The effect of amiloride on the resting potential and the electrical constants of frog skeletal muscle fibres.

1. The effects of amiloride on the membrane potential of frog skeletal muscle fibres were investigated with a single intracellular microelectrode. Two microelectrode current- and voltage-clamp experiments were also performed to determine the effects of amiloride on the electrical constants and membrane current near the resting potential. 2. Amiloride reversibly hyperpolarized muscle fibres up to ca 12 mV in 2.5 mM-K+, in a concentration-dependent manner, with a half-maximum effect at 0.2 mM. Amiloride (0.4 mM) also significantly increased the membrane resistance of muscle fibres. 3. The effects of amiloride were consistent with the classical theory of the resting potential and could be described by assuming that it removes the Na+ permeability factor in the Goldman-Hodgkin-Katz equation for [K+]o > or = 2.5 mM. 4. Replacement of [Na+]o by N-methyl-glucamine, choline or Mg2+ produced smaller effects on the resting potential and on membrane resistance than those induced by amiloride. 5. It is concluded that an amiloride-sensitive poorly selective conductance continuously depolarizes the cellular membrane thus playing a role in the resting potential of frog skeletal muscle.