DEPLETION AND REFILLING OF ACETYLCHOLINE-SENSITIVE AND CAFFEINE-SENSITIVE CA++ STORES IN TRACHEAL MYOCYTES

We have previously shown that acetylcholine (ACh) induces oscillations in Ca++ and Ca++-activated Cl- currents (Cl-ca) in tracheal myocytes. These oscillations are initiated by Ca++ release from inositol 1,4,5- trisphosphate-sensitive Ca++ stores and maintained by Ca++ influx, in part, through voltage-operated Ca++ channels. In the current study who le-cell Cl-ca was measured in isolated tracheal smooth muscle cells as an index of changes in intracellular Ca++ concentration. We demonstra te that ACh-sensitive Ca++ stores and caffeine-sensitive Ca++ stores a re functionally linked but are refilled through distinct pathways. Two pathways responsible for replenishing ACh-sensitive Ca++ stores were identified. Ca++ influx through verapamil-sensitive voltage-operated C a++ channels and Cafe uptake through cyclopiazonic acid-sensitive Ca+ pumps accounted for 80% of the response. The other 20% of the respons e was both cyclopiazonic acid- and verapamil-insensitive. In contrast, the refilling of caffeine-sensitive Ca++ stores was not inhibited by 10 mu M cyclopiazonic acid or 10 mu M verapamil, but was dependent on extracellular Ca++ concentration. However, 0.2 mu M thapsigargin, anot her more potent Ca++ pump inhibitor, completely and irreversibly elimi nated ACh-induced transient Cl-ca, whereas it reduced caffeine-induced Cl-ca by 57%. The differences in refilling mechanisms and the functio nal overlap of ACh- and caffeine-sensitive Ca++ pools suggest that mul tiple interactive Ca++ stores play an important role in the generation of Ca++ signals in airway smooth muscle cells.