Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...
Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...
Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...
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132 SUSAN WRAY AND THEODOR BURDYGA<br />
suppression of RyR3 did not alter Ca spark activity. In<br />
ur<strong>in</strong>ary bladder myocytes, RyR2s are required for generation<br />
of Ca sparks (314). Native RyR3s are either not<br />
<strong>in</strong>volved (133, 314) or may even have an <strong>in</strong>hibitory effect<br />
on Ca sparks (407). However, when both RyR1 and RyR2<br />
are <strong>in</strong>hibited with antisense oligonucleotides, and under<br />
conditions of <strong>in</strong>creased SR Ca load<strong>in</strong>g, RyR3 can be activated<br />
by caffe<strong>in</strong>e or agonists (475, 476). RyR3 gene knockout<br />
significantly <strong>in</strong>hibits the contractile response to hypoxia<br />
but not the norep<strong>in</strong>ephr<strong>in</strong>e-<strong>in</strong>duced Ca and contractile<br />
responses, <strong>in</strong> pulmonary artery smooth muscle cells<br />
(805).<br />
The function of RyR3 <strong>in</strong> Ca signal<strong>in</strong>g is complicated<br />
by alternative splic<strong>in</strong>g of RyR3. The expression of a short<br />
isoform of RyR3 (RyR3s) <strong>in</strong> HEK293 cells can <strong>in</strong>hibit both<br />
the full-length RyR3 (RyR3L) and RyR2 subtypes (315). In<br />
native smooth muscle, the dom<strong>in</strong>ant negative effect of the<br />
spliced isoform of RyR3 <strong>in</strong>hibits RyR2 activation and Ca<br />
signals <strong>in</strong> duodenum (138) and the activity of RyR3 <strong>in</strong><br />
mouse pregnant myometrium (139). These observations<br />
reveal a novel mechanism by which a splice variant of one<br />
RyR3 isoform may heteromerize with the other RyR variants<br />
and suppress the activity of these RyR isoforms via a<br />
dom<strong>in</strong>ant negative effect. Thus tissue-specific expression<br />
of RyR3 splice variants is likely to account for some of the<br />
pharmacological and functional heterogeneities of RyR3,<br />
for example, the lack of Ca sparks and caffe<strong>in</strong>e sensitivity<br />
<strong>in</strong> rat myometrium, despite expression of all three forms<br />
of RyRs (88).<br />
On the basis of the available <strong>in</strong>formation, RyR2 appears<br />
to be the most important component of Ca sparks <strong>in</strong><br />
smooth muscle, and recent f<strong>in</strong>d<strong>in</strong>gs on RyR3 splice variants<br />
have helped unravel some of the perplexity <strong>in</strong> Ca<br />
signal<strong>in</strong>g <strong>in</strong> smooth muscle. Calcium sparks are discussed<br />
<strong>in</strong> detail <strong>in</strong> section IX.<br />
C. IP 3-Sensitive Ca release channels<br />
B<strong>in</strong>d<strong>in</strong>g of G prote<strong>in</strong>-coupled receptor agonists to their<br />
receptors <strong>in</strong> smooth muscles leads to activation of phosphatidyl<strong>in</strong>ositol-specific<br />
phospholipase C (PLC), which catalyzes<br />
the hydrolysis of phosphatidyl<strong>in</strong>ositol 4,5-bisphosphate<br />
(PIP 2) to yield IP 3 and diacylglycerol (DAG) (48). The<br />
IP 3 releases Ca from <strong>in</strong>tracellular stores by b<strong>in</strong>d<strong>in</strong>g to IP 3R,<br />
which are IP 3-gated Ca release channels, while DAG activates<br />
prote<strong>in</strong> k<strong>in</strong>ase C (PKC). Activation of IP 3Rs via activation<br />
of L-type Ca channels <strong>in</strong>dependent of Ca entry, socalled<br />
metabotropic Ca channel-<strong>in</strong>duced Ca release, has<br />
been suggested <strong>in</strong> arterial myocytes but requires confirmation<br />
by other studies (149), and is not discussed further.<br />
1. Expression<br />
Three dist<strong>in</strong>ct IP3R gene products (types 1–3) have<br />
been identified <strong>in</strong> different mammalian species (283, 546).<br />
All three isoforms of IP 3R are structurally and functionally<br />
related (193). The IP 3R isoforms differ <strong>in</strong> their sensitivities<br />
to breakdown by cellular proteases (IP 3R2 relatively<br />
more resistant that IP 3R1 and IP 3R3) (765). These<br />
receptors exist as tetrameric structures, with a monomeric<br />
molecular mass of 300 kDa. Unlike RyRs, IP 3Rs<br />
are poorly expressed <strong>in</strong> skeletal and cardiac muscle but<br />
extensively expressed <strong>in</strong> bra<strong>in</strong> and other tissues <strong>in</strong>clud<strong>in</strong>g<br />
smooth muscles (317, 471, 498, 546, 695, 760, 794). In<br />
smooth muscles, the density of IP 3Ris100 times less<br />
than <strong>in</strong> bra<strong>in</strong> (437, 802). In visceral smooth muscles (356,<br />
379–381), IP 3Rs are expressed <strong>in</strong> greater quantity than<br />
RyRs with an overall stoichiometric ratio of IP 3Rs to RyRs<br />
of 10–12:1, while <strong>in</strong> vascular smooth muscles the ratio<br />
is 3-4:1 (62).<br />
<strong>Smooth</strong> muscle cells express multiple IP 3R isoforms,<br />
although as with RyRs, the level of expression of different<br />
subtypes is tissue and species dependent. Us<strong>in</strong>g reversetranscriptase<br />
PCR analysis, Morel et al. (493) showed that<br />
rat portal ve<strong>in</strong> expressed IP 3R1 and IP 3R2, whereas rat<br />
ureter expressed predom<strong>in</strong>antly IP 3R1 and IP 3R3 (493),<br />
although <strong>in</strong> an earlier study it was reported that rat ureteric<br />
myocytes expressed all three isoforms of IP 3Rs (60).<br />
In the thoracic aorta and mesenteric arteries, IP 3R1 was<br />
the only isoform found, while IP 3R1 and weak expression<br />
of IP 3R2 were reported for smooth muscle cells of basilar<br />
artery (324). The type 1 isoform predom<strong>in</strong>ates <strong>in</strong> gu<strong>in</strong>ea<br />
pig <strong>in</strong>test<strong>in</strong>al smooth muscle cells (222) as well as <strong>in</strong> adult<br />
porc<strong>in</strong>e (297) and rat aorta (693), whereas type 3 predom<strong>in</strong>ates<br />
<strong>in</strong> neonatal aorta (693). Subtype-specific IP 3R antibodies<br />
revealed that the expression of IP 3R1 was similar<br />
<strong>in</strong> cultured aortic cells and aorta homogenate, but expression<br />
of IP 3R2 and IP 3R3 types was <strong>in</strong>creased threefold <strong>in</strong><br />
cultured cells (693). Immunosta<strong>in</strong><strong>in</strong>g and functional studies<br />
performed on isolated cells and cells <strong>in</strong> situ <strong>in</strong> rat<br />
portal ve<strong>in</strong> showed two subpopulations of cells, one<br />
which predom<strong>in</strong>antly expressed IP 3R1 and a second predom<strong>in</strong>antly<br />
express<strong>in</strong>g IP 3R2. Dist<strong>in</strong>ct patterns of the Ca<br />
responses were generated by the two subpopulations of<br />
myocytes <strong>in</strong> response to agonist (493).<br />
2. Localization<br />
Physiol Rev VOL 90 JANUARY 2010 www.prv.org<br />
In some smooth muscles, IP 3R1 has been localized<br />
throughout the cell, i.e., central and peripheral SR, as<br />
visualized us<strong>in</strong>g EM (191, 515, 693, 729), and <strong>in</strong> the others<br />
it had a ma<strong>in</strong>ly subplasmalemmal location (222). IP 3R2<br />
has a diffuse cytoplasmic distribution, similar to IP 3R1<br />
(and IP 3R3), but also occurs as dense patches <strong>in</strong> the<br />
peripheral cytoplasm, a pattern that IP 3R1 did not exhibit<br />
(668). IP 3Rs were also arranged <strong>in</strong> clusters <strong>in</strong> rat ureteric<br />
myocytes, as well as throughout the cell (60). In vascular<br />
myocytes, IP 3R2 is distributed peripherally and associated<br />
with the nucleus <strong>in</strong> proliferat<strong>in</strong>g cells (668, 694). In<br />
portal ve<strong>in</strong>, IP 3R2 was also found closely associated with