Preponderance of postsystolic period in the positive inotropic effects of increases in (Ca++).

If the frequency of stimulation of isolated kitten right ventricular papillary muscles is progressively reduced, the size of contractions decreases until a frequency is reached (rested state rate) where further reduction produces no change (rested state contraction, Koch-Weser and Blinks, 1963). At 30 degrees C and 2.25 mM Ca++, the rested state rate was about one per 2-5 min, the peak tension of 0.2-0.6 g/mm2 was about 5-15% of the tension of 20/min beats, and times to peak tension and 80% repolarization about doubled. Sudden calcium increases in the perfusion fluid, [Ca++]p, from 2.25 to 6.75 mM were produced at random during intervals between rested state contractions, and time and amplitude of peak isometric tension, max +/- dT/dt of subsequent beats were studied. The greatest positive inotropic response of the initial beats following increments in [Ca++]p resulted when [Ca++]p was increased immediately after a rested state contraction: as the postsystolic interval before the [Ca++]p increment was increased, the positive inotropic effects decreased approximately exponentially, attaining a plateau level in about 30 sec. This plateau was then maintained until the period prior to the initial test beat in high calcium was less than the approximately 2-min diffusion equilibrium time between [Ca++]p and the interior of the fiber.