Resting potential and electrical properties of frog slow muscle fibres. Effect of different external solutions.

1. The electrical properties of frog slow muscle fibres were investigated with intracellular micropipettes to determine their characteristic length (lambda), specific membrane resistance (R(m)) and specific membrane capacitance.2. The value of lambda was about 1 cm in fibres of 1.2 cm length. The ;short cable model' was used to calculate R(m). Its mean value was 1.12 x 10(5) ohm cm(2), about 10-20 times larger than the value for twitch fibres. The mean value for C(m) was 3.24 x 10(-6) F/cm(2).3. Resting potentials measured immediately after penetration with a single micropipette were about - 80 mV. Lower values can be attributed to the effects of damage or leakage produced by micropipette insertion.4. Changes in external K concentration produced changes in the initially recorded resting potentials which follow the constant field theory using a ratio of Na: K permeabilities P(Na)/P(K) = 0.02. Changes in external Cl concentration produced little or no change in the resting potential or membrane resistance, indicating a low Cl permeability.5. In agreement with previous work, slow fibres showed a time-dependent decrease in resistance (;delayed rectification') for membrane potentials more positive than - 60 mV. ;Anomalous rectification' observed in twitch fibres was not seen in slow fibres. In high external K concentrations the resistance of slow fibres is almost unaffected by changes in membrane potential.6. Increasing the concentration of external Ca (up to isotonic) has two distinct effects on slow fibres. It increases R(m) up to ten times, and it improves the stability of trans-membrane recordings, probably by reducing the leakage due to micropipette penetrations. Magnesium does not appear to have either of these effects.