Ion Conductance

Ions move across the cell membrane through specific ion channels. When these channels open, the permeability and electrical conductance to their respective ions increases, which leads to a change in the membrane potential. Although often used interchangeably, permeability and conductance describe different, but related, properties. Permeability refers to the physical characteristics of the membrane and how easily a substance (ion or non-ion) can move across the membrane. In contrast, conductance refers to how easily a charged particle moves across the membrane, thereby generating an electric current. For a given electrochemical force acting on an ion, conductance increases as membrane permeability to that ion increases.

The membrane potential in a resting cell is near the equilibrium potential for K⁺. That occurs because the membrane is relatively more permeable in the resting state to K⁺ than to other ions such as Na⁺ and Ca⁺⁺. Therefore, the membrane potential reflects not only the concentration gradients of individual ions (i.e., the equilibrium potentials), but also the relative permeability of the membrane to those ions. If the membrane has a very high permeability to one ion over all the others, then that ion will have a greater influence on determining the membrane potential. One way to express this relationship is by the Goldman-Hodgkin-Katz equation, which can be expressed is as follows:

E_m = g'_K (E_K) + g'_Na (E_Na) + g'_Ca (E_Ca) + g'_Cl (E_Cl)

The membrane potential (E_m) depends on the sum of the individual equilibrium potentials multiplied by the relative membrane conductance for the ionic species. The relative conductance (g’_X) of a specific ion is the conductance for that ion divided by the total conductance for all the ionic species (i.e., g’_X = g_X/g_Total).

If the equilibrium potential values for a typical cardiomyocyte are incorporated into the above equation describing E_m, then

E_m = g'_K (-96 mV) + g'_Na (+52 mV) + g'_Ca (+134 mV) + g'_Cl (-90 mV)

In a cardiac cell, the individual ion concentration gradients change very little, even when Na⁺ and Ca⁺⁺ enter the cell, and K⁺ leaves the cell during action potentials. Therefore, changes in E_m primarily occur because of changes in ion conductances and the associated changes in ion currents. For example, in a resting cell, g'_K is very high relative to all the other ion conductances, so the E_m (about -90 mV) is near the E_K (-96 mV) in a resting, repolarized cell. At the peak of an action potential, g'_Na is very high relative to the other ions, therefore the E_m (about +20 mV in cardiomyocytes) approaches E_Na (+52 mV). In the heart, the most important ions determining the membrane potential are Na⁺, K⁺ and Ca⁺⁺.

Ion conductances are altered by antiarrhythmic drugs that block specific ion channels. Sodium-channel blockers such as quinidine inactivate fast-sodium channels and reduce the conductance of sodium ions into the cell. Calcium-channel blockers such as verapamil and diltiazem decrease calcium conductance into the cell. Potassium-channel blockers decrease potassium conductance.

Revised 12/30/2024