[Autoregulation of the contractile function of the myocardium in response to changes in the concentration of the activating cation]

In guinea-pig isolated papillary muscles, dependence of the isotonic contraction velocity on the perfusate Ca++ concentration was studied. The dependence was the steepest at the lowest rate of stimulation. At a high rate (2 Hz), changes in contraction velocity as well as in extent of the muscle shortening were minimal. Replacement of a half of Ca++ ions by Sr++ resulted in an augmentation of steepness of the dependence on sum of [Ca++] and [Sp++], and in a more prominent fall in relaxation velocity as compared with contraction velocity. Replacement of a half of Ca++ ions with Ba++ or depletion of [Na+] in usual solution to 25 mM resulted in a prominent fall in relaxation velocity while the contraction velocity increased. This dissociation coincided with signs of cellular Ca++ overload and a more steep restitution of contraction velocity during diastole. Results are discussed in terms of participation of Na+-Ca++ exchange in these self-regulatory phenomena.