The renal Na+:Ca2+ exchanger (NCX) plays an important role in regulation of cytosolic calcium concentration [Ca 2+]i in contractile cells such as vascular smooth muscle and mesangial cells (MCs). NCX regulation of [Ca2+]i is accomplished by exchanging one cytosolic Ca2+ ion for three extracellular Na+ ions. Previously, we cloned, sequenced, and expressed MC exchangers from Dahl/Rapp salt resistant (R; RNCX1) and salt sensitive (S; SNCX1) rats. RNCX1 (alternative splice site encoded by B, D; isoleucine at amino acid 218), but not SNCX1 (B, D, and F; phenylalanine at 218), exhibited up-regulation following agonist-induced protein kinase C (PKC) activation. Using RT-PCR, we initiated studies to identify NCX1 isoform(s) from cultured MCs of the parental Sprague Dawley (SD) rat strain. We identified a novel isoform, denoted SDNCX1.10, that contains isoleucine at 218 and exons B, D, E, and F at the alternative splice site. SDNCX1.10 was then stably expressed in opossum proximal tubule kidney (OK-PTH) cells where we were able to evaluate functional activity by assessing, in forward and reverse mode, exchange activity. OK-PTH cells expressing SDNCX1.10 reduced elevations in [Ca2+] i, induced by 1 mM ATP via purinergic receptors, back to baseline levels 5-fold faster than cells expressing vector alone. Down-regulation of PKC, following prolonged exposure (24 h) to PMA resulted in a 66% reduction in ATP-induced increases in [Ca2+]i in SDNCX1.10 transfected cells compared to cells transfected with vector alone. This attenuation suggests that SDNCX1.10 is a PKC-sensitive isoform. KB-R7943, an exchanger-specific inhibitor, attenuated the reverse activity of SDNCX1.10 (by 50% versus vector-only cells) and stimulated the forward mode. Further studies were performed to investigate some of the regulatory properties of SDNCX1.10. We found that this isoform was regulated by [Ca2+]i and was inhibited by decreases in intracellular pH (pHi) and that the activity of this exchanger was not altered by protein kinases A (PKA) and G (PKG). We also compared the regulation of [Ca2+]i between the RNCX1 and SNCX1 isoforms. SNCX1 was much less effective in lowering agonist induced increases in [Ca2+]i back to baseline, suggesting that cells expressing SNCX1 may have an impaired ability to regulate [Ca2+]i. Thus, this finding may help explain the hypertension and renal failure that are a hallmark of salt-sensitive hypertension.
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机译:肾Na + super>:Ca 2 + super>交换子(NCX)在调节胞质钙浓度[Ca 2 + super>] i sub>在诸如血管平滑肌和肾小球膜细胞(MCs)的收缩细胞中。 [Ca 2 + super>] i sub>的NCX调节是通过将一个胞质Ca 2 + super>离子交换为三个胞外Na + 来实现的超离子。以前,我们从Dahl / Rapp抗盐(R; RNCX1)和盐敏感(S; SNCX1)大鼠克隆,测序和表达MC交换子。 RNCX1(由B,D;氨基酸218处的异亮氨酸编码的替代剪接位点)而非SNCX1(B,D和F; 218处的苯丙氨酸)在激动剂诱导的蛋白激酶C(PKC)激活后呈现上调。使用RT-PCR,我们启动了研究,以从亲本Sprague Dawley(SD)大鼠菌株的培养的MC中鉴定NCX1亚型。我们确定了一个新的同种型,表示为SDNCX1.10,其在218位包含异亮氨酸,在另一个剪接位点包含外显子B,D,E和F。然后,SDNCX1.10在负鼠负鼠肾小管肾(OK-PTH)细胞中稳定表达,我们能够通过以正向和反向方式评估交换活性来评估功能活性。表达SDNCX1.10的OK-PTH细胞降低了[Ca 2 + super>] i sub>的升高,这是由嘌呤能受体通过1 mM ATP诱导的,比基线速度快5倍细胞单独表达载体。长时间暴露于PMA(24 h)后,PKC的下调导致SDNCX1中ATP诱导的[Ca 2 + super>] i sub>的增加减少了66%。与仅用载体转染的细胞相比,有10个转染的细胞。这种衰减表明SDNCX1.10是PKC敏感的同工型。交换子特异性抑制剂KB-R7943减弱SDNCX1.10的逆向活性(相对于仅含载体的细胞,降低了50%)并刺激了正向模式。进行了进一步的研究以研究SDNCX1.10的某些调节特性。我们发现这种同工型受[Ca 2 + super>] i sub>的调节,并受到细胞内pH值(pH i sub>)的降低的抑制,并且交换蛋白的活性没有被蛋白激酶A(PKA)和G(PKG)改变。我们还比较了RNCX1和SNCX1亚型之间[Ca 2 + super>] i sub>的调控。 SNCX1在降低激动剂诱导的[Ca 2 + super>] i sub>增加到基线时的效果要差得多,这表明表达SNCX1的细胞调节[Ca 的能力可能受损。 super> 2 + super>] i sub>。因此,这一发现可能有助于解释高血压和肾衰竭,这是盐敏感性高血压的标志。
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