Sjodin R A
Soc Gen Physiol Ser. 1984;38:105-27.
Pumped and transported components of ionic flux have been added to passive electrodiffusive components. This permits the derivation of equations for the resting membrane potential that take account of electrogenic mechanisms in which the transport mechanism or pump itself produces a net ionic current. Such equations are general in that they apply to non-steady-state conditions in which intracellular ionic concentrations are changing. The equations developed allow calculation of resting membrane potentials in terms of ionic concentrations, membrane permeability to ions, and kinetic relations for pumped ionic fluxes. When applied to skeletal muscle fibers, the equations predict a buffering effect of the Na/K pump on the membrane potential over a wide range in the values [K]i and [Na]i such that a fairly constant membrane potential occurs under conditions in which the passive ionic fluxes themselves would produce increasing degrees of depolarization. A plot of the membrane potential versus log [K]o with an electrogenic Na pump present gives a curve with slopes both greater than and less than 58 mV per 10-fold concentration change. Over a middle range of [K]o values, the slope is 58 mV. The slope of Em versus log [K]o curves is, therefore, not a very sensitive test for the presence of an electrogenic pump. For the same internal ionic concentrations, less electrogenic increment in membrane potential is observed the higher the value of [K]o, and the more depolarized the membrane. This is due to a rectification present in the pump current-voltage curve, which requires that more pump current be present to produce a given membrane hyperpolarization at depolarized values of the potential than at hyperpolarized values of the potential. A gain in Na and a loss of K by the fibers affects the rectification curve in such a way that less pump current is required to produce the same degree of hyperpolarization. This mechanism ensures that adequate internal negativity will be maintained at high values of [Na]i and [K]o where saturation of the pumping rates might be expected. In the non-steady state of Na extrusion, the condition for which these equations were developed, it is clearly possible for the Na pump to generate potentials considerably higher than those generated under steady-state conditions. For steady-state conditions, both skeletal muscle and squid giant axon Na pumps generate additional internal negativity amounting to a few millivolts.(ABSTRACT TRUNCATED AT 400 WORDS)
离子通量的泵浦和转运成分已被添加到被动电扩散成分中。这使得能够推导静息膜电位的方程，该方程考虑了电生机制，即转运机制或泵本身会产生净离子电流。此类方程具有普遍性，因为它们适用于细胞内离子浓度正在变化的非稳态条件。所推导的方程允许根据离子浓度、膜对离子的通透性以及泵浦离子通量的动力学关系来计算静息膜电位。当应用于骨骼肌纤维时，这些方程预测在较宽的[K]i和[Na]i值范围内，Na/K泵对膜电位具有缓冲作用，使得在被动离子通量本身会导致不同程度去极化的条件下，膜电位相当恒定。存在电生Na泵时，膜电位与log[K]o的关系图给出的曲线斜率在每10倍浓度变化时既大于又小于58 mV。在[K]o值的中间范围内，斜率为58 mV。因此，Em与log[K]o曲线的斜率并非检测电生泵存在的非常灵敏的测试方法。对于相同的细胞内离子浓度，[K]o值越高且膜去极化程度越高，观察到的膜电位电生增量就越小。这是由于泵电流 - 电压曲线中存在整流现象，即与超极化电位值相比，在去极化电位值时需要更多的泵电流来产生给定的膜超极化。纤维中Na的增加和K的减少会以这样一种方式影响整流曲线，即产生相同程度的超极化所需的泵电流减少。这种机制确保在可能预期泵浦速率饱和的高[Na]i和[K]o值下维持足够的细胞内负电位。在Na外流的非稳态中，即这些方程所推导的条件下，Na泵显然有可能产生比稳态条件下更高的电位。对于稳态条件，骨骼肌和乌贼巨轴突的Na泵都会产生额外的细胞内负电位，幅度为几毫伏。（摘要截于400字）
