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 transp ort in alveolar epithelial cells, the exact cellular mechanism involve d in this process is unknown. We determined whether terbutaline, a bet a-adrenergic agonist, modulated Na+-K+-ATPase in cultured rat alveolar type II cells by measuring the enzyme's activity via an adapted radio metric method. The assay conditions were optimized by evaluating perme abilization techniques and substrate concentrations for Na+-K+-ATPase measurement at maximum velocity enzyme reaction (V-max). Terbutaline a t 10(-2) M increased enzyme activity, with a maximal response at 15 mi n that was completely inhibited by 10(-2) M propranolol. This effect o f 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 acti vity was reproduced by 10(-3) M dibutyryl adenosine 3',5'-cyclic monop hosphate and 5 x 10(-5) M forskolin. Neither 10(-4) M amiloride nor a sodium-free solution influenced the effect of terbutaline. Western blo tting showed that terbutaline did not change the expression of the alp ha(1)-subunit of the enzyme, which is the predominant form in this cel l 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 sod ium concentration.