Abstract
In isolated papillary muscles of guinea pigs the time-dependent changes of the Ca-mediated action potential due to variations of the external Ca concentration were analyzed under different experimental conditions. After an increase of the external Ca concentration from 0.2 mM to 2 mM, upstroke velocity and overshoot of the Ca-mediated action potential attained their steady state values with time constants ranging from 80 to 200 s (τregeneration). A decrease of temperature from 35°C to 30°C led to an increase of τregeneration and temperature coefficients between 1.8 and 2.0 were calculated. The β-stimulating compound isoproterenol (1 mg/l) caused a decrease up to 11 s that is prevented by pretreatment with the β-receptor blocking agent pindolol (2 mg/l). This accelerating effect of isoproterenol on the regeneration of the Ca-mediated action potential was mimicked by theophylline. After switching to a Ca-poor (0.2 mM) solution, the Ca-mediated action potential disappeared within 50–120 s. This time of persistence in a Ca-poor environment was abbreviated by lowering the extracellular pH. Isoproterenol and theophylline prolonged the time of persistence. The same effect occurred after interruption of the continuous stimulation during the washout of Ca. The sensitivity of the Ca-mediated action potential towards Ca-withdrawal decreased when the Ca concentration prior to switching to the Ca-poor solution was higher than 2 mM. But there was no linear dependence of the time of persistence on the initial Ca concentration. Rather a saturation characteristic was found. The results suggest that the actual charge carrier of the slow inward current, Ca ions, might originate from a superficially located Ca pool of the cardiac membrane.
Published Version
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