Angiotensin II contractions in coronary artery

Pig coronary artery rings denuded of endothelium contract to the vasoactive hormone angiotensin II (Ang II). The nature of Ang II receptors and their Ca2+-pool utilization were examined for contraction of the artery rings and for increase in ultracellular [Ca2+] ([Ca2+]i) in smooth muscle cells cultured from them. Ang II contracted the arteries (EC50=7±4 nM) but with a lower maximal force (1.4±0.25 N/g tissue) than the contraction with 60 mM K+ (6.11±0.63 N/g tissue). In the cultured cells it caused a transient increase in [Ca2+]i with an EC50 value of 11±4 nM. The cells bound Ang II with a dissociation constant (Kd) of 7±2 nM. Based on the effects of the Ang II antagonists saralasin, DuPont 753, dithiothreitol and PD123319, the Ang II receptors responsible for contraction, increase in [Ca2+]i and Ang II binding to coronary artery smooth muscle were of type AT1.The contraction to Ang II was abolished by EGTA but not by nitrendipine. The sarcoplasmic Ca2+ pump inhibitors cyclopiazonic acid (10 μM CPA) and thapsigargin (1 μM) produced contractions of 4.35±0.73 and 2.07±0.54 N/g, respectively. Ang II contractions in the control arteries were nearly abolished upon pretreatment with CPA and thapsigargin. CPA and thapsigargin induced contractions were abolished by exposure to EGTA for 1 h but short exposure of the cells to EGTA only modulated the CPA or thapsigargin induced increase in [Ca2+]i; Ang II induced increase in [Ca2+]i was not inhibited by 1 μM nitrendipine but was reduced significantly by a 30–60 sec exposure to EGTA. CPA and thapsigargin caused an increase in [Ca2+]i even after 30–200 sec exposure to EGTA. Ang II when added after CPA or thapsigargin did not cause a further increase in [Ca2+]i but when added before them it caused an increase in [Ca2+]i and reduced the increase caused by subsequent addition of CPA or thapsigargin. These data are consistent with the concept that Ang II utilizes an intracellular Ca2+-pool which is a small component of the CPA or thapsigargin sensitive Ca2+-pool.