Functional Nucleotide Receptor Expression and Sarcoplasmic Reticulum Morphology in Dedifferentiated Porcine Coronary Smooth Muscle Cells

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Abstract
The phenotypic dedifferentiation of vascular smooth muscle cells (SMCs) is an early event associated with cell culturing and vascular injury. The purpose of this study was to evaluate the SMC phenotype underlying the functional responsiveness of SMCs to nucleotides in organ culture. Porcine coronary arteries were either used fresh, cold stored (5°C) 4 days, or organ cultured (37°C) 4 days. Fura-2 digital imaging of single SMCs was used to measure the myoplasmic calcium (Cam) response to 10 µM of the following nucleotide receptor agonists: UTP, UDP, ATP, ADP, and 2-MeSATP. In contrast to the nucleotides UDP, ATP, ADP, and 2-MeSATP, the Cam response increased 10-fold and the number of cells that responded to UTP increased 5-fold in SMCs from organ culture compared to SMCs from fresh or cold-stored arteries. Simultaneous imaging of Cam, DNA content, and SR distribution in SMCs from organ culture indicated that the UTP-induced Cam increase occurred exclusively in SMCs that had a dedifferentiated cell phenotype. Three-dimensional image reconstruction of the nucleus and sarcoplasmic reticulum (SR) revealed a novel transnuclear SR distribution that intertwined with the nucleus in fresh SMCs, while in SMCs from organ culture the SR was predominantly perinuclear and cytoplasmic. This study demonstrates that the functional up-regulation of UTP-sensitive receptors and the disappearance of the transnuclear SR distribution are novel features of dedifferentiated coronary SMCs.