Calcium and alpha-adrenergic regulation of Na-Cl(K) cotransport in rabbit tracheal epithelial cells

Abstract

The marked sensitivity of Cl and fluid secretion in mammalian airways to basolateral application of loop diuretics has led to postulates that electrically neutral Na-Cl entry plays a critical role during secretion. Electrically neutral Na-Cl(K) cotransport was investigated by determining the initial rate of 22Na and 36Cl efflux in epithelial cells isolated from rabbit trachea and preequilibrated with radioactive tracer at 25 degrees C. Tracer transport was initiated by 10-fold dilution of an aliquot of cells in radioisotope-free medium. The initial rate of radiolabeled ion transport was calculated from the linear portion of efflux curves. Base-line Na and Cl transport rates were not affected by furosemide or bumetanide. l-Epinephrine stimulated Na and Cl transport rates 1.8-fold each in Ca2(+)-replete and 2.6- and 2.3-fold, respectively, in Ca2(+)-deficient transport medium. Loop diuretics and yohimbine, an alpha 2-adrenergic antagonist, blocked the effects of l-epinephrine, and, clonidine, an alpha 2-adrenergic agonist, stimulated yohimbine- and furosemide-sensitive Cl transport. The beta-adrenergic agonist l-isoproterenol alone did not affect tracer transport and, in combination with clonidine, did not affect the response to clonidine. Elevation of intracellular Ca2+ with ionomycin stimulated tracer transport, and buffering of intracellular Ca2+ with 1,2-bis(aminophenoxy)ethane- N,N,N',N'-tetraacetic acid blocked the stimulatory effects of alpha-adrenergic agents. These results indicate an alpha 2-adrenergic stimulation of loop diuretic-sensitive Na and Cl transport that requires elevated intracellular Ca2+ as the second messenger. The transport mechanism is probably a Na-Cl or Na-K-2Cl cotransport located in the basolateral membrane.