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. t 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. owing through fast voltage-dependent Na+ channels. (C) 2009 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science in China Press. All rights reserved.National Natural Science Foundation of China [30300269]; Technical Innovation Project of Fujian [2003J018
