Sodium current of neurosecretory cells in the eyestalk from the shrimp Penaeus japonicus

The properties of the inward current of medulla terminalis-X-organ (MTXO) cells isolated from the Penaeus japonicus eyestalk were studied with the whole-cell clamp technique in the presence of Ca2+ and K+ channel blockers. The inward currents had a threshold at about −50 mV and peaked at −10 mV. The reversed potential (Vrev) was very close to VNa, the theoretical Nernst equilibrium potential for Na+. Vrev followed VNa when the external Na+ concentration was varied and the currents were entirely suppressed by 30 nM tetrodotoxin (TTX), indicating that it was carried by Na+. The smooth line of concentration-dependent inhibition of sodium currents by TTX represented the best fit with the Hill equation, yielding an IC50 of 2.1 ± 0.1 nM. The values of the half-maximal activation voltage Vh were −20.6 ± 0.5 and −19.3 ± 0.5 mV, respectively, in the absence and presence of 2 nM TTX. TTX had no significant effect on the voltage dependence of steady-state activation and inactivation of INa. Taken together, the results suggest that the inward current recorded under our experimental conditions was carried by sodium ions flowing through fast voltage-dependent Na+ channels.