Abstract
1. The intracellular Ca2+ concentration ([Ca2+]i) was measured in mesenteric artery smooth muscle cells using the fluorescent indicator indo-1. 2. Noradrenaline (1-10 microM) produced a transient increase in [Ca2+]i. This response was unaffected by the removal of external calcium suggesting that the bulk of the increase in [Ca2+]i produced by noradrenaline is due to release from an intracellular store. 3. The maintained application of caffeine (10 mM) produced a transient rise in [Ca2+]i. The rate of relaxation was slower than that of the noradrenaline response. If caffeine was removed at the peak of the rise in [Ca2+]i then [Ca2+]i recovered more quickly than was the case in both the maintained response to noradrenaline and that to caffeine. 4. In the presence of noradrenaline, caffeine or thapsigargin elevated [Ca2+]i. However, if thapsigargin or caffeine was added first, the subsequent application of noradrenaline did not increase [Ca2+]i, suggesting that only part of the caffeine-sensitive store is sensitive to noradrenaline. 5. The recovery of [Ca2+]i during the application of caffeine was unaffected by the removal of external sodium suggesting that Na+-Ca2+ exchange is not important in the reduction in [Ca2+]i. The addition of lanthanum (1 mM) did, however, greatly slow [Ca2+]i recovery. 6. We conclude that the three major factors responsible for removing Ca2+ ions from the cytoplasm are: (i) a caffeine- and noradrenaline-sensitive store (43%), (ii) a caffeine-sensitive but noradrenaline-insensitive store (36%), and (iii) a sarcolemmal Ca(2+)-ATPase (16%). Finally, a 5% contribution remains to be accounted for.