Role of Calcium Loading in Early Afterdepolarizations Generated by Cs+ in Canine and Guinea Pig Purkinje Fibers

Ca²⁺-Loading and EAD. introduction: Our previous observations indicate that the Na²⁺:Ca²⁺ exchange current (I_Na:Ca) plays an important role in early afterdepolarizations occurring at more negative Vm (L-EAD). The purpose of these studies was to examine the role of Ca²⁺-loading, which stimulates I_Na:Ca, in generation of L-EAD. Methods and Results: Purkinje strands and preparations of ventricular myocardium from dogs and guinea pigs were superfused with oxygenated physiologic buffer solutions at 37°C, To induce EADs, K⁺]₀ was reduced to 2.0 to 3.0 mM and Cs⁺]₀, (3.6 to 4.0 mM) was added at slow rates of ≤ 0.3 Hz. Isometric contraction in canine Purkinje strands and guinea pig papillary muscles doubled in 1-hour exposure to Cs⁺ and low K⁺]₀ at slow rates, and the uptake of ⁴⁵Ca²⁺ was approximately doubled after 30 minutes. Forty-three percent of Purkinje fibers developed L-EAD after a latent period of 17 to 123 minutes of exposure. Ouabain (0.2 μM) suppressed LEAD within 10 minutes reversibly. Ca²⁺-loading (low Na⁺]₀ or high Ca²⁺]₀ for 5 to 10 minutes before exposure to Cs⁺, low K⁺]₀, and slow rates resulted in rapid development of L-EAD in all preparations during subsequent exposure. In Ca²⁺-loaded preparations, delayed afterdepolarizations (DADs) as well as L-EADs developed. Conclusion: Reduction of K⁺ currents with Cs⁺, low K⁺]₀, and slow rates induced L-EAD in a fraction of Purkinje fibers after a latent period during which Ca²⁺-loading of the sarcoplasmic reticulum occurred, while fibers preloaded with Ca²⁺ developed L-EAD rapidly and uniformly. These findings indicate that Ca²⁺-loading is a critical condition for the development of L-EAD. Early suppression of L-EAD by ouabain suggests a dependence of L-EAD on low Na⁺]₁. These findings implicate I_Na:Ca in the generation of L-EAD.