Block of single cardiac sodium channels by intracellular magnesium

Currents through single cardiac sodium channels have been measured in inside-out patches from guinea pig ventricular cells. To abolish the fast inactivation, Na channels were modified by DPI 201-106. In symmetrical Na solutions, a diminution of outward sodium currents can be observed that depends on the intracellular magnesium concentration and the membrane potential. Inward currents were not altered by the concentrations of magnesium used (between 0 and 22.5 mmol/l). In Mg free solutions a linear current-voltage relation can also be measured in the range of outward Na currents. At +60 mV (symmetrical Na solutions, single channel conductance 24 pS) a half maximal block of cardiac Na channels by intracellular magnesium was found at 2.1 mmol/l. From the analysis of single channel current-voltage relationships the concentration and voltage-dependent block by intracellular magnesium of cardiac sodium channels could be described as binding of Mg at one site with a Kd value of 5.1 mmol/l at 0 mV. The site is located at an electrical distance of 0.18 from the inside.