Published online before print February 16, 2006, doi: 10.1161/01.RES.0000209963.02720.70
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Submitted on December 12, 2005
Revised on January 18, 2006
Accepted on February 2, 2006
No Apparent Requirement for Neuronal Sodium Channels in Excitation-Contraction Coupling in Rat Ventricular Myocytes
Fabien Brette * and Clive H. Orchard
From the Department of Physiology, University of Bristol, United Kingdom.
* To whom correspondence should be addressed. E-mail: f.brette{at}bristol.ac.uk.
The majority of Na channels in the heart are composed of the tetrodotoxin (TTX)-resistant (KD, 2 to 6 µmol/L) "cardiac" NaV1.5 isoform; however, TTX-sensitive (KD, 1 to 25 nmol/L) "neuronal" Na channel isoforms have recently been detected in several cardiac preparations. In the present study, we determined the functional subcellular localization of Na channel isoforms (according to their TTX sensitivity) in rat ventricular myocytes by recording INa in control and detubulated myocytes. We found that TTX-sensitive INa (KD, 
8.8 nmol/L) makes up 14±3% of total INa in control and 
4% in detubulated myocytes and calculated that 80% of TTX-sensitive INa is located in the t-tubules, where it generates 1/3 of t-tubular INa. In contrast, TTX-resistant INa is located predominantly (78%) at the surface membrane. We also investigated the possible contribution of TTX-sensitive INa to excitation-contraction coupling, using 200 nmol/L TTX to selectively block TTX-sensitive INa. TTX decreased the rate of depolarization of the action potential by 10% but did not delay the rise of systolic Ca in the center of the cell (transverse confocal line scan), suggesting that TTX-sensitive INa does not play a role in synchronizing Ca release at the t-tubules; the amplitude of the Ca transient and contraction were also unchanged by 200 nmol/L TTX. The quantity of charge entering via ICa elicited by control or TTX action potential waveforms was similar, suggesting that the trigger for Ca release is not altered by blocking TTX-sensitive INa. We conclude that neuronal INa is concentrated at the t-tubules, but there is no evidence of a requirement for these channels in normal excitation-contraction coupling in ventricular myocytes.
Key words: cardiac myocytes • sodium channels • excitation-contraction coupling • electrophysiology
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