Comparative mechanical and electrical actions of A23187 and X-537A in canine Purkinje fibers.

1. At concentrations lower than 10(-5) M, A23187 did not affect the contractile force in canine Purkinje fibers, but at 2 x 10(-5) M, decreased it significantly. 2. X-537A (10(-6) M) slightly increased the contractile force. The increase was not modified by 10(-7) M pindolol. 3. At 20 min after application both ionophores (2 x 10(-5) M) affected little or no changes in the action potential configurations, except for a marked shortening of the action potential duration. A delayed afterdepolarization and an aftercontraction occurred. 4. The electrophysiological effects were potentiated with an increase in extracellular Ca2+ and further by isoproterenol (1-3 x 10(-7) M). 5. A post-rest potentiation was depressed in the presence of these Ca2+ ionophores. 6. When the stimulation frequency was stepped up from 120 to 180 beats/min, the negative staircase of the contractile force was reversed to positive one in the presence of A23187, high Ca2+ and isoproterenol. 7. These results indicate that in canine Purkinje fibers, X-537A produces cellular Ca2+ overload, but A23187 alone does not cause it. Mechanical and electrophysiological effects induced by these ionophores are potentiated by addition of high Ca2+ and isoproterenol.