Do pulmonary ionocytes absorb chloride or secrete chloride?

Pulmonary ionocytes express high levels of cystic fibrosis transmembrane conductance regulator (CFTR) channels. When studied using the short-circuit current technique, ionocytes produce CFTR-dependent short-circuit currents consistent with Cl secretion. However, when studied without a voltage clamp, data indicate that ionocytes absorb Cl. In this review, we resolve these seemingly conflicting findings by considering the different transepithelial voltages and the resultant movement of Cl during short circuit and physiological open-circuit conditions. This analysis indicates that behavior under short-circuit conditions cannot be directly extrapolated to infer behavior under physiologic conditions. Finally, we discuss the potential role of basolateral Cl channels in controlling absorption and secretion in ionocytes.