Role of inositol triphosphate isomer formation in type II pneumocyte signal transduction

Adenosine triphosphate (ATP) is a potent agonist of surfactant secretion from type II pneumocytes. The extracellular ATP signal is transduced by both P1- and P2-purinergic pathways, which respectively initiate cyclic adenosine monophosphate formation and phosphatidyl inositol hydrolysis to inositol phosphates (Ins P). We investigated the role of inositol triphosphate (Ins P3) isomer formation in this signal transduction pathway. Primary cultures of rat type II pneumocytes were labeled with 30 microCi [3H]myoinositol/5 x 10(6) cells for 48 h. After preincubation with 10 mM LiCl for 20 min, the cells were stimulated with ATP (10(-4) M) and then were rapidly frozen with liquid N2. The Ins P3 isomers were analyzed by high performance liquid chromatography. A 4-fold increase in Ins 1,4,5 P3 occurred within 2 s after stimulation with ATP, decreased to half maximum by 60 s, and returned close to baseline values by 2 min. In contrast, Ins 1,3,4 P3 did not increase until 15 s, peaked by 60 s with a 4-fold increase, and returned to baseline values by 2 min. Intracellular calcium [( Ca2+]i), measured as Indo-1 fluorescence, also increased 3-fold within 2 s of exposure to ATP (10(-4)M) and fell to a plateau level 25% above baseline values by 10 s. We conclude that Ins 1,4,5 P3 formation and [Ca2+]i release both occur rapidly after exposure of type II pneumocytes to extracellular ATP. We speculate that these early events in type II pneumocyte signal transduction play a role in the initiation of stimulation of surfactant secretion by extracellular ATP.