Williams James M, White Charles R, Chang Melody M, Injeti Elisha R, Zhang Lubo, Pearce William J
Department of Physiology, Center for Perinatal Biology, Loma Linda University School of Medicine, Loma Linda, CA 92350, USA.
J Appl Physiol (1985). 2006 Jun;100(6):1857-66. doi: 10.1152/japplphysiol.00662.2005. Epub 2006 Feb 9.
The present study tests the hypothesis that chronic hypoxia enhances reactivity to nitric oxide (NO) through age-dependent increases in soluble guanylate cyclase (sGC) and protein kinase G (PKG) activity. In term fetal and adult ovine carotids, chronic hypoxia had no significant effect on mRNA levels for the beta1-subunit of sGC, but depressed sGC abundance by 16% in fetal and 50% in adult arteries, through possible depression of rates of mRNA translation (15% in fetal and 50% in adult) and/or increased protein turnover. Chronic hypoxia also depressed the catalytic activity of sGC, but only in fetal arteries (63%). Total sGC activity was reduced by chronic hypoxia in both fetal (69%) and adult (37%) carotid homogenates, but this effect was not observed in intact arteries when sGC activity was measured by timed accumulation of cGMP. In intact arteries treated with 300 microM 3-isobutyl-1-methylxanthine (IBMX), chronic hypoxia dramatically enhanced sGC activity in fetal (186%) but not adult (89%) arteries. This latter observation suggests that homogenization either removed an sGC activator, released an sGC inhibitor, or altered the phosphorylation state of the enzyme, resulting in reduced activity. In the absence of IBMX, chronic hypoxia had no significant effect on rates of cGMP accumulation. Chronic hypoxia also depressed the ability of the cGMP analog, 8-(p-chlorophenylthio)-cGMP, to promote vasorelaxation in both fetal (8%) and adult (12%) arteries. Together, these results emphasize the fact that intact and homogenized artery studies of sGC activity do not always yield equivalent results. The results further suggest that enhancement of reactivity to NO by chronic hypoxia must occur upstream of PKG and can only be possible if changes in cGMP occurred in functional compartments that afforded either temporal or chemical protection to the actions of phosphodiesterase. The range and age dependence of hypoxic effects observed also suggest that some responses to hypoxia must be compensatory and homeostatic, with reactivity to NO as the primary regulated variable.
慢性低氧通过年龄依赖性增加可溶性鸟苷酸环化酶（sGC）和蛋白激酶G（PKG）的活性来增强对一氧化氮（NO）的反应性。在足月胎儿和成年绵羊的颈动脉中，慢性低氧对sGCβ1亚基的mRNA水平没有显著影响，但通过可能降低mRNA翻译速率（胎儿中降低15%，成年中降低50%）和/或增加蛋白质周转，使胎儿动脉中的sGC丰度降低了16%，成年动脉中降低了50%。慢性低氧也降低了sGC的催化活性，但仅在胎儿动脉中（降低63%）。慢性低氧使胎儿（69%）和成年（37%）颈动脉匀浆中的总sGC活性降低，但在用cGMP的定时积累测量sGC活性时，在完整动脉中未观察到这种效应。在用300μM 3-异丁基-1-甲基黄嘌呤（IBMX）处理的完整动脉中，慢性低氧显著增强了胎儿动脉（186%）而非成年动脉（89%）中的sGC活性。后一观察结果表明，匀浆要么去除了一种sGC激活剂，释放了一种sGC抑制剂，要么改变了该酶的磷酸化状态，导致活性降低。在没有IBMX的情况下，慢性低氧对cGMP积累速率没有显著影响。慢性低氧还降低了cGMP类似物8-（对氯苯硫基）-cGMP在胎儿（8%）和成年（12%）动脉中促进血管舒张的能力。总之，这些结果强调了这样一个事实，即对sGC活性的完整动脉和匀浆动脉研究并不总是产生等效的结果。结果进一步表明，慢性低氧对NO反应性的增强必须发生在PKG的上游，并且只有当cGMP在为磷酸二酯酶的作用提供时间或化学保护的功能区室中发生变化时才有可能。观察到的低氧效应的范围和年龄依赖性还表明，对低氧的一些反应必须是代偿性和稳态性的，以对NO的反应性作为主要调节变量。
