KB-R7943 Block of Ca 2+ Influx Via Na + /Ca 2+ Exchange Does Not Alter Twitches or Glycoside Inotropy but Prevents Ca 2+ Overload in Rat Ventricular Myocytes

BACKGROUND: The Na(+)/Ca(2+) exchange (NCX) extrudes Ca(2+) from cardiac myocytes, but it can also mediate Ca(2+) influx, load the sarcoplasmic reticulum with Ca(2+), and trigger Ca(2+) release from the sarcoplasmic reticulum. In ischemia/reperfusion or digitalis toxicity, increased levels of intracellular Na(+) (Na(+)(i)) may raise levels of intracellular Ca(2+) (Ca(2+)(i)) via NCX, leading to cell injury and arrhythmia. METHODS AND RESULTS: We used KB-R7943 (KBR) to selectively block Ca(2+) influx via NCX to study the role of NCX-mediated Ca(2+) influx in intact rat ventricular myocytes. Removing extracellular Na(+) caused Ca(2+)(i) to rise, due to Ca(2+) influx via NCX, and this was blocked by 90% with 5 micromol/L KBR. However, KBR did not alter Ca(2+)(i) decline due to NCX. Thus, we used 5 micromol/L KBR to selectively block Ca(2+) entry but not efflux via NCX. Under control conditions, 5 micromol/L KBR did not alter steady-state twitches, Ca(2+) transients, Ca(2+) 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 Ca(2+)(i) and abolished the spontaneous Ca(2+) oscillations, but it did not prevent inotropy. CONCLUSIONS: In rat ventricular myocytes, Ca(2+) influx via NCX is not important for normal excitation-contraction coupling. Furthermore, the inhibition of Ca(2+) efflux alone (as Na(+)(i) rises) may be sufficient to cause glycoside inotropy. In contrast, Ca(2+) overload and spontaneous activity at high Na(+)(i) was blocked by KBR, suggesting that net Ca(2+) influx (not merely reduced efflux) via NCX is involved in potentially arrhythmogenic Ca(2+) overload.