Yamanishi Shigeki, Katsumura Kozo, Kobayashi Takatoshi, Puro Donald G
Department of Ophthalmology and Visual Sciences, University of Michigan, 1000 Wall St., Ann Arbor, Michigan 48105, USA.
Am J Physiol Heart Circ Physiol. 2006 Mar;290(3):H925-34. doi: 10.1152/ajpheart.01012.2005. Epub 2005 Nov 18.
We tested the hypothesis that extracellular lactate regulates the function of pericyte-containing retinal microvessels. Although abluminally positioned pericytes appear to adjust capillary perfusion by contracting and relaxing, knowledge of the molecular signals that regulate the contractility of these mural cells is limited. Here, we focused on lactate because this metabolic product is in the retinal extracellular space under both physiological and pathophysiological conditions. In microvessels freshly isolated from the adult rat retina, we used perforated-patch pipettes to monitor ionic currents, fura-2 to measure calcium levels, and time-lapse photography to visualize changes in mural cell contractility and lumen diameter. During lactate exposure, pericyte calcium rose; these cells contracted, and lumens constricted. This contractile response appears to involve a cascade of events resulting in the inhibition of Na+/Ca2+ exchangers (NCXs), the decreased of which function causes pericyte calcium to increase and contraction to be triggered. On the basis of our observation that gap junction uncouplers minimized the lactate-induced rise in pericyte calcium, we propose that the NCXs inhibited by lactate are predominately located in the endothelium. Indicative of the importance of endothelial/pericyte gap junctions, uncouplers of these junctions switched the pericyte response to lactate from contraction to relaxation. In addition, we observed that hypoxia, which closes microvascular gap junctions, also switched lactate's effect from vasocontraction to vasorelaxation. Thus the response of pericyte-containing retinal microvessels to extracellular lactate is metabolically modulated. The ability of lactate to serve as a vasoconstrictor when energy supplies are ample and a vasodilator under hypoxic conditions may be an efficient mechanism to link capillary function with local metabolic need.
我们验证了细胞外乳酸调节含周细胞的视网膜微血管功能这一假说。尽管位于血管腔外的周细胞似乎通过收缩和舒张来调节毛细血管灌注，但调节这些壁细胞收缩性的分子信号的相关知识却很有限。在此，我们聚焦于乳酸，因为这种代谢产物在生理和病理生理条件下均存在于视网膜细胞外空间。在从成年大鼠视网膜新鲜分离的微血管中，我们使用穿孔膜片吸管监测离子电流，用fura-2测量钙水平，并通过延时摄影观察壁细胞收缩性和管腔直径的变化。在乳酸暴露期间，周细胞内钙升高；这些细胞收缩，管腔变窄。这种收缩反应似乎涉及一系列事件，导致钠/钙交换体（NCXs）受到抑制，其功能降低导致周细胞内钙增加并引发收缩。基于我们观察到缝隙连接解偶联剂可最小化乳酸诱导的周细胞内钙升高，我们提出受乳酸抑制的NCXs主要位于内皮细胞。内皮细胞/周细胞缝隙连接的重要性表明，这些连接的解偶联剂将周细胞对乳酸的反应从收缩转变为舒张。此外，我们观察到缺氧会关闭微血管缝隙连接，这也会将乳酸的作用从血管收缩转变为血管舒张。因此，含周细胞的视网膜微血管对细胞外乳酸的反应受到代谢调节。当能量供应充足时乳酸作为血管收缩剂，而在缺氧条件下作为血管舒张剂的能力，可能是一种将毛细血管功能与局部代谢需求相联系的有效机制。
