Role of sarcoplasmic reticulum and mitochondria in calcium signaling in vascular smooth muscle

Superficial sarcoplasmic reticulum (SR) regulates smooth muscle force development directly and indirectly. In the rabbit basilar artery (BA), relative contributions of direct effects and those mediated through activation of K _Ca were evaluated by measuring force and intracellular Ca2+ concentration ([Ca2+]_i) in response to the SR-depleting agents thapsigargin (Tg) and ryanodine and the large conductance K_Ca (BK_Ca) blockers iberiotoxin (IbTx) and tetraethylammonium ion (TEA). It appears that a significant fraction of K_Ca remains activated in the absence of SR function and that SR contributes to relaxation after blockade of K_Ca. We found that depletion of SR before stimulating Ca2+ influx through voltage-gated Ca2+ channels markedly reduced force development rate and that thapsigargin abolished this effect. We conclude that the SR of rabbit cerebral arteries modulates constriction by direct and indirect mechanisms.; Next, we investigated the role of mitochondria (MT) in calcium signaling in a primary culture of rat aortic smooth muscle cells. We have used the calcium photoprotein, aequorin, selectively targeted to the mitochondrial matrix to measure [Ca2+] in this organelle. Our results reveal that smooth muscle cell stimulation with 1 mM ATP or 1 μM vasopressin (AVP) causes a large, transient increase in mitochondrial [Ca2+] ([Ca 2+]_m). This large transient can be blocked with 100 μM cyclopiazonic acid (CPA) or 1 μM Tg, suggesting a close relationship between the SR and MT. Thus, in addition to SR, MT are also important in Ca 2+ homeostasis of smooth muscle.; Finally, gene expression studies using RT-PCR were performed in 3 types of smooth muscle; rabbit BA, inferior vena cava (IVC), and a rat aortic smooth muscle cell (RASMC) line. Expression of BK_Ca channels, and VGCC differed between rabbit BA and IVC, and was compared to functional data using various inhibitors. Taken together, this data suggests an association of RyR to BK_Ca in BA, and store-operated Ca2+ channels and IP₃ in IVC. We hypothesize that SR and MT interactions with channels and pumps on the PM, and with each other, are critical in the formation of cytoplasmic Ca2+ microdomains, contributing to the diversity of Ca2+ signaling in different smooth muscles.