FLOSEQUINAN, A VASODILATOR WITH A NOVEL MECHANISM OF ACTION

1 The mechanism of action of flosequinan was investigated in ferret aortic smooth muscle by the simultaneous measurement of aequorin luminescence and isometric force. 2 The control calcium-force curve was obtained by plotting the calibrated aequorin luminescence against the force from potassium-depolarized muscles. Flosequinan relaxed potassium-depolarized muscles by causing parallel changes in [Ca2+]i and force with no shift in the control [Ca2+]i-force relationship. 3 The [Ca2+]i-force relationship in the presence of a maximally effective concentration of phenylephrine was significantly shifted to the left of that for the control, potassium-depolarized muscle. Flosequinan relaxed the phenylephrine-contracted muscle by causing a large decrease in force with only a minimal decrease in [Ca2+]i, resulting in an apparent rightward shift of the [Ca2+]i-force relationship, toward the control curve. 4 In comparison, sodium nitroprusside caused relaxation of either the potassium- or phenylephrine-induced contraction solely by a decrease in [Ca2+]i with no shift in either calcium-force relationship. 5 Milrinone caused no significant rightward shift of the calcium-force relationship during phenylephrineor potassium-induced contractions, but when milrinone was added in the absence of vasoconstrictors, relaxation was obtained with no significant decrease in [Ca2+]i. 6 Flosequinan appears to differ in mechanism of action from both nitroprusside and milrinone. It relaxes depolarization-mediated contractions solely by decreasing [Ca2+]i but also appears to be capable of reversing the apparent calcium sensitizing action of phenylephrine.