Fast Na+ current in circular smooth muscle cells of the large intestine

Whole-cell voltage clamp was carried out on freshly dispersed single smooth muscle cells from adult rat and human colons to investigate the regulation of the Ca²⁺ channels. In this study, we unexpectedly discovered the existence of a fast Na⁺ channel current. With normal physiological salt solution (PSS) plus 4-amino-pyridine (3 mM) in the bath and high-Cs⁺ solution in the pipette to inhibit outward K⁺ currents, an inward current possessing fast and slow components was observed when the cell membrane was depolarized to a value more positive than –20 mV from a holding potential of –100 mV. When Ca²⁺ ions were removed from the PSS, or when nifedipine (10 M) and Ni²⁺ (30 M) were simultaneously applied, the slow component disappeared and the fast component remained. The fast current component became almost completely inactivated within 10 ms. This fast component was dependent on extracellular Na⁺ concentration and was inhibited by tetrodotoxin (TTX) dose dependently (IC₅₀ of 130 nM in rat and 14 nM in human). These results suggest that the slow component of inward current was a Ca²⁺ channel current, whereas the fast component was a TTX-sensitive fast Na⁺ channel current. The threshold voltage, the voltage for peak current, and the reversal potential for the fast Na⁺ current were, respectively, about –50, –20, and + 50 mV in rats, and –40, 0, and + 60 mV in humans. The incidence of cells possessing fast Na⁺ currents depended on the region of the colon. In rat proximal colon, the incidence was 64% (14 out of 22 cells tested); in distal colon, it was 10% (2 out of 21 cells tested). In humans, the incidence in the ascending colon was 73% (16 out of 22 cells tested), and in the descending colon was 22% (7 out of 32 cells tested). The densities of fast Na⁺ and Ca²⁺ currents were 3.2 and 4.5 pA/pF in rats and 1.0 and 1.4 pA/pF in humans, respectively. The ratio of both current densities (Na⁺ vs Ca²⁺) was 0.71, in both rats and humans. We conclude that the major ion channels associated with the generation of inward currents in the circular smooth muscle cells of rat and human colon are voltage-dependent Ca²⁺ channels and TTX-sensitive Na⁺ channels. The fast Na⁺ current may facilitate propagation of excitation.