Effect of phlorizin on the action potential and voltage-dependent ionic conductances in frog skeletal muscle.

The effects of phlorizin on the action potential and voltage-dependent ionic conductances were investigated in frog (Rana esculenta) sartorius muscles. In order to study the ionic currents, phase plane trajectories of the action potentials were analysed. As a consequence of lowering the overshoot by 19%, the amplitude of the action potential was decreased on the effect of phlorizin applied at a concentration of 2 mmol/l. The maximum rate of rise of the action potential (V +max) was reduced on the average by 23%. The maximum rate of fall of the spike potential (V -max) was also diminished by about 25% and the repolarization was prolonged consequently. Due to phlorizin treatment the early negative after-potential was significantly lowered or was not noticeable at all. Analysing the phase plane trajectories of the action potentials the peak inward (Na) current was found to be decreased by about 16% despite the unchanged maximum Na conductance. At the Na equilibrium potential a 9.2 mV shift towards negative direction was observed. Both the peak outward (K) current and the maximum outward (K) conductance were reduced by 25 and 29%, resp. tau K, the time constant reflecting the kinetic properties of K conductance was increased by 36%. Phlorizin treatment prevented the repetitive activity induced by cevadine. The inhibiting actions of phlorizin on fast ionic currents developed also in cevadine-treated muscles. All of the effects of phlorizin were reversible. The presented data show that phlorizin, the well-known inhibitor of phosphorylase and a competitive antagonist of sugar uptake, is also capable of modifying the electrical properties of the frog skeletal muscle membrane.