SODIUM-INDEPENDENT MODULATION OF NA+-K+-ATPASE ACTIVITY BY BETA-ADRENERGIC AGONIST IN ALVEOLAR TYPE-II CELLS

Although beta-adrenergic agonists are known to stimulate sodium transport in alveolar epithelial cells, the exact cellular mechanism involved in this process is unknown. We determined whether terbutaline, a beta-adrenergic agonist, modulated Na+-K+-ATPase in cultured rat alveolar type II cells by measuring the enzyme's activity via an adapted radiometric method. The assay conditions were optimized by evaluating permeabilization techniques and substrate concentrations for Na+-K+-ATPase measurement at maximum velocity enzyme reaction (V-max). Terbutaline at 10(-2) M increased enzyme activity, with a maximal response at 15 min that was completely inhibited by 10(-2) M propranolol. This effect of terbutaline was dependent on the presence of serum as well as on the time the cells were in culture. The enhancement of Na+-K+-ATPase activity was reproduced by 10(-3) M dibutyryl adenosine 3',5'-cyclic monophosphate and 5 x 10(-5) M forskolin. Neither 10(-4) M amiloride nor a sodium-free solution influenced the effect of terbutaline. Western blotting showed that terbutaline did not change the expression of the alpha(1)-subunit of the enzyme, which is the predominant form in this cell type. We conclude that beta-adrenergic agonists can modulate Na+-K+-ATPase activity partially through adenosine 3',5'-cyclic monophosphate and this process is not secondary to an increase in intracellular sodium concentration.