Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...

142 SUSAN WRAY AND THEODOR BURDYGA
to a depletion of SR Ca. Previous work had investigated
TRPC channels as the putative Ca entry mechanism
opened by store Ca depletion, but the growing consensus
is that they are not responsible for I crac. Therefore, it may
be argued that TRPCs are unlikely to be responsible for
SOCE in smooth muscles. This has already been suggested
by some (153, 722, 738) but contested by others
(e.g., Ref. 7). TRPC1, the TRP most thought to be involved
in SOCE and highly expressed in smooth muscles, had no
effect on vascular function or SOCE when knocked out
(12). In TRPC6 knockout mice, enhanced DAG and agonist-induced
current in smooth muscle cells was reported,
i.e., the opposite result expected if TRPC6 is involved in
SOCE (188). RNA interference (RNAi)-based high-throughput
screens also revealed no effect on SOCE activation of
TRP channel knockdown, in contrast to Stim (400). As
with studies on nonexcitable cells, caution must be applied
to studies where genes have been knocked out or
overexpressed, and in smooth muscle, when cells have
been cultured, the physiological relevance of such studies
is even harder to interpret (274). Nevertheless, while it is
premature to rule TRPCs out of involvement in SOCE in
smooth muscle cells, especially as much of the Orai work
has been conducted on nonexcitable cells, it nevertheless
does not appear profitable to cover in great depth previous
studies related to a role in SOCE of TRPC, especially
as this has recently been reviewed (7). A recent report by
Dehaven et al. (148) demonstrated that TRPC functions
independently of Orai1 in vascular myocytes (A7r5 and
A10 cell lines) and suggests that TRPCs are activated by
PLC and do not involve Stim1 as the Ca sensor and TRCPs
open nonselective cation channels when stimulated by
arginine vasopressin. Roles for TRCPs other than in storeoperated
Ca entry in smooth muscle have been well demonstrated
for receptor-operated channels, where they
constitute nonspecific cation channels (39, 457) and
stretch channels (658), but these are outside the scope of
this review.
There is already evidence to support the Stim-Orai
model for SOCE in some smooth muscles: airway, Stim-1
(549), Orai (550); vascular, Stim (153, 413, 685), Stim and
Orai (47, 210); and uterus, Stim and Orai (611). The list of
smooth muscles expressing Stim and Orai is expected to
grow rapidly as more studies are performed. To date
however, there is already evidence that capacitative Ca
entry/SOCE in some smooth muscles at least occurs via
the Stim-Orai mechanism. Dietrich et al. (153) inhibited
SOCE using siRNA against Stim1, whereas TRCP1 knockout
did not affect it. Further support for a physiological
function for these proteins in smooth muscles comes
from data showing that their expression levels are altered
at different sites and in disease. Thus Orai and Stim were
upregulated in proliferating arterial myocytes, leading the
authors to suggest that they play a critical role in the
altered Ca handling that occurs during vascular growth
Physiol Rev VOL 90 JANUARY 2010 www.prv.org
and remodeling (47). Stim1 expression was greater in the
distal than proximal part of the pulmonary artery, where
SOCE is also greatest (413). Stim1 and Orai1 were also
found to be upregulated in aortas from hypertensive rats
(210). Knockdown of Orai1 reduced thapsigargin-induced
Ca influx and reduced I crac (210).
Thus, in summary, Orai and Stim appear to constitute
the molecular mechanism underlying capacitative Ca entry
and I crac in nonexcitable cells, and there is mounting
evidence for this also to be the case in smooth muscle. To
us, the most urgent questions revolve around whether
there is only one form of SOCE in smooth muscle and if
this involves only the Stim-Orai mechanism. The relative
simplicity of Ca entry in nonexcitable cells falls away in
studies of smooth muscle, as SOCE is a variable feature
between myocytes, and several other Ca entry pathways
exist. In addition, there are only relatively few studies that
have gone beyond demonstrating a putative SOCE, i.e.,
mechanistic details, measurement of I crac, and biophysical
and imaging studies are sparse, leading to interpretation
based on only a few myocyte preparations. The interaction
of TRPCs and Orai channels, if any, also requires
resolution.
4. Evidence for store-operated Ca entry
in smooth muscle
As one might anticipate from knowing that the contribution
of voltage-gated Ca entry varies between smooth
muscles, being high for example in uterine (771) and
intestinal (64) and low in many large blood vessels and
tracheal myocytes (7), so the contribution of Ca entry
through SOCE and/or other voltage-independent routes
such as nonspecific receptor-operated cation channels
(ROCs) shows considerable variation between smooth
muscles (44). Although ROC activity is a feature of agonist
binding, e.g., norepinephrine, and is important for
their physiological effects (39), it does not involve SR Ca
depletion, and therefore, these channels are not discussed
in detail further.
In smooth muscles, SOCE has been associated with a
second, delayed Ca response to agonist applications; the
IP 3 formed causes an initial fast and transient release of
Ca from the SR, followed by the slower process of SOCE
which produces a more or less sustained Ca signal, often
of lower amplitude than the initial transient rise. If Ca
sensitization mechanisms are also stimulated by the agonist,
then the force responses will not be directly related
to the Ca signal and force may continue long after [Ca]
has fallen.
A sustained increase in [Ca] due to extracellular Ca
entry following agonist application has been taken as
evidence for SOCE by many investigators, sometimes augmented
by data from putative blockers of SOCE such as
SKF96365 and La 3 . It is generally accepted that there are