pH dependence of acid secretion of primary human tracheal cultures. Serosal pH 7.4 and mucosal target pH were set to step-wise increasing values over a range of pH 6.0 to 8. (A) Cultures expressing wild-type HVCN1. Filled symbols, untreated; open symbols, 10 µM ZnCl₂ mucosally. (B) Primary human tracheal cultures expressing M91T HVCN1. (C) Activation of Zn-sensitive acid secretion by mucosal alkalinization in wild-type (filled symbols) and M91T HVCN1 (open symbols). Data were calculated as the difference of values before-minus-after Zn addition from data in A and B. *, significantly different from control; t test.