MAINTENANCE OF BRADYKININ-INDUCED INTRACELLULAR CALCIUM RESPONSE OF MDCK CELLS DEPENDS ON EXTRACELLULAR CALCIUM

Bradykinin (BK) induces intracellular calcium ([Ca2+](i)) release in M adin-Darby canine kidney (MDCK) cells. During long-term continuous BK exposure, cells become desensitized and fail to respond to a new BK st imulus. We used a protocol of repeated short-term BK addition and remo val. MDCK cells were loaded with the Ca-indicator indo-1 and were expo sed to BK (100 nmol/L) for 10 seconds, followed by BK removal. This cy cle was repeated four to eight times while [Ca2+](i) was continuously recorded. In a Ca-free bath, the cells gradually became completely des ensitized to repeated BK stimuli. In the presence of 1 mmol/L or 10 mm ol/L Ca-e, however, repeated addition of BK caused repeated [Ca2+](i) transients with partial decrease of peak heights (327 and 436 nmol/L D elta[Ca2+](i final)) (partial desensitization). Repeated BK stimuli al so led to partial desensitization (70% to 85%) to adenosine triphospha tase and carbachol (heterologous desensitization). BK also reduced pea k thapsigargin response (70%), consistent with partial depletion of en doplasmic reticulum Ca pools. Our results show that MDCK cells maintai n their sensitivity to BK during repeated short-term BK exposures. Ava ilable Ca-3 plays a major role in modulating the degree of cellular re sponsiveness.