Different temperature sensitivity of cardiac Na+ channels in cell-attached and cell-free conditions

At 19 degrees C and after cooling to 9 degrees C, elementary Na+ currents were recorded in cell-attached and inside-out patches from cultured neonatal rat myocytes. The study aimed to define the temperature sensitivity of single cardiac Na+ channels and to detect a possible modulating influence arising from the recording conditions. Cooling prolonged the open state by reducing the exit rate into a nonconducting configuration [tau open; temperature coefficient (Q10) = 1.90 +/- 0.02] and reduced the unitary current size (i(unit); Q10 = 1.26 +/- 0.03). Reopening proved to be only slightly temperature dependent; the mean number of sequential openings decreased to 91 +/- 2.1% of the value at 19 degrees C. Cooled Na+ channels are still capable of switching to ultralong sequential activity. Cooling reduced reconstructed peak Na+ current (INa) strongly, with Q10 values between 1.85 and 3.11 (mean, 2.36 +/- 0.29), if Na+ channels remained in their natural environment as was the case in cell-attached recording conditions. However, after disrupting the cellular contact, in inside-out patches, the same 10 degrees C drop in temperature diminished peak INa only slightly to 91 +/- 8.1% of the initial control value at 19 degrees C. This suggests a modulating influence on the open probability of cardiac Na+ channels arising from the cytosol of cooled myocytes.