The sodium pump modulates the influence of I-Na on [Ca2+](i) transients inmouse ventricular myocytes

To investigate whether activity of the sarcolemmal Na pump modulates the in fluence of sodium current on excitation-contraction (E-C) coupling, we meas ured [Ca2+](i) transients (fluo-3) in single voltage-clamped mouse ventricu lar myocytes([Na+](pip) = 15 or 0 mM) when the Na pump was activated (4.4 m M Kb(o)(+)) and during abrupt inhibition of the pump by exposure to 0 K-o() with a rapid solution-switcher device. After induction of steady state [C a2+](i) transients by conditioning voltage pulses (0.25 Hz), inhibition of the Na pump for 1.5 s immediately before and continuing during a voltage pu lse (200 ms, -80 to 0 mV) caused a significant increase (15 +/- 2%; n = 16; p < 0.01) in peak systolic [Ca2+](i) when [Na+](pip) was 15 mM. In the abs ence of sodium current (I-Na, which was blocked by 60 <mu>M tetrodotoxin (T TX)), inhibition of the Na pump immediately before and during a voltage pul se did not result in an increase in peak systolic [Ca2+](i). Abrupt blockad e of I-Na during a single test pulse with mt caused a slight decrease in pe ak [Ca2+](i), whether the pump was active (9%) or inhibited (10%). With the reverse-mode Na/Ca exchange inhibited by KB-R 7943, inhibition of the Na p ump failed to increase the magnitude of the peak systolic [Ca2+](i) (4 +/- 1 %; p = NS) when [Na+](pip) was 15 mM. When [Na+](pip) was 0 mM, the ampli tude of the peak systolic [Ca2+](i) was not altered by abrupt inhibition of the Na pump immediately before and during a voltage pulse. These findings in adult mouse ventricular myocytes indicate the Na pump can modulate the i nfluence of I-Na on E-C coupling in a single beat and provide additional ev idence for the existence of Na fuzzy space, where [Na+] can significantly m odulate Ca2+ influx via reverse Na/Ca exchange.