Arachidonate-Regulated Ca2+ Influx in Human Airway Smooth Muscle

Plasma membrane Ca2+ influx, especially store-operated Ca2+ entry triggered by sarcoplasmic reticulum (SR) Ca2+ release, is a key component of intracellular calcium concentration ([Ca2+](i)) regulation in airway smooth muscle (ASM). Agonist-induced Ca2+ oscillations in ASM that involve both influx and SR mechanisms have been previously demonstrated. In nonexcitable cells, [Ca2+](i) oscillations involve Ca2+ influx via arachidonic acid (AA) -stimulated channels, which show similarities to store-operated Ca2+ entry, although their molecular identity remains undetermined. Little is known about AA-regulated Ca2+ channels or their regulation in ASM. In enzymatically dissociated human ASM cells loaded with the Ca2+ indicator, fura-2, AA (1-10 mu M) triggered [Ca2+](i) oscillations that were inhibited by removal of extracellular Ca2+. Other fatty acids, such as the diacylglycerol analog, 1-oleoyl-2-acetyl-SN-glycerol, oleic acid, and palmitic acid (10 mu M each), failed to elicit similar [Ca2+](i) responses. Preincubation with LaCl3 (1 mu M or 1 mM) inhibited AA-induced oscillations. Inhibition of receptor-operated channels (SKF96,365 [10 mu M]), lipoxygenase (zileuton [10 mu M]), or cyclooxygenase (indomethacin [10 mu M]) did not affect oscillation parameters. Inhibition of SR Ca2+ release (ryanodine [10 mu M] or inositol 1,4,5-trisphosphate receptor inhibitor, xestospongin C [1 mu M]) decreased [Ca2+](i) oscillation frequency and amplitude. Small interfering RNA against caveolin-1, stromal interaction molecule 1, or Orai3 (20 nM each) reduced the frequency and amplitude of AA-induced [Ca2+](i) oscillations. In ASM cells derived from individuals with asthma, AA increased oscillation amplitude, but not frequency. These results are highly suggestive of a novel AA-mediated Ca2+ -regulatory mechanism in human ASM, reminiscent of agonist-induced oscillations. Given the role of AA in ASM intracellular signaling, especially with inflammation, AA-regulated Ca2+ channels could potentially contribute to increased [Ca2+](i) in diseases such asthma.