Sodium current in single myocardial mouse cells.

Morphologically intact single myocardial cells of the adult mouse show a length of 132 +/- 20 microns, a width of 21 +/- 5 microns, and a height of 10 +/- 4 microns (all mean +/- SD) and are brick-like in shape. A one suction pipette method is used for voltage clamp of those single cells. The determined time constant of capacitive current tau = 35 +/- 14 microseconds is very short. Series resistance rs, membrane resistance rm, and membrane capacity Cm are calculated to be 192 +/- 48 k omega, 6.1 +/- 1.1 M omega, and 186 +/- 92 pF (all mean +/- SD), respectively. Assuming the specific unit membrane capacitance of 1 microF/cm2, a total membrane area of 1.86 X 10(-4) cm2 is determined yielding a specific membrane resistance Rm of 1,134 omega cm2. Settling time of voltage clamp is 30 microseconds. TTX-block of sodium current is described by 1:1 binding with a KD value of 1.4 X 10(-6) M. Using a reduced extracellular sodium concentration the maximum Na current is between 25 and 40 nA at voltages between -40 and -30 mV. Currents of between +20 and +30 mV reverse in an outward direction. Inward currents are approximated by a m3h model. The time constant of activation decreases from 0.7 ms at -60 mV to 0.12 ms at +20 mV. The time constant of inactivation falls from 9.1 ms at -60 mV to 0.6 ms at + 20 mV.(ABSTRACT TRUNCATED AT 250 WORDS)