β<SUB>2</SUB>-Agonist regulation of cell volume in fetal distal lung epithelium by CAMP-independent Ca<SUP>2+</SUP> release from intracellular stores

Abstract

The effects of beta 2-adrenoceptor agonist (beta 2 agonist) and cAMP on cytosolic Ca2+ concentration ([Ca2+]c) and cell volume were studied in fetal distal lung epithelial cells. Both terbutaline (a specific beta 2 agonist, 10 microM) and dibutyryl cAMP (DBcAMP, 1 mM) increased [Ca2+]c in the presence of extracellular Ca2+. Even in the absence of extracellular Ca2+, the terbutaline-induced increase in [Ca2+]c was still observed, although the increase was transient. However, DBcAMP caused no significant change in [Ca2+]c. In the presence of 1 mM extracellular Ca2+, terbutaline and DBcAMP induced quinine (a blocker of K+ channel) sensitive cell shrinkage. However, in a Ca2(+)-free solution, terbutaline induced rapid cell shrinkage, followed by benzamil (a specific blocker of Na+ channel, an analogue of amiloride) sensitive transient cell swelling. In a Ca2(+)-free solution, DBcAMP induced benzamil-sensitive transient cell swelling without cell shrinkage. Taken together, our observations indicate that the beta 2 agonist induced an elevation of [Ca2+]c by increasing both a Ca2+ influx from the extracellular space and a Ca2+ release from intracellular Ca2+ stores, whereas DBcAMP only stimulated Ca2+ influx from the extracellular space. Furthermore, it is suggested that terbutaline and DBcAMP activated benzamil-sensitive channels independently of an increase in [Ca2+]c.