Clarifying the role of (apical) K+ channels in Na+ and Cl- transport.

A colleague once stated that ‘Heaven is a place where you can deal with just one issue at a time.’ His statement reflected the fact that, in a complex system, one cannot define or even investigate the role of one component without simultaneously defining the activities of other complimentary, competing, or supporting components. This ‘un-heavenly state’ is the situation as one defines epithelial ion transport. It is impossible to study one ion transport mechanism without defining the rest of the system and changes in one mechanism reverberate to affect all other components. In a recent issue of The Journal of Physiology, Palmer and colleagues (Palmer et al. 2011) add clarity to the contribution that a particular K+ channel, KCa3.1, makes to net epithelial ion transport. The outcome is intriguing because it addresses both K+ secretion and Na+ absorption in a particular epithelial cell system, the human mammary gland. In this and previous work (Lee et al. 2007) the authors report that KCa3.1, the product of the KCNN4 gene, is expressed in both the apical and basolateral membrane of mammary epithelial cells and, importantly, that it appears to be regulated independently at the two locations. These observations have the greatest impact when placed in the context of a complex system that is known to support the absorption and/or secretion of multiple ions. Mechanisms to account for the secretion and composition of milk are poorly understood. Milk is isotonic, but varies widely in composition between species. In all cases, however, the concentration of K+ is greater and Na+ is less than in plasma. Humans are at the lowest end of the mammalian spectrum in milk cation concentration. The paper by Palmer et al. perhaps provides an explanation for these ion concentrations and ratios. The epithelial Na+ channel ENaC can account for Na+ absorption from the apical (milk) compartment. Basolateral KCa3.1 can recycle K+ that enters with activity of the Na+/K+ ATPase, hyperpolarize the cell, and thus provide the driving force for electrogenic Na+ absorption. Additionally, apical KCa3.1 could account for K+ secretion into milk that would also tend to hyperpolarize the cell. In the presence of ENaC, apical KCa3.1 would support ongoing Na+ absorption and a component of the net K+ and Na+ flux would be electroneutral. The authors show changes in net ion flux measured as short circuit current that are consistent with transient electrogenic K+ secretion. Because of ongoing Na+ absorption, one wonders whether this K+ secretion is maintained. There is certainly that possibility. Four decades ago it was suggested that mammary apical membranes were freely permeable to Na+ and K+ (Linzell & Peaker, 1971), although this opinion was questioned consistently. While numerous laboratories have documented ENaC in mammary epithelial cells, Palmer and colleagues have provided the best evidence for a K+ secretory mechanism that can be regulated and can work in concert with ENaC to account for milk cation composition. One naturally wonders whether apical KCa3.1 and basolateral KCa3.1 are, in fact, identical. This suspicion becomes more pressing with apparently independent regulation at the two membranes. One possible explanation is that second messenger pathways can be targeted to localized regions of a cell. Certainly, Palmer et al. show that at least two types of purinergic receptors can be targeted to modulate apical KCa3.1 and that one of the receptors affecting the apical conductance does not modify KCa3.1 activity at the basolateral membrane. An alternative concept regarding targeted regulation is presented by Rajendran and co-workers (Barmeyer et al. 2010; Nanda Kumar et al. 2010). These authors suggested that the molecular identity of the apical K+ conductance is distinct from that expressed at lateral membranes of rat and human colonic epithelia. Three isoforms of mRNA coding for KCa3.1 were identified in rat colon. The authors suggested that KCa3.1a was expressed in smooth muscle, that KCa3.1b was expressed in epithelial lateral membranes and that KCa3.1c was expressed in epithelial apical membranes. The epithelial isoforms reportedly exhibited distinct pharmacological profiles with a tenfold difference in the potency of a selective inhibitor, TRAM-32. Like the current work from Palmer et al. the reported observations support a model that can account for tightly regulated K+ secretion and that can contribute to the regulation of other ion transport processes, in this case anion secretion mediated by CFTR or other apical anion channels. The authors concluded that apical K+ exit could hyperpolarize the cell and drive anion secretion. So, is there a simple, perhaps heavenly, model that can incorporate these potentially disparate, yet potentially complimentary observations? Certainly, there is. Figure 1 provides a generic epithelial cell model that incorporates observations from Palmer et al., Rajendran and colleagues, and others that have placed KCa3.1 in the apical and/or basolateral membranes of other ion transporting epithelia. The depicted model allows for K+ secretion through apical KCa3.1c that would hyperpolarize the cell and promote Na+ absorption and/or anion secretion, depending upon the tissue and prevailing conditions. Basolateral KCa3.1b could recycle K+ to the interstitial space while hyperpolarizing the cell to drive the vectoral transport of other ions. This hypothetical, idealized epithelial cell model works in theory, but remains to be tested. Selective modulation by purinergic agonists, differences in pharmacological sensitivities, and knockout or knockin systems provide new and powerful tools to demonstrate the overall contributions of KCa3.1 to net K+, Na+ and anion transport. Critical experiments to quantify simultaneous K+ secretion and Na+ absorption or anion secretion are required for each tissue system. Perhaps the best evidence that KCa3.1 participates in physiological K+ secretion would come from a measurable change in milk composition following selective inhibition or stimulation. When it comes to defining epithelial ion transport mechanisms, heaven may be unachievable, but perhaps by testing this model thoroughly we can arrive at a land flowing with milk (and honey). Figure 1 Epithelial cell model to account for K+ secretion and vectoral transport of other ions.