A continuous 400 pA Na current does not cause a detectable rise of subsarcolemmal Na. (A) The cartoon summarizes patch-clamp conditions and the predicted 5.6 mM cytoplasmic Na concentration during a continuous 0.38 nA Na current. (B) Veratridine was applied in the absence of Na, and the subsequent application of 120 mM Na activated a 0.38 nA inward current. Na/K pump currents activated by applying 7 mM K were at the threshold of reliable detection, indicating that subsarcolemmal Na remains with good certainty less than 10 mM. (C) Current–voltage relations given by the indicated subtractions do not reverse up to 80 mV, verifying that subsarcolemmal Na remains <10 mM.