Lotshaw D P, Li F
Department of Biological Sciences, Northern Illinois University, DeKalb 60115, USA.
Am J Physiol. 1996 Nov;271(5 Pt 1):C1705-15. doi: 10.1152/ajpcell.1996.271.5.C1705.
A Ca(2+)-permeant, nonselective cation channel was observed in cell-attached and inside-out membrane patches from rat adrenal glomerulosa cells maintained in primary cell culture. In cell-attached patches under near physiological ionic conditions, single-channel currents exhibited a reversal potential near -10 mV, inward rectification, a nearly linear slope conductance between 0 and -80 mV of 17.4 pS, and voltage-dependent block at potentials more negative than -80 mV. Channels exhibiting identical conductance and gating properties were observed in inside-out patches; however, channel gating ran down within minutes in this configuation. In the inside-out configuration, channel gating did not require cytosolic Ca2+ (Ca2+ < 10(-9) M), and inward rectification was relieved by removal of intracellular Mg2+. Relative ionic permeability was calculated using reversal potential measurements from inside-out patches under bi-ionic conditions. The channel discriminated poorly among monovalent cations (PLi > PK > PCs > PNa) and was not significantly permeable to anions. The channel was permeable to Ca2+, exhibiting a relative permeability ratio of 0.29 PCa/PNa) when measured with 110 mM Ca2+ on the intracellular face and a permeability ratio of 4.38 (PCa/PNa) with 110 mM Ca2+ on the extracellular face. Channel gating behavior was episodic with open times ranging from milliseconds to tens of seconds and closed times lasting up to several minutes or longer. Channel gating appeared to be relatively voltage independent except that mean channel open time and open probability were reduced by membrane hyperpolarization. In cell-attached patches, bath application of 1 nM angiotensin II (ANG II) increased the channel open probability, primarily affecting channels exhibiting a low open probability, primarily affecting channels exhibiting a low open probability before stimulation. With the use of nystatin perforated-patch current clamp to measure membrane potential, ANG II was observed to induce large transient membrane depolarizations, consistent with activation of an inward current. We hypothesize that this channel is an important component of ANG II-induced membrane depolarization and Ca2+ influx during stimulation of aldosterone secretion.
在原代细胞培养的大鼠肾上腺球状带细胞的细胞贴附式和内面向外式膜片上观察到一种Ca(2+)通透的非选择性阳离子通道。在接近生理离子条件下的细胞贴附式膜片中，单通道电流在-10 mV附近呈现反转电位，具有内向整流特性，在0至-80 mV之间的斜率电导近乎线性，为17.4 pS，且在比-80 mV更负的电位下存在电压依赖性阻滞。在内面向外式膜片中观察到具有相同电导和门控特性的通道；然而，在此配置下通道门控在数分钟内就会衰减。在内面向外式配置中，通道门控不需要胞质Ca2+（Ca2+ < 10(-9) M），并且通过去除细胞内Mg2+可缓解内向整流现象。使用双离子条件下内面向外式膜片的反转电位测量值计算相对离子通透性。该通道对单价阳离子的区分能力较差（PLi > PK > PCs > PNa），且对阴离子的通透性不显著。该通道对Ca2+通透，当胞内面为110 mM Ca2+时，相对通透率为0.29（PCa/PNa），胞外面为110 mM Ca2+时，通透率为4.38（PCa/PNa）。通道门控行为呈间歇性，开放时间从毫秒到数十秒不等，关闭时间长达数分钟或更长。通道门控似乎相对不依赖电压，只是平均通道开放时间和开放概率会因膜超极化而降低。在细胞贴附式膜片中，浴加1 nM血管紧张素II（ANG II）会增加通道开放概率，主要影响刺激前开放概率较低的通道。使用制霉菌素穿孔膜片电流钳测量膜电位时，观察到ANG II会诱导大的瞬时膜去极化，这与内向电流的激活一致。我们推测该通道是ANG II刺激醛固酮分泌过程中诱导膜去极化和Ca2+内流的重要组成部分。
