Ca2+ influx triggered by depletion of sarcoplasmic reticulum (SR) Ca2+ stores [mediated via store-operated Ca2+ channels (SOCC)] was characterized in enzymatically dissociated porcine airway smooth muscle (ASM) cells. When SR Ca2+ was depleted by either 5 muM cyclopiazonic acid or 5 mM caffeine in the absence of extracellular Ca2+, subsequent introduction of extracellular Ca2+ further elevated [Ca2+](i). SOCC was insensitive to 1 muM nifedipine- or KCl-induced changes in membrane potential. However, preexposure of cells to 100 nM - 1 mM La3+ or Ni2+ inhibited SOCC. Exposure to ACh increased Ca2+ influx both in the presence and absence of a depleted SR. Inhibition of inositol 1,4,5-trisphosphate (IP3)-induced SR Ca2+ release by 20 muM xestospongin D inhibited SOCC, whereas ACh-induced IP3 production by 5 muM U-73122 had no effect. Inhibition of Ca2+ release through ryanodine receptors (RyR) by 100 muM ryanodine also prevented Ca2+ influx via SOCC. Qualitatively similar characteristics of SOCC-mediated Ca2+ influx were observed with cyclopiazonic acid- vs. caffeine-induced SR Ca2+ depletion. These data demonstrate that a Ni2+/ La3+-sensitive Ca2+ influx via SOCC in porcine ASM cells involves SR Ca2+ release through both IP3 and RyR channels. Additional regulation of Ca2+ influx by agonist may be related to a receptor-operated, noncapacitative mechanism.