The collecting duct has two types of cells: principal cells (the aldosterone-responsive, ENac-expressing cells which also mediate water reabsorption via ADH) and the intercalated cells. The intercalated cells come in two different varieties: alpha-intercalated cells (which secrete acid) and beta-intercalated cells (which secrete base). The alpha-intercalated cells (on the left in the figure) are tall, columnar epithelial cells which contain apical H+ ATPase, explaining its ability to secrete protons. The beta-intercalated cells (on the right) in contrast are shorter, flatter cells which contain an apical chloride-bicarbonate exchanger (called pendrin) which enables it to secrete base.
The interesting thing is that inducing metabolic acidosis results in the conversion of beta-intercalated cells to alpha-intercalated cells--giving the kidney a greater ability to secrete protons and return pH to the normal range. This conversion event is regulated by a secreted extracellular matrix molecule called hensin. Since these cells evidently retain the ability to switch between markedly different cellular phenotypes, it has become of interest to the ever-evolving field of stem cell biology.