DIDS and the Janus-faced Na⁺-K⁺-activated ATPase. Focus on "DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism".

THE MAINTENANCE OF MAMMALIAN cell volume fundamentally rests on net cation extrusion through Na -K -activated ATPase, the Na pump. The publication by Shahidullah et al. (11) persuasively identifies the series of events by which a widely used inhibitor of anion transport, DIDS, can reduce Na pump activity of porcine nonpigmented ciliary epithelial (NPE) cells. DIDS sequentially acidifies the NPE cells and activates Src family kinases (SFK), thereby triggering phosphorylation and inhibition of Na -K -activated ATPase (Fig. 1). A pH dependence of pump activity has been long known; Na -K -ATPase activity of Xenopus membrane homogenates displays a pH maximum at pH 7 (10). The authors’ results may be clinically relevant. By inhibiting Na pump activity, DIDS can reduce porcine aqueous humor formation and thereby lower intraocular pressure. Reducing intraocular pressure remains the only documented approach for delaying the onset and slowing the progression of irreversible blindness in glaucoma. In addition to this potential clinical implication, the authors’ observations are broadly instructive in two additional ways. First, even when its direct action is limited to inhibition of anion transport, DIDS can initiate a wide range of unexpected downstream effects. That the stilbene can exert nonspecific effects at concentrations in excess of 100 M is well known. However, the work of Shahidullah et al. (11) provides a further cautionary note. Their conclusions suggest that even were DIDS applied at concentrations less than 10 M, a range over which only anion transporters and not Cl channels are thought inhibited, unanticipated downstream signaling events could result. Second, the observations of Shahidullah et al. (11) highlight yet another action of the SFK. SFK comprises three groups of protein tyrosine kinases, some of which are alternatively spliced in different cells. These kinases play major roles in signaling an enormous spectrum of cellular activities, apart from regulation of sodium pump activity. These roles are subject to considerable complexities (14). The interaction observed by Shahidullah et al. (11) is directionally opposite to the effect of the ouabain-treated Na pump to activate SFK, which has been documented by multiple investigators (4). Thus, the Na pump both responds to intracellular regulators (backwardlooking), and in turn can modify those same regulators (forward-looking), hence a Janus-faced aspect of Na -K -activated ATPase (Fig. 1). The specific action of SFK on the Na pump of these NPE cells cannot be directly extrapolated to other cells. In the present work, the authors found that SFK inhibits Na -K activated ATPase of porcine NPE cells. In previous work from this group, SFK was found to exert an opposite effect on Na -K -activated ATPase of rabbit lens epithelial cells (13). Such a discrepancy is not entirely surprising. In the present work, SFK was activated within the framework of the signaling cascade triggered by cellular acidosis, whereas in the previous study SFK was activated as part of the P2-receptor purinergic signaling cascade. Multiple downstream and cross-reacting actions could be playing roles. The central focus of the current publication has been the ion transport function of Na -K -activated ATPase. The Na /K exchange activity and energy requirement of a Na pump had been anticipated from results in multiple tissues, even before the Nobel Prize-winning discovery of Na -K -activated ATPase by Skou in 1957. The kinetics of that exchange activity have also been long understood within the framework of Address for correspondence: M. M. Civan, Dept. of Physiology, A303 Richards Bldg., Univ. of Pennsylvania, Philadelphia, PA 19104-6085 (e-mail: [email protected]).