Koldkjaer Pia, Berenbrink Michael
School of Biological Sciences, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK.
J Exp Biol. 2007 Oct;210(Pt 19):3451-60. doi: 10.1242/jeb.008524.
Haemoglobin concentrations in vertebrate red blood cells are so high that in human sickle cell disease a single surface amino acid mutation can result in formation of large insoluble haemoglobin aggregates at low oxygen levels, causing peculiar cell deformations or 'sickling'. This may cause vascular occlusion and thereby severe pain, organ failure and death. Here, using light and transmission electron microscopy, we demonstrate extensive in vivo sickling of whiting red blood cells after capture stress without any apparent haemolysis and show its subsequent recovery. We show exceptionally high cooperative proton binding during the sickling process in vitro and identify the reduction of extracellular pH below resting values as the primary cause for in vivo sickling, although the response is modulated to a lesser extent also by oxygen tension. Using isotope tracer fluxes, we further show that beta-adrenergic hormones, which are released under capture stress, activate a powerful endogenous Na/H exchanger in these fish red blood cells, which is known to elevate intracellular pH. beta-adrenergic treatment further leads to a marked reduction of acid-induced in vitro sickling, which is impaired when Na/H exchange is inhibited by amiloride. We propose that this mechanism protects red blood cells of some fishes against the problem of haemoglobin aggregation and red blood cell sickling, except under most severe acidosis. This system offers a unique example of how, over evolutionary time, nature may have overcome what is still a deadly disease in humans.
脊椎动物红细胞中的血红蛋白浓度非常高，以至于在人类镰状细胞病中，单个表面氨基酸突变就可能导致在低氧水平下形成大量不溶性血红蛋白聚集体，从而引起特殊的细胞变形或“镰变”。这可能导致血管阻塞，进而引发剧痛、器官衰竭甚至死亡。在此，我们通过光学显微镜和透射电子显微镜观察发现，牙鳕红细胞在捕捞应激后会在体内大量镰变，但没有明显的溶血现象，并且随后会恢复。我们还发现，在体外镰变过程中存在异常高的协同质子结合现象，并确定细胞外pH值降至静息值以下是体内镰变的主要原因，不过氧张力也会在较小程度上调节这种反应。利用同位素示踪通量，我们进一步表明，在捕捞应激下释放的β - 肾上腺素能激素会激活这些鱼类红细胞中一种强大的内源性钠/氢交换体，已知该交换体会提高细胞内pH值。β - 肾上腺素能处理还会导致酸诱导的体外镰变显著减少，而当钠/氢交换被氨氯地平抑制时，这种减少就会受到损害。我们认为，这种机制可以保护一些鱼类的红细胞免受血红蛋白聚集和红细胞镰变问题的影响，除非在最严重的酸中毒情况下。这个系统提供了一个独特的例子，展示了在进化过程中，大自然可能是如何克服人类至今仍面临的致命疾病的。
