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 (INa) 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 (Kd = 11.2 nM), mibefradil (Kd = 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 INa. The values for half-maximal (Vhalf) inactivation and activation of INa 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 NaV1.6 were detected in smooth muscle layers. Using NaV1.6-null mice (NaV1.6−/−) lacking the expression of the Na+ channel gene, Scn8a, INa were not detected in dispersed smooth muscle cells from the vas deferens, while TTX-sensitive INa were recorded in their wild-type (NaV1.6+/+) littermates. This study demonstrates that the molecular identity of the voltage-gated Na+ channels responsible for the TTX-sensitive INa in murine vas deferens myocytes is primarily NaV1.6.