Goldman W F, Blaustein M P
Department of Physiology, University of Maryland School of Medicine, Baltimore 21201.
J Cardiovasc Pharmacol. 1988;12 Suppl 5:S13-9.
Contraction in vascular smooth muscle is normally triggered by an increase in the cytosolic free Ca2+ concentration. We employed the fluorescent Ca indicator, fura-2, and digital imaging microscopy to study the spatial distribution of intracellular Ca2+ in arterial myocytes and the changes elicited by norepinephrine (NE) and by alterations in the Na+ electrochemical gradient. In modified Kreb's solution containing 1.8 mM Ca2+, these bovine tail artery myocytes were relaxed and able to maintain normal membrane potentials. The cells contracted rapidly when exposed to NE or high K+ solution. They then relaxed slowly when the activator was washed away. A rise in Ca2+ concentration, [Ca2+], was elicited by NE as well as by Ca2+ entry via Na+/Ca2+ exchange. Digital analysis of images of cellular fura-2 fluorescence revealed that the intracellular [Ca2+] was relatively uniformly distributed in resting cells prior to activation. During NE-evoked contractions, intracellular [Ca2+] increased an average of two- to threefold, and the distribution of [Ca2+] became much more heterogeneous. Upon recovery from activation, the cells relaxed, usually attaining greater than 90% of their original resting length. In contrast to the relatively uniform Ca2+ distribution observed prior to NE activation, discrete regions of elevated [Ca2+] were observed throughout the recovered cells. Similarly, reducing the Na+ electrochemical gradient and, presumably, activating Na+/Ca2+ exchange elicited a rise in [Ca2+]. In cells exposed to increased [Na+]i and reduced [Na+]o, as in cells following recovery from NE stimulation, discrete areas of high [Ca2+] were apparent. The large spatial variation induced by cell activation implies that Ca2+ was sequestered at localized sites throughout the cell.
血管平滑肌的收缩通常由胞质游离钙离子浓度升高触发。我们使用荧光钙指示剂fura-2和数字成像显微镜来研究动脉肌细胞内钙离子的空间分布以及去甲肾上腺素（NE）和钠离子电化学梯度改变所引发的变化。在含有1.8 mM钙离子的改良克雷布斯溶液中，这些牛尾动脉肌细胞处于舒张状态，并能够维持正常的膜电位。当暴露于NE或高钾溶液时，细胞迅速收缩。当去除激活剂后，它们随后缓慢舒张。NE以及通过钠/钙交换的钙离子内流均可引发钙离子浓度[Ca2+]升高。对细胞fura-2荧光图像的数字分析显示，在激活前，静息细胞内的[Ca2+]相对均匀分布。在NE诱发的收缩过程中，细胞内[Ca2+]平均增加两到三倍，且[Ca2+]的分布变得更加不均匀。从激活状态恢复后，细胞舒张，通常达到其原始静息长度的90%以上。与NE激活前观察到的相对均匀的钙离子分布不同，在整个恢复后的细胞中均观察到钙离子浓度升高的离散区域。同样，降低钠离子电化学梯度并推测激活钠/钙交换会引发[Ca2+]升高。在暴露于细胞内钠离子浓度升高和细胞外钠离子浓度降低的细胞中，如在NE刺激恢复后的细胞中，高[Ca2+]的离散区域很明显。细胞激活引起的大空间变化表明钙离子被隔离在整个细胞的局部位点。
