The basis for the membrane potential of quiescent cells of the canine coronary sinus.

During prolonged periods of quiescence, the membrane potential of cells in the isolated canine coronary sinus, exposed to normal Tyrode solution containing 4 mM-K, declines to about -60 mV. The nature of the resting potential was investigated, in small strips of coronary sinus tissue mounted in a fast-flow system, by recording the membrane potential responses to sudden changes in the extracellular ionic environment. At extracellular K concentrations (Ko) from 0 to 64 mM the resting potential was little affected by replacing all but 1 mM of external Cl ions with isethionate and methylsulphate ions. At Ko levels from 4 to 150 mM the resting potential was reasonably well described by the Goldman-Hodgkin-Katz equation on the assumption that the intracellular K concentration (Ki) was 155 mM and that the ratio of membrane permeability coefficients for Na and K, PNa/PK, was 0.07. In the presence of a high concentration of acetylcholine or carbachol (greater than or equal to 1 microM), the resting potentials at Ko levels from 1 to 150 mM approximated K equilibrium potentials (EK) calculated on the assumption that Ki was 155 mM. At Ko levels less than or equal to 8 mM replacing most of the external Na with sucrose or Tris caused a substantial hyperpolarization, whereas application of 1-2 microM-tetrodotoxin caused only slight hyperpolarization. A transient hyperpolarization, due to enhanced electrogenic Na extrusion, was recorded on switching back to 4 mM-K following brief exposures to K-free solution; no transient hyperpolarization was recorded in the presence of 5 microM-acetylstrophanthidin. The acetylstrophanthidin itself caused a rapid depolarization of several millivolts. Preliminary conductance measurements made with two micro-electrodes in some smaller preparations indicate that the steady-state current-voltage relationship is N-shaped. We conclude that the low membrane potential of quiescent coronary sinus cells reflects not a low Ki but rather a relatively high ratio PNa/PK, of about 0.07: the Na ions flow into the cells via predominantly TTX-insensitive pathways and are extruded by the electrogenic Na/K exchange pump, which thereby makes a substantial contribution to the resting potential.