FIG. 1.

In a single cell (a), Na-K homeostasis is achieved automatically because Na,K-ATPase activity increases or decreases in response to fluctuations in cytoplasmic sodium concentration at the Na⁺ binding site on the cytoplasmic face. Na-K homeostasis is more complicated when the cell is coupled to one (b) or more cells that lack Na,K-ATPase (c). It must export Na⁺ ions that enter at a distant site. This is the case in the lens (d) where Na,K-ATPase is located in the epithelium but not fibers. (d) Is adapted from Mathias et al.¹⁶ It depicts how the arrangement of Na,K-ATPase, K channels, and gap junctions at the lens surface creates a circulating flow of current/ions through coupled cells of the fiber mass. In an intact lens, the flow exits at the equator and enters at the anterior and posterior poles.