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 intracellular calcium levels ([Ca²⁺](i)) were studied in MDCK cells. U73122 elevated [Ca²⁺](i) dose-dependently, Ca²⁺ influx contributed to 75% of 20 μM U73122-induced Ca²⁺ signals. U73122 pretreatment abolished the [Ca²⁺](i) transients evoked by ATP and bradykinin, suggesting that U73122 inhibited PLC. The Ca²⁺ signals among individual cells varied considerably. The internal Ca²⁺ source for the U73122 response was the endoplasmic reticulum (ER) since the response was abolished by thapsigargin. The depletion of the ER Ca²⁺ store triggered a La³⁺-sensitive capacitative Ca²⁺ entry. Independently of the internal release and capacitative Ca² entry, U73122 directly evoked Ca²⁺ influx through a La³⁺-insensitive pathway. The U73122 response was augmented by pretreatment of carbonylcyanide m-chlorophynylhydrozone (CCCP), but not by Na⁺ removal, implicating that mitochondria contributed significantly in buffering the Ca²⁺ signal, and that efflux via Na⁺/Ca²⁺ exchange was insignificant.