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 Kca were
evaluated by measuring force and intracellular Ca²⁺ concentration ([Ca²⁺]j) in response to
the SR-depleting agents thapsigargin (Tg) and ryanodine and the large conductance Kca
(BKca) blockers iberiotoxin (IbTx) and tetraethylammonium ion (TEA). It appears that a
significant fraction of Kca remains activated in the absence of SR function and that SR
contributes to relaxation after blockade of Kca . 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 uM vasopressin (AVP) causes a large, transient increase in mitochondrial [Ca ]
([Ca²⁺][sub m]). This large transient can be'blocked with 100 μM cyclopiazonic acid (CPA) or
1 uM Tg, suggesting a close relationship between the SR and MT. Thus, in addition to
SR, MT are also important in Ca²⁺ 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 BKca 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 BKca 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 Ca signaling in different smooth
muscles.Medicine, Faculty ofAnesthesiology, Pharmacology and Therapeutics, Department ofGraduat
