View of K+ secretion through the apical K channel of cortical collecting duct.

The apical small-conductance K+ channel plays an important role in renal K+ secretion, as evidenced by the presence of the extensive modulatory pathways. Figure 3 summarizes the current understanding of the mechanisms that modulate the apical small-conductance K+ channel. Stimulation of adenylate cyclase enhances channel activity and consequently K+ secretion. In contrast, increases in intracellular Ca2+ concentration and activation of Ca(2+)-dependent signal transduction pathways inhibit the K+ channel and thus decrease K+ secretion. The vasopressin-induced stimulation of K+ secretion in CCD results at least in part from cAMP-dependent signal transduction pathways. The Ca(2+)-dependent signal transduction pathway is responsible for modulatory coupling between Na+ pump turnover and apical K+ conductance when the Na+ pump is inhibited.