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 (V_rev) was very close to V_Na, the theoretical Nernst equilibrium potential for Na⁺. V_rev followed V_Na 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 V_h 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 I_Na. 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.