Heterogeneity of calcium stores and elementary release events in canine pulmonary arterial smooth muscle cells

To examine the nature of inositol 1,4,5-trisphosphate (IP3)-sensitive and r yanodine (Ryn)-sensitive Ca2+ stores in isolated canine pulmonary arterial smooth cells (PASMC), agonist-induced changes in global intracellular Ca2concentration ([Ca2+](i)) were measured using fura 2-AM fluorescence. Prope rties of elementary local Ca2+ release events were characterized using fluo 3-AM or fluo 4-AM, in combination with confocal laser scanning microscopy. In PASMC, depletion of sarcoplasmic reticulum Ca2+ stores with Ryn (300 mu M) and caffeine (Caf; 10 mM) eliminated subsequent Caf-induced intracellula r Ca2+ transients but had little or no effect on the initial IP3-mediated i ntracellular Ca2+ transient induced by ANG II (1 muM). Cyclopiazonic acid ( CPA; 10 muM) abolished IP3-induced intracellular Ca2+ transients but failed to attenuate the initial Caf-induced intracellular Ca2+ transient. These r esults suggest that in canine PASMC, IP3-, and Ryn-sensitive Ca2+ stores ar e organized into spatially distinct compartments while similar experiments in canine renal arterial smooth muscle cells (RASMC) reveal that these Ca2 stores are spatially conjoined. In PASMC, spontaneous local intracellular Ca2+ transients sensitive to modulation by Caf and Ryn were detected, exhib iting spatial-temporal characteristics similar to those previously describe d for "Ca2+ sparks" in cardiac and other types of smooth muscle cells. Afte r depletion of Ryn-sensitive Ca2+ stores, ANG II (8 nM) induced slow, susta ined [Ca2+](i) increases originating at sites near the cell surface, which were abolished by depleting IP3 stores. Discrete quantal-like events expect ed due to the coordinated opening of IP3 receptor clusters ("Ca2+ puffs") w ere not observed. These data provide new information regarding the function al properties and organization of intracellular Ca2+ stores and elementary Ca2+ release events in isolated PASMC.