Function and control of Na-K-ATPase in single nephron segments of the mammalian kidney

Abstract

Measuring Na-K-ATPase activity at the level of discrete nephron segments shows that besides its previously recognized role as a constitutive enzyme, Na-K-ATPase may be a primary hormonal effector as it is controlled by several hormones within minutes to hours (control of the pump synthesis, turnover rate or recycling in the cell). This breakthrough, which gives additional importance to Na-K-ATPase in transporting epithelia, was made possible thanks to the following two advantages of the micromethods for Na-K-ATPase measurements. First, they permit to study the enzyme at the level of the target segments, thus avoiding masking of the effects by unresponsive cells. Furthermore, the nephron heterogeneity has revealed itself as an advantage since it allowed to discriminate specific hormonal effects (which are localized in some segments of nephron only) from nonspecific ones. Secondly, they preserve some integrity of cellular architecture, which may be important when observing in vitro changes in the turnover rate or recycling of the enzyme within the cell. The characterization of these primary effects of hormones on Na-K-ATPase activity should permit a better understanding of the long-term alterations of Na-K-ATPase observed under pathological circumstances. Furthermore, the recent cloning of the genes encoding for the subunits of Na-K-ATPase will certainly soon give invaluable information on the molecular mechanisms involved in the transcriptional control of Na-K-ATPase synthesis by such hormones.