Effect of urea on some electrophysiological properties of the frog muscle cell membrane.
The effect of urea on the electrophysiological properties of the frog muscle cell membrane was studied with intracellular microelectrodes, together with an analysis of the electrolyte composition of the muscle. The different parameters were measured and evaluated after soaking the muscle for 60 min at urea concentrations up to 2.25 M. The resting membrane potential was markedly decreased above 1.50 M and fell to about -50mV at 2.25 M. The specific membrane resistance (Rm) was almost unaffected at concentrations of 0.75 M and 1.50 M but was reduced after 60 min in 2.25 M to very low values indicating a leaky membrane. The maximum rate of rise of the action potential (VA) was unaffected up to 0.50 M but was reduced to about 50% of control value at 0.75 M. Between 0.75 M and 1.25 M it was constant and at higher concentrations reduced to almost zero. The reduction of VA at the lower concentrations was not accompanied by changes in neither the resting membrane potential nor Rm. It is proposed that urea perturbs protein systems concerned with the generation of the muscle action potential without affecting the passive electrical properties of the muscle membrane. The electrolyte analysis revealed an increase in intracellular Na+ and K+ concentrations, mainly due to loss of intracellular water.