Abstract
The sarco/endoplasmic reticulum (SER) Ca2+ pool is refilled by the SER Ca2+ pump (SERCA) using cytosolic Ca2+ and/or extracellular Ca2+ entering the cell. The effects of the SERCA pump inhibitor cyclopiazonic acid (CPA) were studied in pig coronary artery smooth muscle using two protocols. In protocol A, the SERCA pump was inhibited by adding CPA to cells/tissues in Ca2+-containing solution, whereas in protocol B, CPA was added to cells/tissues in Ca2+-free solution, followed by reintroduction of extracellular Ca2+. Addition of CPA increased cytosolic Ca2+ in cultured smooth muscle cells and elicited contraction in de-endothelialized coronary arteries in both protocols. Based on pharmacological experiments, the CPA-induced contraction of de-endothelialized arteries in protocol B resulted from store operated Ca2+ entry (SOCE). Reactive oxygen species such as peroxides are known to damage the SERCA pump in this tissue. Consistently, CPA-induced contractions were decreased in arteries pre-treated with hydrogen peroxide in protocol A. However, this pretreatment also decreased the force of contraction due to SOCE in protocol B, suggesting that it closed SOCE. We propose that the closure of SOCE triggered by exposure to reactive oxygen species may be a protective mechanism, so that Ca2+ entry by this pathway is disallowed when SERCA is damaged in pathologies such as ischemia–reperfusion.
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