Sodium current in single myocardial mouse cells

Abstract

Morphologically intact single myocardial cells of the adult mouse show a length of 132±20 μm, a width of 21±5 μ, and a height of 10±4 μm (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 τ=35±14 μs is very short. Series resistancer _s, membrane resistancer _m, and membrane capacityc _m are calculated to be 192±48 kΩ, 6.1±1.1 MΩ, and 186±92 pF (all mean ± SD), respectively. Assuming the specific unit membrane capacitance of 1 μF/cm², a total membrane area of 1.86×10⁻⁴ cm² is determined yielding a specific membrane resistanceR _m of 1,134 Ωcm². Settling time of voltage clamp is 30 μs. TTX-block of sodium current is described by 1:1 binding with aK _D value of 1.4×10⁻⁶M. 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.

Steady state inactivationh _∞ is characterized by the half maximum valueV _H=−76.1±4.3 mV and the slope parameters=−6.3±1.1 mV (mean ± SD). A prepulse duration of 500 ms is essential for real steady state inactivation. Steady state activationm _∞ and inactivationh _∞ overlap each other defining a maximum window current at −65 mV.