Key result
The RT4-B8 cell line expresses voltage-gated sodium channels of the cardiac phenotype, with a current activation half point of -41 mV and tetrodotoxin block half concentration of 0.74 microM.
The RT4-B8 cell line expresses voltage-gated sodium channels of the cardiac phenotype, providing a useful model for studying cardiac sodium channel properties.
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May facilitate preclinical cardiac Na channel studies; leaves open human translation and clinical relevance.
RT4-B is one of several cell lines derived from a multipotent stem cell line, RT4-AC, which originated from a rat peripheral neurotumor. Based on Northern blot and ribonuclease protection experiments, RT4-B8 cells have been proposed to express rat cardiac Na channel mRNA as the major isoform. We report here direct electrophysiological evidence that the expressed voltage-gated Na channels in the RT4-B8 cell line are of the cardiac phenotype with no evidence for subpopulations expressing other Na channel isoforms. Current activation half point (conductance) was -41 +/- 5 mV (n = 7) and the steady-state voltage-dependent availability half point was -89 +/- 1 mV. As expected for cardiac Na channels, the half concentration of block for tetrodotoxin block was 0.74 microM, for saxitoxin (STX) was 0.15 microM, and for the class 2B divalent cation Cd2+ was 67 microM. Block was well described by single-site dose-response relationships with no indication of a subpopulation with "neuronal" affinity. Single-channel conductance (140 mM Na+) was 10 pS and predicted the average number of channels open at peak Na current to be 3 channels/microns2. [3H]STX binding data were also consistent with a single population of low-affinity STX binding sites and predicted channel density to be 11 sites/microns2. No inwardly or outwardly rectifying K or Ca currents were detected electrophysiologically, although in some cells a small time-independent Cl current was detected. Reverse transcription-polymerase chain reaction of mRNA isolated from RT4-B8 cells demonstrated the presence of rat cardiac (rH1) and brain IIa alpha-subunit mRNA, as well as mRNA for the Na channel beta 1-subunit. Northern blot analysis confirmed the predominance of the rat cardiac Na mRNA compared with brain IIa. The beta 1-subunit mRNA levels were significantly lower than those detected in rat brain and heart mRNA but were comparable to the low level of beta 1-subunit mRNA detected in isolated rat ventricular myocytes.
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Zeng et al. (1996) studied In vitro cell line (RT4-B8). RT4-B8 cell line electrophysiological characterization was evaluated on Electrophysiological properties of voltage-gated Na channels. The RT4-B8 cell line expresses voltage-gated sodium channels of the cardiac phenotype, with a current activation half point of -41 mV and tetrodotoxin block half concentration of 0.74 microM.
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