Electrical characteristics of smooth muscles

Abstract

Analysis of electrical properties of smooth muscle is difficult due to a small size of single fibres and electrical connections between fibres. When an intracellular electrode is used for membrane polarization, it is almost impossible to estimate the membrane parameters from electrotonic potentials, because of three dimensional current spread. However, when preparations are polarized longitudinally by large extracellular electrode, many smooth muscles have been shown to behave like a cable, the electrical properties of which can be analyzed. With this method, the membrane resistance is obtained to be about 50 × 1−3 ohm cm2 and the membrane capacity about 2 × 10−6 F/cm2, the time constant being about 100 msec. An estimation based on ion fluxes also suggests a similar value for the membrane resistance, but the calculated membrane potentials vary from 35 to 60 mV depending on interpretation of the Na flux. If a low membrane potential (i.e., a high Na permeability) were chosen, a significant contribution of the electrogenic Na pump has to be assumed. However, at least in a steady state, a theoretical consideration is in favour of a relatively small contribution of the pump and a low permeability of Na. In many smooth muscles, removal of the external Na depolarizes the membrane, in contrast to prediction from the Goldman equation. An increase in Ca conductance could be a factor, but some other mechanisms may be involved in this depolarization.