Effects of cyclopiazonic acid on cytosolic calcium in bovine airway smooth muscle cells

In many cells, inhibition of sarcoplasmic reticulum (SR Ca2+-ATPase activity induces a steady-state increase in cytosolic calcium concentration ([Ca2+](i)) that is sustained by calcium influx. The goal was to characterize the response to inhibition of SR Ca2+-ATPase activity in bovine airway smooth muscle cells. Cells were dispersed from bovine trachealis and loaded with fura 2-AM (0.5 muM) for imaging of single cells. Cyclopiazonic acid (CPA; 5 muM) inhibited refilling of both caffeine- and carbachol-sensitive calcium stores. In the presence of extracellular calcium, CPA caused a transient increase in [Ca2+](i) from 166 +/- 11 to 671 +/- 100 nM, and then [Ca2+](i) decreased to a sustained level (CPA plateau; 236 +/- 19 nM) significantly above basal. The CPA plateau spontaneously declined toward basal levels after 10 min and was attenuated by discharging intracellular calcium stores. When CPA was applied during sustained stimulation with caffeine or carbachol, decreases in [Ca2+](i) were observed. We concluded that the CPA plateau depended on the presence of SR calcium and that SR Ca2+-ATPase activity contributed to sustained increases in [Ca2+](i) during stimulation with caffeine and, to a lesser extent, carbachol.