Sensitivity of G‐protein involved in endothelin‐1‐induced Ca2+ influx to pertussis toxin in porcine endothelial cells in situ

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Abstract
1 We designed a new method to determine quantitatively the intracellular Ca2+ concentration ([Ca2+]i) in endothelial cells in situ, using front-surface fluorometry and fura-2-loaded porcine aortic valvular strips. Using this method, we investigated the characteristics of the G-protein involved in endothelin-1 (ET-1)-induced changes in [Ca2+]i of endothelial cells in situ. 2 Endothelial cells were identified by specific uptake of acetylated-low density lipoprotein labelled with 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl-indocarbocyanine perchlorate (DiI-Ac-LDL). Double staining with DiI-Ac-LDL and fura-2 showed that the valvular strip was covered with a monolayer of endothelial cells and that the cellular component which contributed to the fura-2 fluorescence, [Ca2+]i signal, was exclusively endothelial cells. 3 ET-1 (10−7 m) induced an elevation of [Ca2+]i consisting of two components: the first was a rapid and transient elevation to reach a peak, followed by a second, sustained elevation (the second phase). The first phase was composed of extracellular Ca2+-independent and -dependent components, while the second phase was exclusively extracellular Ca2+-dependent. The extracellular Ca2+-independent component of the first phase was due to the release of Ca2+ from intracellular storage sites. The second phase and part of the first phase of [Ca2+]i elevation were attributed to the influx of extracellular Ca2+. The Ca2+ influx component was completely inhibited by 10−3 m Ni2+ but was not affected by 10−5 m diltiazem. 4 Pertussis toxin (IAP) markedly inhibited the extracellular Ca2+-dependent elevation of [Ca2+]i, but had no effect on the extracellular Ca2+-independent elevation of [Ca2+]i caused by ET-1 (10−7 m). 5 Bradykinin (10−7 m) or ATP (10−5 m) elevated [Ca2+]i and these responses also consisted of extracellular Ca2+-independent and extracellular Ca2+-dependent components. IAP had no effect on either component of the [Ca2+]i elevation induced by bradykinin or ATP. 6 From these findings we conclude that, in porcine endotheliel cells in situ, ET-1 elevates [Ca2+]i as a result of a Ca2+ influx component from the extracellular space and release of intracelluarly stored Ca2+. The Ca2+ influx is regulated by an IAP-sensitive G-protein, while the release of Ca2+ from the intracellular store is not.