The role of inward Na+-Ca2+ exchange current in the ferret ventricular action potential

1. Inward Na+-Ca2+ exchange current (i(NaCa)) was either blocked in ferret ventricular cells by replacing extracellular Na+ with Li+ or substantially reduced by the almost complete elimination of the Ca2+ transient by buffering intracellular Ca2+ with the acetoxymethyl ester form of BAPTA (BAPTA AM). 2. During square wave voltage clamp pulses to 0 mV, replacing extracellular Na+ with Li+ or buffering intracellular Ca2+ with BAPTA AM resulted in the loss of a transient inward current. This current was increased by the application of isoprenaline (expected to increase the underlying Ca2+ transient) and displayed the voltage-dependent characteristics of inward i(NaCa). 3. Replacing extracellular Na+ with Li+ or buffering intracellular Ca2+ caused a significant shortening of the action potential (at -65 mV, 44 +/- 2% with Li+ and 20 + 2% with BAPTA AM). The shortening can be explained by changes in i(NaCa). 4. The action potential clamp technique was used to measure the BAPTA-sensitive current (putative i(NaCa)) and the Ca2+ current (i(Ca); measured using nifedipine) during the action potential. Under control conditions, the inward BAPTA-sensitive current makes approximately the same contribution as i(Ca) during much of the action potential plateau. These results suggest an important role for inward i(NaCa) in the ferret ventricular action potential.