Gamperl A K, Vijayan M M, Pereira C, Farrell A P
Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada.
Am J Physiol. 1998 Feb;274(2):R428-36. doi: 10.1152/ajpregu.1998.274.2.R428.
We examined the in vivo effect of acute hypoxemia on myocardial cell-surface (sarcolemmal) beta-adrenoreceptor density (Bmax) and binding affinity (KD) and on stress protein 70 (sp70) expression by exposing rainbow trout (Oncorhynchus mykiss; 2.1-2.7 kg) to hypoxic water (3 mg/l O2) at 15 degrees C for 6 h. This degree of hypoxia was the minimum O2 level that these trout could tolerate without losing equilibrium and struggling violently. Hypoxic exposure reduced arterial PO2 (PaO2) from 98 to 26 mmHg and arterial oxygen content (CaO2) from 10.8 to 7.4 vol/100 vol, but did not elevate epinephrine and norepinephrine levels above 10 and 30 nM, respectively. Despite the substantial reduction in blood oxygen status, the Bmax and KD of myocardial cell-surface beta-adrenoreceptors were unaffected by 6 h of hypoxic exposure. In addition, acute hypoxemia did not increase myocardial sp70 expression. The failure of short-term hypoxia to decrease trout myocardial beta-adrenoreceptor density clearly contrasts with the established hypoxia-mediated down-regulation shown for mammals. To further investigate the influence of low PO2 on salmonid myocardial beta-adrenoreceptors, binding studies were performed on the spongy (continuously exposed to deoxygenated venous blood) and compact (perfused by oxygenated blood supplied by the coronary artery) myocardia of chinook salmon. The spongy myocardium has adapted to its microenvironment of continuous low PO2 by having 14% more cell-surface beta-adrenoreceptors compared with the compact myocardium. There was no tissue-specific difference in KD and no evidence of sexual dimorphism in Bmax or KD. We conclude from our studies that the salmonid heart is well adapted for sustained performance under hypoxic conditions. We found that wild chinook salmon had 2.8 x more cell-surface beta-adrenoreceptors compared with hatchery-reared rainbow trout. This difference suggests a significant degree of plasticity exists for fish myocardial beta-adrenoreceptors. The signals underlying such differences await further study, but are not likely to include moderate hypoxia and sexual dimorphism.
我们通过将虹鳟鱼（Oncorhynchus mykiss；2.1 - 2.7千克）在15摄氏度下暴露于含氧量为3毫克/升的缺氧水中6小时，研究了急性低氧血症对心肌细胞表面（肌膜）β - 肾上腺素能受体密度（Bmax）、结合亲和力（KD）以及应激蛋白70（sp70）表达的体内影响。这种缺氧程度是这些虹鳟鱼在不失平衡且不剧烈挣扎的情况下所能耐受的最低氧气水平。缺氧暴露使动脉血氧分压（PaO2）从98毫米汞柱降至26毫米汞柱，动脉血氧含量（CaO2）从10.8体积/100体积降至7.4体积/100体积，但肾上腺素和去甲肾上腺素水平分别未升高至10纳摩尔和30纳摩尔以上。尽管血氧状态大幅降低，但缺氧暴露6小时并未影响心肌细胞表面β - 肾上腺素能受体的Bmax和KD。此外，急性低氧血症并未增加心肌sp70的表达。短期缺氧未能降低虹鳟鱼心肌β - 肾上腺素能受体密度，这与哺乳动物中已确立的缺氧介导的下调明显不同。为了进一步研究低氧分压对鲑科动物心肌β - 肾上腺素能受体的影响，我们对奇努克鲑的海绵状心肌（持续暴露于脱氧静脉血）和致密心肌（由冠状动脉供应的含氧血液灌注）进行了结合研究。与致密心肌相比，海绵状心肌通过增加14%的细胞表面β - 肾上腺素能受体，适应了其持续低氧分压的微环境。KD不存在组织特异性差异，且在Bmax或KD方面没有性别二态性的证据。我们从研究中得出结论，鲑科动物的心脏在缺氧条件下能很好地适应持续的功能。我们发现，野生奇努克鲑的细胞表面β - 肾上腺素能受体比养殖的虹鳟鱼多2.8倍。这种差异表明鱼类心肌β - 肾上腺素能受体存在显著程度的可塑性。造成这种差异的信号有待进一步研究，但不太可能包括中度缺氧和性别二态性。
