Calmidazolium-induced rises in cytosolic calcium concentrations in Madin Darby canine kidney cells

The effect of calmidazolium on Ca(2+) signaling in Madin Darby canine kidney (MDCK) cells was investigated using fura-2 as a Ca(2+) probe. Calmidazolium at 2-5 microM increased Ca(2+)](i) concentration dependently. The Ca(2+)](i) rise induced by 2-5 microM calmidazolium comprised an immediate rise and a slow decay. External Ca(2+) removal partly inhibited the Ca(2+) signals, suggesting that calmidazolium activated external Ca(2+) influx and internal Ca(2+) release. In Ca(2+)-free medium, pretreatment with 3 microM calmidazolium abolished the Ca(2+) release induced by 1 microM thapsigargin, an endoplasmic reticulum Ca(2+) pump inhibitor, and, vice versa, pretreatment with thapsigargin inhibited calmidazolium-induced Ca(2+) release. This indicates that thapsigargin-sensitive Ca(2+) store was the source of calmidazolium-induced Ca(2+) release. Calmidazolium (3 microM) induced Mn(2+) quench of fura-2 fluorescence at 360 nm excitation wavelength, which was suppressed by 0.1 mM La(3+). Addition of 3 mM Ca(2+) increased Ca(2+)](i) after pretreatment with 3-5 microM calmidazolium in Ca(2+)-free medium. This implies that calmidazolium activated concentration-dependent capacitative Ca(2+) entry. Calmidazolium (3 microM) augmented the capacitative Ca(2+) entry induced by 1 microM thapsigargin or 0.1 mM ATP by 38%. Calmidazolium (3 microM)-induced Ca(2+) release was blocked by pretreatment with 40 microM aristolochic acid and 2 microM U73122 (2 microM) to inhibit phospholipase A(2) and phospholipase, respectively, but pretreatment with 0.1 mM propranolol to inhibit phospholipase D had no effect. This suggests that calmidazolium induced internal Ca(2+) release in a manner dependent on phospholipases C- and A(2)-coupled events.