[Effect of free fatty acids on the heart: effects of nonanoic acid on contractility and postextrasystolic potentiation in the rat papillary muscle].

1. Nonanoic acid causes a depression in the contractile force of rat papillary muscles working isometrically. The effect depends on both the concentration of the fatty acid (0.1-1 mM) and the time of exposure (2-12 min), and shows a nearly complete reversibility. 2. Vmax-values derived from the force-velocity relation as well as the index of relation suggested by Meerson do not change in muscles exposed to nonanoic acid. During paired pulse stimulation with a 400 ms-delay of the second impulse, both the postextrasystolic potentiation and the time to reach a new steady level in the contraction amplitude are significantly increased. The speed of restituting the contraction after a twitch (resitution) is descreased. 4. The results suggest that the action site of nonanoic acid may be the excitation-contraction coupling system (including the action potential) rather than the contractile element or the relaxation of the muscle.