CORRELATION BETWEEN LEUKOTRIENE D-4-INDUCED CONTRACTION AND CYTOSOLICCALCIUM ELEVATION - A QUANTITATIVE AND SIMULTANEOUS EVALUATION IN SMOOTH-MUSCLE

The role of Ca++ as an intracellular messenger in leukotriene (LT)D-4- induced muscle contraction was investigated by measuring force develop ment and elevation in cytosolic Ca++ concentration simultaneously in s trips of guinea pig ileal longitudinal muscle loaded with the fluoresc ent calcium indicator Fura 2. Upon addition of LTD(4), a simultaneous increase in tension and cytosolic calcium concentration, [Ca++](i), wa s observed. Cumulative applications of LTD(4), induced concentration-d ependent increases in both muscle tension and [Ca++](i), being the hal f-maximal effect reached at approximately 6 to 9 nM. A statistically s ignificant positive correlation (r = 0.993, P < .001) exists between t he two parameters examined. Removal of calcium in the bathing solution , accompanied by addition of 7.5 mM EGTA, completely prevented any inc rease in either calcium levels or force development, thus indicating a role for Ca++ influx, rather than a release from intracellular stores . All of the LTD(4) antagonists tested were able to counteract the eff ect of the leukotriene on both [Ca++](i) and tension increase. However , although LY171883 shifted both of the LTD(4) curves to the right in a parallel fashion, FPL 55712 and ICI 198,615 behaved as noncompetitiv e antagonists in reversing the effect of LTD(4) on [Ca++](i) and tensi on. Thus, these results strongly suggest that changes in muscle tensio n induced by LTD(4) are attributable to changes in cytosolic free Ca+ concentrations in guinea pig ileum.