Molecular and biophysical properties of voltage-gated Na⁺ channels in murine vas deferens

The biological and molecular properties of tetrodotoxin (TTX)-sensitive voltage-gated Na⁺ currents (I_Na) in murine vas deferens myocytes were investigated using patch-clamp techniques and molecular biological analyses. In whole-cell configuration, a fast, transient inward current was evoked in the presence of Cd²⁺, and was abolished by TTX (K _d = 11.2 nM), mibefradil (K_d=3.3 μM), and external replacement of Na⁺ with monovalent cations (TEA⁺, Tris⁺, and NMDG⁺). The fast transient inward current was enhanced by veratridine, an activator of voltage-gated Na⁺ channels, suggesting that the fast transient inward current was a TTX-sensitive I _Na. The values for half-maximal (V_half) inactivation and activation of I_Na were -46.3 mV and -26.0 mV, respectively. RT-PCR analysis revealed the expression of Scn1a, 2a, and 8a transcripts. The Scn8a transcript and the α-subunit protein of Na_V1.6 were detected in smooth muscle layers. Using Na_V1.6-null mice (Na_V1.6 ^-/-) lacking the expression of the Na⁺ channel gene, Scn8a, I_Na were not detected in dispersed smooth muscle cells from the vas deferens, while TTX-sensitive I_Na were recorded in their wild-type (Na_V1.6^+/+) littermates. This study demonstrates that the molecular identity of the voltage-gated Na⁺ channels responsible for the TTX-sensitive I_Na in murine vas deferens myocytes is primarily Na_V1.6.