Reduced Sarco/Endoplasmic Reticulum Ca 2+ Uptake Activity Can Account for the Reduced Response to NO, but Not Sodium Nitroprusside, in Hypercholesterolemic Rabbit Aorta

Abstract

Background Hypercholesterolemia (HC) impairs acetylcholine-induced relaxation but has little effect on that caused by the NO donor sodium nitroprusside (SNP), suggesting that acetylcholine releases less NO from the endothelium in HC. The relaxation to authentic NO gas, however, is also impaired in HC aortic smooth muscle, indicating an abnormal smooth muscle response. NO relaxes arteries by both cGMP-dependent and -independent mechanisms, and the response involves calcium (Ca ²⁺ ) store refilling via the sarco/endoplasmic reticulum calcium ATPase (SERCA). We studied the involvement of cGMP and SERCA in the smooth muscle response to NO and SNP in HC rabbit aorta. Methods and Results A selective guanylyl cyclase inhibitor, 1H-[1,2,4]-oxadiazole-[4,3-a]quinoxalin-1-one, eliminated SNP-induced relaxation but only partially blocked NO-induced relaxation in both normal and HC aorta. The residual relaxation to NO was still less in HC and, in both normal and HC aorta, was abolished by concomitant administration of the SERCA inhibitor cyclopiazonic acid (CPA). In contrast, CPA did not affect SNP-induced relaxation in either normal or HC aorta. SERCA activity measured by ⁴⁵ Ca ²⁺ uptake was markedly decreased in HC, although SERCA2 protein expression did not change significantly. Conclusions These data suggest that NO-induced relaxation but not that to SNP is partially mediated by cGMP-independent Ca ²⁺ uptake into sarco/endoplasmic reticulum and that reduced sarco/endoplasmic reticulum Ca ²⁺ pump function can account for the impaired response to NO in HC.