Irving M, Maylie J, Sizto N L, Chandler W K
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510.
Biophys J. 1990 Apr;57(4):717-21. doi: 10.1016/S0006-3495(90)82592-3.
Junge and McLaughlin (1987) derived an expression for the apparent diffusion constant of protons in the presence of both mobile and immobile buffers. Their derivation applies only to cases in which the values of pH are considerably greater than the largest pK of the individual buffers, a condition that is not expected to hold in skeletal muscle or many other cell types. Here we show that, if the pH gradients are small, the same expression for the apparent diffusion constant of protons can be derived without such constraints on the values of the pK's. The derivation is general and can be used to estimate the apparent diffusion constant of any substance that diffuses in the presence of both mobile and immobile buffers. The apparent diffusion constant of protons is estimated to be 1-2 x 10(-6) cm2/s at 18 degrees C inside intact frog twitch muscle fibers. It may be smaller inside cut fibers, owing to a reduction in the concentration of mobile myoplasmic buffers, so that in this preparation a pH gradient, if established within a sarcomere following action potential stimulation, could last 10 ms or longer after stimulation ceased.
容格和麦克劳克林（1987年）推导出了在存在可移动和不可移动缓冲剂的情况下质子表观扩散常数的表达式。他们的推导仅适用于pH值远大于各缓冲剂最大pK值的情况，而在骨骼肌或许多其他细胞类型中预计不存在这种情况。在此我们表明，如果pH梯度较小，则无需对pK值有此类限制即可推导出质子表观扩散常数的相同表达式。该推导具有普遍性，可用于估计在存在可移动和不可移动缓冲剂的情况下扩散的任何物质的表观扩散常数。在完整青蛙抽搐肌纤维内部，18摄氏度时质子的表观扩散常数估计为1 - 2×10^(-6) 平方厘米/秒。在切断的纤维内部，由于可移动肌浆缓冲剂浓度降低，其值可能更小，因此在该制剂中，如果在动作电位刺激后肌节内建立pH梯度，那么在刺激停止后该梯度可能持续10毫秒或更长时间。
