Scheid C R, Fay F S
J Gen Physiol. 1980 Feb;75(2):163-82. doi: 10.1085/jgp.75.2.163.
We describe a technique for examining unidirectional ion movements in suspensions of enzymatically disaggregated smooth muscle cells derived from stomach muscle of the toad. This technique has been used to analyze the movement of 42K across these cells. This analysis was greatly simplified by the finding that the cells were in a steady state with respect to K+ distribution after isolation. The potassium contents of the isolated cells were identical to those of intact smooth muscle (131 mM/liter intracellular fluid) and stable for over 4 h; moreover, the unidirectional influx and efflux rates were equal. An additional simplification was provided by the finding that virtually all the K+ exchanges in a manner predicted for a simple two-compartment system consisting of an extracellular and an intracellular space. Transmembrane K+ flux in these cells averaged 1.2 pmol.cm-2.s-1 at room temperature. A large portion (approximately 80%) of 42K influx appeared to be mediated by a saturable transport system with an apparent Km of 0.6 mM and an apparent Vmax of 1.3 pmol.cm-2.s-1. The calculated resting membrane permeability to K+ in these isolated smooth muscle cells, assuming a membrane potential of -50 mV, was 2.9 X 10(-8) cm/s. The calculated gK+ was 2.7 mumho/cm2 constituting only a small fraction of the total membrane conductance as measured electrophysiologically. The latter finding suggests that the resting membrane potential in the isolated cells must be determined by ions in addition to K+. We propose that these methods for studying ion movements in smooth muscle should aid in unraveling the mechanisms responsible for controlling the distribution of ions both at rest, as in the present study, as well as in response to neurotransmitters.
我们描述了一种用于检测源自蟾蜍胃肌的酶解平滑肌细胞悬液中单向离子运动的技术。该技术已用于分析(^{42}K)在这些细胞中的运动。由于发现分离后的细胞在(K^+)分布方面处于稳态，这一分析得以大大简化。分离细胞的钾含量与完整平滑肌的钾含量相同（细胞内液为(131 mM/L)），且在(4)小时以上保持稳定；此外，单向流入和流出速率相等。另一个简化之处在于，几乎所有的(K^+)交换都符合由细胞外和细胞内空间组成的简单双室系统所预测的方式。在室温下，这些细胞中的跨膜(K^+)通量平均为(1.2 pmol·cm^{-2}·s^{-1})。(^{42}K)流入的很大一部分（约(80%)）似乎由一个可饱和转运系统介导，其表观(Km)为(0.6 mM)，表观(Vmax)为(1.3 pmol·cm^{-2}·s^{-1})。假设膜电位为(-50 mV)，计算得出这些分离的平滑肌细胞对(K^+)的静息膜通透性为(2.9×10^{-8} cm/s)。计算得出的(gK^+)为(2.7 μmho/cm^2)，仅占电生理测量的总膜电导的一小部分。后一发现表明，分离细胞中的静息膜电位必定由除(K^+)之外的离子所决定。我们认为，这些研究平滑肌中离子运动的方法应有助于阐明负责控制离子在静息状态下（如本研究）以及对神经递质作出反应时的分布的机制。
