THE EFFECTS OF A NOVEL VASODILATOR, LP-805, ON CYTOSOLIC CA2+ CONCENTRATIONS AND ON TENSION IN RABBIT ISOLATED FEMORAL ARTERIES

1 LP-805, 5-methylpyrazolo-[1,5a]-pyrimidine-3-carbonitrile, is a newl y synthesized potent vasodilator. To investigate the cellular mechanis ms of vasorelaxation induced by LP-805, we simultaneously determined t he effects of LP-.805 on cytosolic Ca2+ concentrations ([Ca2+](i)) and on tension of smooth muscle of rabbit femoral arterial strips, with o r without the endothelium, using front-surface fluorometry and fura-2. 2 In the absence of the endothelium, LP-805, in a concentration-depen dent manner, decreased [Ca2+](i) and tension during the contraction in duced by K+-depolarization, at relatively low concentrations ([K+](o) less than or equal to 30 mM). The decreases in [Ca2+](i) and tension w ere fully antagonized by treatment with 2 x 10(-6) M glibenclamide. Th e [Ca2+](i)-tension relationship in the LP-805-induced relaxation was similar to that of K+-depolarization-induced contractions. 3 LP-805, i n a concentration-dependent manner (IC50 for inhibition of tension; 1. 7 x 10(-6) M), decreased both [Ca2+](i) and tension during the steady- state of contractions induced by 1 x 10(-7) M noradrenaline (NA) in th e strips without the endothelium. Glibenclamide completely inhibited t hese reductions of [Ca2+](i) and tension. At the steady-state of relax ation induced by LP-805 during NA-induced contraction. [Ca2+](i)-tensi on relation was shifted to the left of that obtained with high K+-indu ced contraction. 4 NA induced transient increases in [Ca2+](i) and ten sion in the absence of extracellular Ca2+. LP-805 (up to 3 x 10(-6) M) had no effect on these intracellular Ca2+ mobilisation and tension de velopment induced by NA. 5 In strips with an intact endothelium, LP-80 5 decreased both [Ca2+](i) and tension during contraction induced by 1 x 10(-7) M NA. The concentration-response curve for inhibition of [Ca 2+](i) and tension obtained in the presence of the endothelium was shi fted to the left from that obtained in the absence of endothelium. IC5 0 for the inhibition of tension obtained in the strips with the endoth elium was 4.0 x 10(-7) M. Treatment with 1 x 10(-4) M N-G-nitro-L-argi nine (L-NOARG) attenuated reductions of both [Ca2+](i) and tension ind uced by LP-805 and the concentration-response curve shifted to the rig ht and overlapped that obtained in the absence of the endothelium. Tre atment with glibenclamide almost fully overcame the reduction of [Ca2](i) induced by LP-805, while the reversion of tension was 50% at most . 6 In the presence of the endothelium with L-NOARG, LP-805 reduced th e tension to the extent of that expected from the reduction of [Ca2+]( i), as based on the [Ca2+](i)-tension relationship obtained with LP-80 5 in the absence of endothelium. On the contrary, in the presence of t he endothelium without L-NOARG, LP-805 induced a greater reduction of tension than expected from the reduction of [Ca2+](i). This effect bec ame more apparent after treatment with glibenclamide. 7 These results suggest that: (1) LP-805 relaxes smooth muscle mainly by activating AT P-sensitive K+ channels of smooth muscle and by releasing endothelium- derived relaxing factor (EDRF). (2) Activation of ATP-sensitive K+ cha nnels decrease [Ca2+](i) and thereby relax smooth muscle with no effec t on Ca2+-sensitivity of the contractile apparatus of smooth muscle or on the agonist-induced Ca2+-release process. (3) EDRF induced by LP-8 05 relaxes smooth muscle not only by decreasing [Ca2+](i) but also dec reasing Calf-sensitivity of the contractile apparatus of smooth muscle . In the presence of an intact endothelium, a decrease in Ca2+-sensiti vity of the contractile apparatus may play an important role in LP-805 -induced relaxation.