Aging and interaction of natriuretic factors on renal and vascular sodium pump

MBG是一种内源性Na/K-ATPase(NKA)抑制剂，通过抑制肾小管NKA促进钠尿，但也可能通过抑制血管内NKA而引起血管收缩。最近我们证实，ANP通过cGMP/PKG-2依赖的肾α-1 NKA的磷酸化，使肾NKA对MBG增敏，并可能增强MBG的利钠作用。相反，在血管系统，ANP通过依赖PKG-1的机制减少NKA的磷酸化，并可能抵消MBG引起的过度血管收缩。由于衰老与cGMP/PKG信号的下调有关，我们假设在老年大鼠，ANP不会增强MBG的肾脏效应，也不会对抗MBG的血管效应。我们比较了青年(3月龄)和老年(24月龄)SD大鼠在急性盐负荷(20%，2.5ml/kg)后的血压、尿钠、肾髓质和血管肌膜NKA活性、MBG和α-ANP水平，以及MBG和ANP对NKA的体外相互作用。与青年大鼠相比，在α-ANP和cGMP水平变化相似的情况下，盐负荷老年大鼠表现出更大的MBG反应、更大的血压升高和更大的主动脉NKA抑制，更少的钠尿和肾髓质NKA的抑制。老年大鼠主动脉和肾脏中PKG-1和PKG-2水平显著降低，而肾脏PDE-V水平显著升高。在老年大鼠的主动脉肌膜和肾髓质中，1nmol/Lα-心钠素对α-1NKA的磷酸化水平没有影响。因此，在老年动物中，低浓度的α-ANP不能增强MBG对肾髓质NKA的抑制作用，也不能降低MBG对主动脉肌膜NKA的影响。在3月龄大鼠主动脉血管平滑肌细胞的原代培养中，2nmolMBG刺激胶原合成。24月龄大鼠血管平滑肌细胞前胶原-1和胶原的基础水平高于3月龄大鼠。来自幼年动物的血管平滑肌细胞中PKG-1基因的沉默产生了基础胶原水平的增加，并导致这些细胞对MBG促纤维化作用的敏感性增强。因此，(I)在老年大鼠，cGMP/PKG依赖信号的下调是心钠素对肾和血管NKA调节作用的转变，从而促进盐敏感性；(Ii)在血管平滑肌细胞中，与年龄相关的PKG依赖信号的下调是对内源性强心类固醇促纤维化作用敏感性增加的基础。

MBG, an endogenous Na/K-ATPase (NKA) inhibitor, promotes natriuresis via inhibition of renotubular NKA, but may cause vasoconstriction via inhibition of the NKA in the vasculature. Recently we demonstrated that ANP, via cGMP/PKG-2-dependent phosphorylation of renal alpha-1 NKA, sensitizes renal NKA to MBG and may potentiate natriuretic action of MBG. In the vasculature, on the opposite, ANP, via PKG-1 dependent mechanism, reduces NKA phosphorylation and may offset the excessive vasoconstriction induced by MBG. Since aging is associated with a down-regulation of cGMP/PKG signaling, we hypothesized that in aged rats, ANP would not potentiate renal effects of MBG and would not oppose vascular effects of MBG. In young (3 month old) and aged (24 months old) Sprague-Dawley rats, we compared systolic blood pressure (BP), natriuresis, NKA activity in renal medulla and in vascular sarcolemma, and levels of MBG and alpha-ANP following acute NaCl loading (20%, 2.5 ml/kg, intraperitoneally), and the in vitro interactions of MBG and ANP on the NKA. As compared to young rats, NaCl-loaded aged rats exhibited greater MBG response, greater BP elevation and greater inhibition of NKA in aortae, less natriuresis and less inhibition of NKA in renal medulla in the presence of comparable changes in alpha-ANP and cGMP levels. Levels of PKG-1 in aorta and PKG-2 in the kidney in aged rats were markedly reduced, while levels of PDE-V in the kidney were increased. In aortic sarcolemma and renal medulla from aged rats, 1 nmol/L alpha-ANP did not affect level of alpha-1 NKA phosphorylation. Accordingly, in aged animals low concentrations of alpha-ANP did not potentiate MBG-induced inhibition of NKA from renal medulla and did not reduce the effect of MBG on the NKA from aortic sarcolemma. In the primary culture from vascular smooth muscle cells from aorta from 3 month old rats, 2 nmoles MBG stimulated synthesis of collagen. In 24 month old rats, baseline levels of procollagen-1 and collagen in vascular smooth muscle cells were greater than those in the cells from 3 month old animals. Silencing of the PKG-1 gene in vascular smooth muscle cells from young animals produced an increase in the baseline levels of collagen and resulted in the enhanced sensitivity of these cells to pro-fibrotic action of MBG. Thus, (i) in aged rats, down-regulation of cGMP/PKG dependent signaling underlies a shift in ANP modulation of the effect of MBG on renal and vascular NKA, which promotes salt-sensitivity, and (ii) in vascular smooth muscle cells age-associated down-regulation of PKG-dependent signaling underlies the increase sensitivity to pro-fibrotic effects of endogenous cardiotonic steroids.