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

MBG是一种内源性Na/K-ATP酶（NKA）抑制剂，通过抑制肾小管NKA促进尿钠排泄，但可能通过抑制血管系统中的NKA引起血管收缩。最近，我们证明，ANP，通过cGMP/PKG-2依赖性磷酸化肾α-1 NKA，肾NKA敏感MBG，并可能加强利钠作用的MBG。相反，在血管系统中，ANP通过PKG-1依赖性机制减少NKA磷酸化，并可能抵消MBG诱导的过度血管收缩。由于衰老与cGMP/PKG信号的下调有关，我们假设在老年大鼠中，ANP不会增强MBG的肾脏作用，也不会对抗MBG的血管作用。在年轻（3个月大）和老年（24个月大）Sprague-Dawley大鼠中，我们比较了收缩压（BP）、尿钠排泄、肾髓质和血管肌膜中的NKA活性以及急性氯化钠负荷（20%，2.5 ml/kg，腹腔内）后MBG和α-ANP的水平，以及MBG和ANP对NKA的体外相互作用。与年轻大鼠相比，NaCl负荷的老年大鼠表现出更大的MBG反应，更大的BP升高和更大的抑制NKA在肾脏，尿钠排泄减少和NKA在肾髓质中的抑制较少的存在下，在α-ANP和cGMP水平的变化相当。老年大鼠主动脉PKG-1和肾脏PKG-2水平明显降低，而肾脏PDE-V水平升高。1 nmol/L α-ANP对老年大鼠主动脉肌膜和肾髓质α-1 NKA磷酸化水平无影响。因此，在老年动物中，低浓度的α-ANP不增强MBG诱导的对来自肾髓质的NKA的抑制，也不降低MBG对来自主动脉肌膜的NKA的作用。在来自3月龄大鼠的主动脉的血管平滑肌细胞的原代培养物中，2nmol MBG刺激胶原蛋白的合成。在24个月大的大鼠中，血管平滑肌细胞中的前胶原-1和胶原的基线水平大于3个月大动物的细胞中的水平。来自年轻动物的血管平滑肌细胞中PKG-1基因的沉默产生了胶原蛋白基线水平的增加，并导致这些细胞对MBG的促纤维化作用的敏感性增强。因此，（i）在老年大鼠中，cGMP/PKG依赖性信号传导的下调是ANP调节MBG对肾和血管NKA作用的转变的基础，这促进了盐敏感性，（ii）在血管平滑肌细胞中，PKG依赖性信号传导的年龄相关性下调是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.