KB-R7943 block of Ca²⁺ influx via Na⁺/Ca²⁺ exchange does not alter twitches or glycoside inotropy but prevents Ca²⁺ overload in rat ventricular myocytes

Background - The Na⁺/Ca²⁺ exchange (NCX) extrudes Ca²⁺ from cardiac myocytes, but it can also mediate Ca²⁺ influx, load the sarcoplasmic reticulum with Ca²⁺, and trigger Ca²⁺ release from the sarcoplasmic reticulum. In ischemia/reperfusion or digitalis toxicity, increased levels of intracellular [Na⁺] ([Na⁺](i)) may raise levels of intracellular [Ca²⁺] ([Ca²⁺](i)) via NCX, leading to cell injury and arrhythmia. Methods and Results - We used KB-R7943 (KBR) to selectively block Ca²⁺ influx via NCX to study the role of NCX-mediated Ca²⁺ influx in intact rat ventricular myocytes. Removing extracellular Na⁺ caused [Ca²⁺](i) to rise, due to Ca²⁺ influx via NCX, and this was blocked by 90% with 5 μmol/L KBR. However, KBR did not alter [Ca²⁺](i), decline due to NCX. Thus, we used 5 μmol/L KBR to selectively block Ca²⁺ entry but not efflux via NCX. Under control conditions, 5 μmol/L KBR did not alter steady-state twitches, Ca²⁺ transients, Ca²⁺ load in the sarcoplasmic reticulum, or rest potentiation, but it did prolong the late low plateau of the rat action potential. When Na⁺/K⁺ ATPase was inhibited by strophanthidin, KBR reduced diastolic [Ca²⁺](i) and abolished the spontaneous Ca²⁺ oscillations, but it did not prevent inotropy. Conclusions - In rat ventricular myocytes, Ca²⁺ influx via NCX is not important for normal excitation-contraction coupling. Furthermore, the inhibition of Ca²⁺ efflux alone (as [Na⁺](i) rises) may be sufficient to cause glycoside inotropy. In contrast, Ca²⁺ overload and spontaneous activity at high [Na⁺](i) was blocked by KBR, suggesting that net Ca²⁺ influx (not merely reduced efflux) via NCX is involved in potentially arrhythmogenic Ca²⁺ overload.