Ammann H, Noël J, Tejedor A, Boulanger Y, Gougoux A, Vinay P
Department of Physiology, Notre-Dame Hospital, Montréal, Canada.
Can J Physiol Pharmacol. 1995 Apr;73(4):421-35. doi: 10.1139/y95-055.
In renal cells, the Na+ pump maintains a transmembrane concentration gradient for sodium ensuring the net reabsorption of sodium with or without cotransported species. This process requires a significant fraction of the ATP turnover of proximal tubules and thick ascending limbs. To understand the potential regulatory influences of Na+ and ATP on the activity of the Na+ pump in these nephron segments, the apparent kinetics of the membrane-bound Na+-K+ ATPase and of the cellular Na+ pump were studied in different preparations of dog proximal tubules and thick ascending limbs (tubular suspensions, tissue homogenates, and basolateral membrane vesicles) obtained from dog kidney cortex and red medulla. Two determinant kinetic parameters, i.e., the apparent Michaelis constant (Km) and the saturating concentrations for sodium and ATP, were compared with the intracellular concentrations of Na+ and ATP measured under physiological conditions. In both types of tubules, the apparent Km value for Na+ (5-15 mM) is set well below the measured mean intracellular concentration of sodium (50-60 mM), suggesting that the Na+ pump should be saturated by sodium ions under normal conditions. Nevertheless, a modest increment of the Na concentration in the vicinity of the pump, obtained by equilibrating the intra- and extra-cellular sodium concentrations at various extracellular [Na+] with nystatin, increases the activity of the Na+ pump in intact cortical tubules and thick ascending limbs, even when the extracellular [Na+] is set at the estimated intracellular [Na+], demonstrating that the pump is not saturated by sodium in situ. Similarly, the kinetics of the renal Na+ pump as a function of the ATP concentration suggested that the pump should be saturated by ATP in physiological conditions, since in both tissues the cellular ATP level (3-6 mM) is higher than the concentration required to achieve saturation of this activity (< 2.5 mM). However, in renal cortical tubules, the steady-state intracellular [Na+] is affected by modest changes of ATP concentration, suggesting that the Na+ pump is not functionally saturated by ATP. Our data suggest that concentration gradients for Na+ and ATP may exist in the cytosol of renal cells. These gradients would be related to the polarity of sodium transport and of the ATP-consuming and ATP-regenerating processes in intact cells.(ABSTRACT TRUNCATED AT 400 WORDS)
在肾细胞中，钠泵维持着钠的跨膜浓度梯度，确保钠在有或没有共转运物质的情况下的净重吸收。这一过程需要近端小管和髓袢升支粗段相当一部分的ATP周转。为了了解钠和ATP对这些肾单位节段中钠泵活性的潜在调节影响，在从狗肾皮质和髓质获得的狗近端小管和髓袢升支粗段的不同制剂（肾小管悬浮液、组织匀浆和基底外侧膜囊泡）中研究了膜结合的钠钾ATP酶和细胞钠泵的表观动力学。将两个决定性动力学参数，即表观米氏常数（Km）以及钠和ATP的饱和浓度，与在生理条件下测得的细胞内钠和ATP浓度进行了比较。在这两种类型的小管中，钠的表观Km值（5 - 15 mM）设定得远低于测得的细胞内钠的平均浓度（50 - 60 mM），这表明在正常条件下钠泵应该被钠离子饱和。然而，通过用制霉菌素使细胞内和细胞外钠浓度在各种细胞外[Na⁺]下达到平衡，从而在泵附近适度增加钠浓度，即使细胞外[Na⁺]设定为估计的细胞内[Na⁺]，也会增加完整皮质小管和髓袢升支粗段中钠泵的活性，这表明泵在原位并未被钠饱和。同样，肾钠泵作为ATP浓度函数的动力学表明，在生理条件下泵应该被ATP饱和，因为在这两种组织中细胞内ATP水平（3 - 6 mM）高于实现该活性饱和所需的浓度（< 2.5 mM）。然而，在肾皮质小管中，稳态细胞内[Na⁺]会受到ATP浓度适度变化的影响，这表明钠泵在功能上并未被ATP饱和。我们的数据表明，肾细胞胞质溶胶中可能存在钠和ATP的浓度梯度。这些梯度将与完整细胞中钠转运以及ATP消耗和ATP再生过程的极性相关。（摘要截断于400字）
