Tetrodotoxin-sensitive voltage-dependent Na currents recorded from Xenopus oocytes injected with mammalian cardiac muscle RNA

Voltage-sensitive sodium (Na) channel currents recorded from mammalian cardiac muscle are blocked by tetrodotoxin (TTX) with a K_d of 1–3 μM. We have observed a K_d for TTX of 4–10 nM for Na currents recorded from Xenopus oocytes injected with RNA extracted from rabbit cardiac muscle. This result suggests that the degree of TTX sensitivity of Na channels encoded by cardiac muscle mRNA is in part determined by post-translational modification(s) or of associations with accessory proteins in the membrane.