The inotropic memory of amphibian myocardium. I.-Identification of two stimultaneous mechanisms and statement of a model.
(1) The effects of previous contractions on the actual contractile strength is studied in strips of toad ventricle. The inotropic effect is quantified by superposing a conditioning contraction to a rhythm of definite frequency, its measure being the difference in strength between a rhythm contraction in the presence and in the absence of the conditioning one. (2) The inotropic effect is studied as a function of the interval between the actual and conditioning contractions. Depressed sections of the curve, associated to shortened action potentials, are detected and excluded. (3) The inotropic effect is always positive at low frequencies, but at higher frequencies and long intervals it becomes negative. (4)In high calcium concentration the inotropic effect is always negative and does not depend on frequency. Morever, the joint effect of two previous contractions is equal to the sum of the individual effects of each one. (5) The results are interpreted in terms of two independent elementary processes, one of which potentiates whereaas the other inhibits the strength of contraction. The former disappears in high calcium. Assuming some simple properties for these processes a mathematical expression has been achieved. This expression describes the inotropic effect of any sequence of contractions as a function of intervals involved.
D000200:Action Potentials; D000818:Animals; D002022:Bufo arenarum; D002118:Calcium / Q000494:pharmacology; D004558:Electric Stimulation; D006321:Heart / Q000502:physiology*; D008433:Mathematics; D008954:Models, Biological; D009200:Myocardial Contraction / Q000187:drug effects*
D002118:Calcium