Correlation Between Leukotriene D4-lnduced Contraction and Cytosolic Calcium Elevation: A Quantitative and Simultaneous Evaluation in Smooth Muscle

The role of Ca as an intracellular messenger in leukotriene (LT)D4-induced muscle contraction was investigated by meas-unng force development and elevation in cytosolic Ca4 concen-tration simultaneously in strips of guinea pig ileal longitudinal muscle loaded with the fluorescent calcium indicator Fura 2. Upon addition of LTD4, a simultaneous increase in tension and cytosolic calcium concentration, [CaJ, was observed. Cumu-lative applications of LTD4 induced concentration-dependent in-creases in both muscle tension and [CaJ, being the half-maximal effect reached at approximately 6 to 9 nM. A statistically significant positive correlation (r = 0.993, P <.001) exists between the two parameters examined. Removal of calcium in the bathing solution, accompanied by addition of 7.5 mM EGTA, completely prevented any increase in either calcium levels or force development, thus indicating a role for Ca influx, rather than a release from intracellular stores. All of the LTD4 antago-nists tested were able to counteract the effect of the leukotnene on both [Ca] and tension increase. However, although LY1 71 883 shifted both of the LTD4 curves to the right in a parallel fashion, FPL 5571 2 and ICI 198,61 5 behaved as non-competitive antagonists in reversing the effect of LTD4 on [Ca] and tension. Thus, these results strongly suggest that changes in muscle tension induced by LTD4 are attributable to changes in cytosolic free Ca concentrations in guinea pig ileum. Since the demonstration that the sulfidopeptide leukotrienes LTC4, LTD4 and LTE4 are the components of SRS-A, a variety of studies have shown that these LTs are potent constrictors of smooth muscle, both vascular and extravascular (Feuerstein, 1985; Piper, 1983). In particular, the ability of SRS-A to con-tract guinea pig ileal longitudinal muscle has been known fo