Defective regulation of apical membrane chloride transport and exocytosis in cystic fibrosis

A biochemical link is proposed between recent observations on defective regulation of Cl⁻ transport in CF respiratory epithelial cells and studies showing altered biological activity of calmodulin in exocrine glands from CF patients. A consensus is emerging that defective β-adrenergic secretory responsiveness in CF cells is caused by a defect in a regulator protein at a site distal to cyclic AMP formation. Our results indicate that this protein might be a specific calmodulin acceptor protein which modifies the activity of calmodulin in epithelial cells. Alteration in Ca²⁺/calmodulin dependent regulation of Cl⁻ transport and protein secretion could explain (i) alterations in Ca²⁺ homeostasis seen in CF, (ii) defective β-adrenergic responses of CF cells, and (iii) the observed inability of cyclic AMP (acting via its specific protein kinase, A-kinase) to open apical membrane Cl⁻ channels in CF epithelial cells. Most of the physiological abnormalities of CF including elevated sweat electrolytes and hyperviscous mucus can be explained on this basis.