Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...

128 SUSAN WRAY AND THEODOR BURDYGA
not to occur in most smooth muscles. Few studies on
smooth muscle have considered the more complex role of
the SR in this tissue, e.g., providing negative feedback on
Ca entry via Ca sparks, as opposed to it acting as a Ca
store to augment contraction. We tentatively conclude
that phospholamban is not a major physiological regulator
of SERCA in most smooth muscles; its effects on
SERCA, for example, are not as large as those of luminal
[Ca] alteration. This difference between smooth and cardiac
muscles may be due to the relative expression of
SERCA isoforms between the two muscles, but we suggest
it also reflects the differences in the role of SERCA
between them.
D. Sarcolipin
This 31-amino acid protein appears to be a homolog
of phospholamban (522); it associates with the SR membrane,
and its transmembrane domains are structurally
similar. Sarcolipin inhibits SERCA by lowering its apparent
Ca affinity (increasing K m) and V max (27, 205, 435,
523). It has been found in greatest abundance in fast- (199,
425, 511) and slow-twitch muscles (523) as well as cardiac
muscles (720). To date, we can find no data describing
sarcolipin presence in any smooth muscle.
E. Calsequestrin
1. Introduction
Calsequestrin and calreticulin (discussed next) are
the two most important luminal buffers of Ca in the SR.
They share many protein properties, as well as fulfilling
the requirement of being a Ca buffer, i.e., binding Ca with
a high capacity and low affinity. Calsequestrin was the
first SR Ca binding protein to be identified (103, 429, 432).
Its crystal structure has been elucidated from rabbit skeletal
muscle (734). These studies suggest that three very
negative thioredoxin-like domains underlie the high Ca
binding capacity of calsequestrin.
2. Isoforms and expression
There is evidence for calsequestrin expression, at
varying levels, in the following smooth muscles: stomach
(585, 730, 778), vas deferens (729, 730), aorta (730), ileum
(585), and trachea (585). It may not be present in bladder
(730) or pulmonary artery (585) and is either absent or in
trace amounts in uterus (472).
It is now appreciated that two genes exist for calsequestrin
expression, leading to two isoforms which are
65% identical (622). These isoforms are often referred to
as the fast-twitch skeletal and cardiac forms (182, 622),
but slow skeletal muscles express the fast form (181) and
it appears that many smooth muscles express both iso-
Physiol Rev VOL 90 JANUARY 2010 www.prv.org
forms (730). The functional significance of smooth muscles
expressing the two isoforms of calsequestrin or neither
remains to be established.
Calsequestrin in skeletal muscle binds 40–50 Ca
ions with a binding constant of 1 mM and a high off-rate
(430). The cardiac isoform binds 20 Ca ions with an
affinity of 0.5 mM (51). In striated muscles calsequestrin
binds to other proteins, including triadin and junctin (see
sect. IVG), leading to a physical association with RyRs
(233, 443). It may be that this arrangement enables the Ca
of the SR to be in the right place, i.e., close to the release
channel (187). Regular arrays of calsequestrin with Ca
attached appear as crystalline structures in the SR lumen
(607). Calsequestrin has been shown to increase the RyR
open probability in skeletal muscle (343).
Wuytack et al. (778) were the first to report that
calsequestrin was present in SR from smooth muscle.
They examined pig antrum and concluded it contained
the cardiac form of calsequestrin in low amounts compared
with heart, although commensurate with the lower
SERCA expression in smooth muscle compared with
heart. These authors also noted that calsequestrin in portal
vein smooth muscles had first been proposed, on
morphological grounds, by Somlyo (652) and Franzini-
Armstrong (187). Vas deferens expresses both calsequestrin
isoforms in about equal amounts (730). Its distribution
was reported to be clustered at discrete luminal sites
and on SR located superficially below the plasma membrane
and rich in IP 3R (729). Thus a specific distribution
of calsequestrin in smooth muscle as reported also for
striated muscles may occur (322, 323). This contrasts with
the nondiscrete distribution of calreticulin (see below).
The colocalization of IP 3R with calsequestrin presumably
aids rapid release of messenger Ca in smooth muscles
expressing both these elements. Moore et al. (491) demonstrated
in toad stomach that calsequestrin was not only
present in the SR in a discrete manner, but that it was
associated with the superficial SR. They also demonstrated
a close association between calsequestrin and the
NCX and Na pump distribution on the plasma membrane,
in caveolae. Thus not only is calsequestrin close to SR Ca
release channels but also to exchangers important to Ca
homeostasis in smooth muscle, and contributing to microdomains
(as discussed in sect. IID).
Despite these several publications from a number of
different laboratories, a paper appeared stating that calreticulin
and not calsequestrin was the major Ca binding
protein in smooth muscle SR (472). This study however
only examined one type of smooth muscle, pig uterus, and
actually reported that calsequestrin could be detected in
uterine tissue extracts but not SR vesicles. They also used
calsequestrin antibodies on cultured uterine cells and
reported negligible staining, but given the culturing and a
switch to a noncontractile, synthetic state, these data are
difficult to interpret. This publication appears to have