Differential tyrosine phosphorylation of plasma membrane Ca2+-ATPase and regulation of calcium pump activity by carbachol and bradykinin

We investigated the effects of thapsigargin (TG), bradykinin (BK), and carbachol (CCh) on Ca2+ entry via endogenous channels in human embryonic kidney BKR21 cells. After depletion of Ca2+ stores by either TG, BK, or CCh, the addition of Ca2+ gave a much larger rise in Ca2+ levels in CCh-treated and TG-treated cells than in cells treated with BK. However, in experiments performed with Ba2+, a cation not pumped by Ca2+-ATPases, only a modest difference between CCh- and BK-stimulated Ba2+ entry levels was observed, suggesting that the large difference in the Ca2+ response is mediated by a differential regulation of Ca2+ pump activity by CCh and BK. This hypothesis is supported by the finding that when Ca2+ is removed during the stable, CCh-induced Ca2+ plateau phase, the decline of cytosolic Ca2+ is much faster in the absence of CCh than in its presence. In addition, if Ca2+ is released from a caged Ca2+ compound after a UV pulse, the resulting Ca2+ peak is much larger in the presence of CCh than in its absence. Thus, the large increase in Ca2+ levels observed with CCh results from both the activation of Ca2+ entry pathways and the inhibition of Ca2+ pump activity. In contrast, BK has the opposite effect on Ca2+ pump activity. If Ca2+ is released from a caged Ca2+ compound, the resulting Ca2+ peak is much smaller in the presence of BK than in its absence. An investigation of tyrosine phosphorylation levels of the plasma membrane Ca2+-ATPase (PMCA) demonstrated that CCh stimulates an increase in tyrosine phosphorylation levels, which has been reported to inhibit Ca2+ pump activity, whereas in contrast, BK stimulates a reduction of PMCA tyrosine phosphorylation levels. Thus, BK and CCh have a differential effect both on Ca2+ pump activity and on tyrosine phosphorylation levels of the PMCA.