MECHANISM OF ENDOTHELIUM-DEPENDENT RELAXATION INDUCED BY SUBSTANCE-P IN THE CORONARY-ARTERY OF THE PIG

1 Using front-surface fluorometry of fura-2-loaded porcine coronary arterial strips with the endothelium intact, we investigated the mechanisms of vasorelaxation induced by substance P (SP). Fura-2 fluorescence signals which indicated the cytosolic Ca2+-concentration ([Ca2+](i)), were observed to arise exclusively from the smooth muscle cells in these strips. 2 During the contractions induced by U46619 (100 nM), a thromboxane A(2) analogue, an SP-induced endothelium-dependent, biphasic vasorelaxation was observed, which consisted of an initial rapid relaxation phase followed by a sustained phase, with a transient decrease in [Ca2+](i). Pretreatment with indomethacin (Ind) had no effect on the SP-induced relaxation; however, pretreatment with N-G-nitro-L-arginine (L-NOARG) partially, but significantly inhibited the decrease in both the [Ca2+](i) and tension induced by SP; in addition, the sustained phase of SP-induced relaxation was almost completely abolished. Thus, part of the relaxation was considered to be mediated by L-NOARG-sensitive relaxing factor (endothelium-derived relaxing factor : EDRF). 3 During the 40 mM K+-depolarization-induced contraction which may eliminate the effects of endothelium-derived hyperpolarizing factor (EDHF), the vasorelaxation induced by SP was completely inhibited by L-NOARG. 4 During the vasorelaxation induced by SP, the [Ca2+](i)-tension relationships shifted to the right of the contractions induced by either U46619 or high K+-depolarization. 5 Using front-surface fluorometry of fura-2 loaded porcine aortic valvular strips, we examined the effects of SP on [Ca2+](i) in endothelial cells in situ. SP induced a rapid increase in [Ca2+](i) of endothelial cells in situ followed by a small sustained phase in normal PSS (5.9 mM K+). The increase in extracellular K+ had no apparent effect on the SP-induced [Ca2+](i) elevation of endothelial cells. 6 We thus conclude that: (I) SP-induced vasorelaxation is mediated by an L-NOARG-sensitive factor (EDRF) and an L-NOARG-resistant factor; and (2) the first, rapid, phase of the relaxation is mediated by both factors while the sustained phase seems to be mediated mainly by EDRF. The underlying mechanisms of L-NOARG-resistant relaxation have yet to be elucidated, but EDHF appears to be a potentially contributing factor.