Sarcoplasmic Reticulum Function in Smooth Muscle - Physiological ...

150 SUSAN WRAY AND THEODOR BURDYGA
action potential, and both are inhibited by L-type Ca
channel blockers, indicating that Ca influx is a key factor
in the generation of the global Ca transients. Is CICR
contributing to the global Ca transients associated with
the action potential in smooth muscles?
In the guinea pig ileum where Ca sparks have been
observed (220), ryanodine and thapsigargin produced
stimulant rather than inhibitory effects on the Ca transients
associated with the action potential by increasing
the amplitude and duration of the spike (203, 288, 312,
494). Voltage-clamp experiments performed on ileum
(361), cerebral arteries (330), portal vein (133), vas deferens
and bladder (288), and pregnant myometrium (630)
demonstrated that Ca entry via L-type Ca channels can
trigger some Ca release via RyRs. At the same time, other
workers have failed to detect CICR in voltage-clamped
smooth muscle cells isolated from other or even the same
type of smooth muscles. For example, CICR was not
detected in voltage-clamped smooth muscle cells of portal
vein (329), gastrointestinal tract (79, 781), and airways
(178).
A discussion of the physiological role of CICR in
urinary bladder smooth muscle cells illustrates the complexity
in this area and the data which are hard to unify.
Elegant work performed on voltage-clamped guinea pig
urinary bladder myocytes by Ganitkevich and Isenberg
(203) showed that Ca transients induced by long depolarizing
voltage steps were inhibited 70% by ryanodine.
This suggested that a relatively high gain “cardiac type” of
CICR operates in these myocytes, at least under these
experimental conditions. The importance of a functional
contribution of CICR to E-C coupling and contraction in
urinary bladder smooth muscles has been assessed by
several research groups, using different species and experimental
models (127, 247, 254, 490, 494, 525). Results
from these studies addressing the susceptibility to contraction
in bladder myocytes treated with ryanodine are
not in agreement. For example, Hashitani and Brading
(247) showed the intracellular [Ca] rise induced by spontaneous
action potentials in the guinea pig was reduced to
60% by ryanodine. In a later publication (494), the stimulant
action of ryanodine on the mechanical activity of the
guinea pig bladder was reported. Reduction of spontaneous
or electrically evoked contractions by ryanodine have
been reported for mouse bladder myocytes (494). However,
ryanodine has no significant effect on the [Ca] and
spontaneous contractions in rat bladder myocytes (252).
These data suggest that the contribution of RyRs and
CICR to E-C coupling in the bladder differs between
species and experimental conditions.
Imaizumi et al. (288) in guinea pig urinary bladder
myocytes using brief (50 ms) depolarizing voltage steps,
obtained the first direct evidence that Ca “hot spots”
(equivalent to Ca sparks) could be transiently activated in
cells depolarized to 20 mV. These events lasted for 0.5
s and spread as Ca waves when depolarization was to 0
mV. The increase in global [Ca] elicited by 50 ms depolarization
to 0 mV reached a peak 60–100 ms after the
start of depolarization, indicating that the spread of Ca
hot spots continued even after most voltage-dependent
Ca channels had closed. Recently, Kotlikoff and co-workers
(127) have also investigated the temporal relationship
between the Ca current and local and global Ca signaling
in rabbit urinary bladder myocytes, under voltage- and
current-clamp conditions with physiologically relevant
protocols of stimulation. Localized photolysis of caged Ca
initiated CICR, but multiple excitation pulses were
needed and IP 3R channels were also activated (313). Calcium
channels were shown to activate RyRs to produce
CICR in the form of Ca sparks and propagated Ca waves.
Both the initial Ca spark and the subsequent Ca waves
occurred through the opening of RyRs, since both were
eliminated by ryanodine, and neither was affected by
dialysis with heparin (127). They concluded that 1) L-type
Ca channels could open without triggering Ca sparks,
2) triggered Ca sparks could be observed after channel
closure, 3) the trigger stimulus for the Ca release process
is not a local but a global rise in intracellular [Ca], and
4) this results in a functional uncoupling of a single action
potential from Ca release.
5. Loose coupling
Physiol Rev VOL 90 JANUARY 2010 www.prv.org
The small number of Ca spark sites evoked by the Ca
current, the delay between Ca channel and RyR channel
openings, as well as the inhibitory effect of BAPTA on Ca
sparks in urinary bladder smooth muscle cells suggested
that compared with cardiac myocytes, a fundamentally
different coupling process operates in smooth muscle,
which was termed “loose coupling” (127, 368). Features of
the loose coupling system are low gain (multiple Ca channels
must open to produce sparks), discriminated responses
(release takes the form of local Ca sparks or
globally propagated Ca waves), and a marked lengthening
of signal duration (Ca waves last far longer than the
action potential). The delayed activation of CICR and the
subsequent propagation of a Ca wave, which could last up
to 1 s, suggests that CICR in contrast to cardiac muscle
(122, 127) cannot be controlled by a brief (40–60 ms)
single action potential, and its functional role is yet to be
established (368). Thus, in urinary bladder myocytes paradoxically,
global rises of [Ca] are required to activate Ca
sparks. The concept of loose coupling can also be explained
by an increase in the SR [Ca] and activation of
[Ca] sparks. An increase in the luminal [Ca] produces
profound effects on both the frequency and the amplitude
of Ca sparks in smooth muscle (811).
Both cytosolic and SR Ca levels would be expected to
increase with enhanced Ca entry. The proximity of the SR
to the plasma membrane and the existence of the uptake