IONIC CURRENTS DURING ACTION-POTENTIALS IN MAMMALIAN SKELETAL-MUSCLE FIBERS ANALYZED WITH LOOSE PATCH-CLAMP

The loose patch-clamp technique was applied to analyze transmembrane c urrents during propagating action potentials in superficial fibers of musculi extensor digitorum longus of the mouse in vitro. Experimentall y three components were identified in the transmembrane current: 1) a capacitive, 2) an inward sodium, and 3) an outward potassium current. Other components were negligible. The capacitive current was similar i n shape to the first derivative of the intracellularly measured action potential. Tetrodotoxin, tetraethylammonium, and 4-aminopyridine, app lied in the pipette, were used to identify the contribution in the cur rent by sodium and potassium ions. With extracellularly applied depola rization steps only a sodium current was observed, not a potassium cur rent. Occasionally found outward currents were artifactual. The behavi or of delayed rectifier potassium channels in muscle fiber membranes i s discussed in the light of these unexpected findings. We conclude tha t potassium channel activity contributing to and measured during actio n potential generation is in some way inaccessible to loose patch extr acellular voltage-clamp stimulation and that loose patch action curren t recording is a useful noninvasive method to analyze membrane conduct ances involved in action potential generation.