightly altered. Time to peak tension (TPT) was reduced by 12.0 +/- 3% (n = 9, P less than 0.001) whereas time to half relaxation (THR) was left unaffected. The rate of rise of force was reduced by 35 +/- 3.2 mN/mm2s i.e. 46 +/- 3%. The action potential duration and amplitude was not significantly changed by the drug. The shape of the curve relating peak twitch force of an extra beat to the preceding test interval, i.e. mechanical restitution, was affected by 2 mM 2,3-butane-dionemonoxime. The curve reached its maximum faster after addition of the drug. Maximum postextrasystolic potentiation (force in response to the prepreceding test interval) was 3.2 +/- 0.4 mN/mm2 in 2 mM 2,3-butane-dionemonoxine and 7.6 +/- 0.7 mN/mm2 in control (n = 6). However the percentage potentiation was very similar in control (82%) and in presence of 2,3-butane-dionemonoxime (91%). Peak twitch force in relation to peak force of the preceding potentiated contraction during decay of postextrasystolic potentiation was analysed. There was a linear relation between the variables, the slope being 0.34 +/- 0.04 in control and 0.30 +/- 0.02 in 2,3-butane-dionemonoxime. This suggests that the drug is without an action on the fraction of calcium recirculating within the cell.(ABSTRACT TRUNCATED AT 250 WORDS)