title: Preponderance of postsystolic period in the positive inotropic effects of increases in (Ca++).
abstract: 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.
mesh_terms: D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002415:Cats; D002417:Cattle; D006339:Heart Rate / Q000187:drug effects; D006801:Humans; D009200:Myocardial Contraction / Q000187:drug effects*; D010210:Papillary Muscles / Q000187:drug effects; D012756:Sheep; D013268:Stimulation, Chemical
chemical_list: D002118:Calcium