THE PHOSPHOLIPASE-C INHIBITOR U73122 INCREASES CYTOSOLIC CALCIUM IN MDCK CELLS BY ACTIVATING CALCIUM INFLUX AND RELEASING STORED CALCIUM

The effects of the phospholipase C (PLC) inhibitor U73122 on intracell ular calcium levels ([Ca2+](i)) were studied in MDCK cells. U73122 ele vated [Ca2+](i) dose-dependently. Ca2+ influx contributed to 75% of 20 mu M U73122-induced Ca2+ signals. U73122 pretreatment abolished the [ Ca2+](i) transients evoked by ATP and bradykinin, suggesting that U731 22 inhibited PLC. The Ca2+ signals among individual cells varied consi derably. The internal Ca2+ source for the U73122 response was the endo plasmic reticulum (ER) since the response was abolished by thapsigargi n. The depletion of the ER Ca2+ store triggered a La3+-sensitive capac itative Ca2+ entry. Independently of the internal release and capacita tive Ca-2 entry, U73122 directly evoked Ca2+ influx through a La3+-ins ensitive pathway. The U73122 response was augmented by pretreatment of carbonylcyanide m-chlorophynylhydrozone (CCCP), but not by Na+ remova l, implicating that mitochondria contributed significantly in bufferin g the Ca2+ signal, and that efflux via Na+/Ca2+ exchange was insignifi cant.