Some properties of Ca(2+)-induced Ca2+ release mechanism in single visceral smooth muscle cell of the guinea-pig

Molecular mechanisms of cholinergic neurotransmission in visceral smooth muscles with a focus on receptor-operated TRPC4 channel and impairment of gastrointestinal motility by general anaesthetics and anxiolytics

Acetylcholine is the primary excitatory neurotransmitter in visceral smooth muscles, wherein it binds to and activates two muscarinic receptors subtypes, M2 and M3, thus causing smooth muscle excitation and contraction. The first part of this review focuses on the types of cells involved in cholinergic neurotransmission and on the molecular mechanisms underlying acetylcholine-induced membrane depolarisation, which is the central event of excitation-contraction coupling causing Ca2+ entry via L-type Ca2+ channels and smooth muscle contraction. Studies of the muscarinic cation current in intestinal myocytes (mICAT) revealed its main molecular counterpart, receptor-operated TRPC4 channel, which is activated in synergy by both M2 and M3 receptors. M3 receptors activation is of permissive nature, while activation of M2 receptors via Gi/o proteins that are coupled to them plays a direct role in TRPC4 opening. Our understanding of signalling pathways underlying mICAT generation has vastly expanded in recent years through studies of TRPC4 gating in native cells and its regulation in heterologous cells. Recent studies using muscarinic receptor knockout have established that at low agonist concentration activation of both M2 receptor and the M2/M3 receptor complex elicits smooth muscle contraction, while at high agonist concentration M3 receptor function becomes dominant. Based on this knowledge, in the second part of this review we discuss the cellular and molecular mechanisms underlying the numerous anticholinergic effects on neuroactive drugs, in particular general anaesthetics and anxiolytics, which can significantly impair gastrointestinal motility. This article is part of the Special Issue on "Ukrainian Neuroscience".

Calcium-induced calcium release in smooth muscle: the case for loose coupling

This article reviews the key experiments demonstrating calcium-induced calcium release (CICR) in smooth muscle and contrasts the biophysical and molecular features of coupling between the sarcolemmal (L-type Ca(2+) channel) and sarcoplasmic reticulum (ryanodine receptor) Ca(2+) channels in smooth and cardiac muscle. Loose coupling refers to the coupling process in smooth muscle in which gating of ryanodine receptors is non-obligate and may occur with a variable delay following opening of the sarcolemmal Ca(2+) channels. These features have been observed in the earliest studies of CICR in smooth muscle and are in marked contrast to cardiac CICR, where a close coupling between T-tubular and SR membranes results in tight coupling between the gating events. The relationship between this "loose coupling" and distinct subcellular release sites within smooth muscle cells, termed frequent discharge sites, is discussed.

FKBP12.6 and cADPR regulation of Ca2+ release in smooth muscle cells

Intracellular Ca2+ release through ryanodine receptors (RyRs) plays important roles in smooth muscle excitation-contraction coupling, but the underlying regulatory mechanisms are poorly understood. Here we show that FK506 binding protein of 12.6 kDa (FKBP12.6) associates with and regulates type 2 RyRs (RyR2) in tracheal smooth muscle. FKBP12.6 binds to RyR2 but not other RyR or inositol 1,4,5-trisphosphate receptors, and FKBP12, known to bind to and modulate skeletal RyRs, does not associate with RyR2. When dialyzed into tracheal myocytes, cyclic ADP-ribose (cADPR) alters spontaneous Ca2+ release at lower concentrations and produces macroscopic Ca2+ release at higher concentrations; neurotransmitter-evoked Ca2+ release is also augmented by cADPR. These actions are mediated through FKBP12.6 because they are inhibited by molar excess of recombinant FKBP12.6 and are not observed in myocytes from FKBP12.6-knockout mice. We also report that force development in FKBP12.6-null mice, observed as a decrease in the concentration/tension relationship of isolated trachealis segments, is impaired. Taken together, these findings point to an important role of the FKBP12.6/RyR2 complex in stochastic (spontaneous) and receptor-mediated Ca2+ release in smooth muscle.

Calcium-induced calcium release in smooth muscle: loose coupling between the action potential and calcium release

Calcium-induced calcium release (CICR) has been observed in cardiac myocytes as elementary calcium release events (calcium sparks) associated with the opening of L-type Ca(2+) channels. In heart cells, a tight coupling between the gating of single L-type Ca(2+) channels and ryanodine receptors (RYRs) underlies calcium release. Here we demonstrate that L-type Ca(2+) channels activate RYRs to produce CICR in smooth muscle cells in the form of Ca(2+) sparks and propagated Ca(2+) waves. However, unlike CICR in cardiac muscle, RYR channel opening is not tightly linked to the gating of L-type Ca(2+) channels. L-type Ca(2+) channels can open without triggering Ca(2+) sparks and triggered Ca(2+) sparks are often observed after channel closure. CICR is a function of the net flux of Ca(2+) ions into the cytosol, rather than the single channel amplitude of L-type Ca(2+) channels. Moreover, unlike CICR in striated muscle, calcium release is completely eliminated by cytosolic calcium buffering. Thus, L-type Ca(2+) channels are loosely coupled to RYR through an increase in global [Ca(2+)] due to an increase in the effective distance between L-type Ca(2+) channels and RYR, resulting in an uncoupling of the obligate relationship that exists in striated muscle between the action potential and calcium release.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Ca2+ images and K+ current during depolarization in smooth muscle cells of the guinea-pig vas deferens and urinary bladder

1. Electrical events and intracellular calcium concentration ([Ca2+]) imaged using fluo-3 and laser scanning confocal microscopy were simultaneously monitored in single smooth muscle cells freshly isolated from guinea-pig vas deferens or urinary bladder. 2. Images obtained every 8 ms, during stepping from -60 to 0 or +10 mV for 50 ms under voltage clamp, showed that a rise in [Ca2+] could be detected within 20 ms of depolarization in five to twenty small (< 2 micrometer diameter) 'hot spots', over 95 % of which were located within 1.5 micrometer of the cell membrane. Depolarization at 30 s intervals activated hot spots at the same places. 3. Cd2+ or verapamil abolished both hot spots and Ca2+-activated K+ current (IK,Ca). Caffeine almost abolished hot spots and markedly reduced IK,Ca. Cyclopiazonic acid, which raised basal global [Ca2+], decreased the rise in hot spot [Ca2+] and IK,Ca amplitude during depolarization. These results suggest that Ca2+ entry caused Ca2+-induced Ca2+ release (CICR). 4. Under voltage clamp, hot spot [Ca2+] closely paralleled the rise in IK,Ca and reached a peak within 20 ms of the start of depolarization, but the rise in global [Ca2+] over the whole cell area was much slower. Step depolarization to potentials positive to -20 mV caused hot spots to grow in size and coalesce, leading to a rise in global [Ca2+] and contraction. Ca2+ hot spots also occurred during the up-stroke of an evoked action potential under current clamp. 5. It is concluded that the entry of Ca2+ in the early stages of an action potential evokes CICR from discrete subplasmalemma Ca2+ storage sites and generates hot spots that spread to initiate a contraction. The activation of Ca2+-dependent K+ channels in the plasmalemma over hot spots initiates IK,Ca and action potential repolarization.

Activation of Ca(2+)-activated K+ (maxi-K+) channel by angiotensin II in myocytes of the guinea pig ileum

We investigated the regulation of the Ca(2+)-activated K+ (maxi-K+) channel by angiotensin II (ANG II) and its synthetic analog, [Lys2]ANG II, in freshly dispersed intestinal myocytes. We identified a maxi-K+ channel population in the inside-out patch configuration on the basis of its conductance (257 +/- 4 pS in symmetrical 150 mM KCl solution), voltage and Ca2+ dependence of channel opening, low Na(+)-to-K+ and Cl(-)-to-K+ permeability ratios, and blockade by external Cs+ and tetraethylammonium chloride. ANG II and [Lys2]ANG II caused an indirect, reversible, Ca(2+)- and dose-dependent activation of maxi-K+ channels in cell-attached experiments when cells were bathed in high-K+ solution. This effect was reversibly blocked by DUP-753, being that it is mediated by the AT1 receptor. Evidences that activation of the maxi-K+ channel by ANG II requires a rise in intracellular Ca2+ concentration ([Ca2+]i) as an intermediate step were the shift of the open probability of the channel-membrane potential relationship to less positive membrane potentials and the sustained increase in [Ca2+]i in fura 2-loaded myocytes. The preservation of the pharmacomechanical coupling of ANG II in these cells provides a good model for the study of transmembrane signaling responses to ANG II and analogs in this tissue.

Efficacy of peak Ca2+ currents (ICa) as trigger of sarcoplasmic reticulum Ca2+ release in myocytes from the guinea-pig coronary artery

1. Increments in cytosolic Ca2+ concentration (delta[Ca2+]c) were measured in single smooth muscle cells from guinea-pig coronary artery together with the density of peak Ca2+ currents (ICa) in response to clamp steps from -50 to 0 mV. The comparison of depolarization- with caffeine-induced delta[Ca2+]c was used to define the efficacy by which ICa can trigger Ca2+ release from the sarcoplasmic reticulum (SR). 2. At 2.5 mM extracellular calcium concentration ([Ca2+]o), depolarization induced a rapid rise of delta[Ca2+]c followed by a slow creep. Peak [Ca2+]c occurred within ca 30 s and could be followed by an undershoot and a second rise in [Ca2+]c. The creep was blocked by ryanodine but was insensitive to block of InsP3 receptors with heparin. The creep was not observed in Cs(+)-filled cells. After disappearance of the creep, a tonic delta[Ca2+]c became unmasked. 3. At 2.5 mM [Ca2+]o, peak ICa was -0.80 +/- 0.17 microA cm-2. delta[Ca2+] peaked at the end of the 6 s pulse at 202 +/- 98 nM while caffeine-induced delta[Ca2+]c peaked at 1330 +/- 410 nM. The ratio of depolarization- to caffeine-induced delta[Ca2+]c was 10 +/- 6%. 4. In media containing 10 mM [Ca2+]o plus 1 microM Bay K 8644, peak ICa was -2.6 +/- 1.1 microA cm-2 and delta[Ca2+]c peaked within 2.5 s at 451 +/- 194 nM. Paired measurements yielded the ratio of depolarization- to caffeine induced delta[Ca2+]c as 30 +/- 10%. Depolarization-induced delta[Ca2+]c was nearly blocked by caffeine and reduced by ryanodine to 30%, suggesting the contribution of Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ stores. 5. Trypsin (1 mg ml-1) in the electrode solution (10 mM [Ca2+]o plus 1 microM Bay K 8644) increased peak ICa up to 12.5 microA cm-2. ICa induced a delta[Ca2+]c of 990 +/- 210 nM and was accompanied by a 'hump' of IK,Ca. When applied briefly after peak delta[Ca2+]c, caffeine increased [Ca2+]c only moderately. The results suggest that a peak ICa can trigger a synchronized whole-cell Ca2+ release only if ICa is strongly augmented. 6. Amplitude and rate of rise of delta[Ca2+]c were graded by test step potentials along a bell-shaped voltage-dependent curve, similar to that of L-type ICa. Steps to +80 mV induced no delta[Ca2+]c when the electrode solution contained 10 mM Na+. However, with 150 mM intrapipette Na+, pulses to +80 mV induced delta[Ca2+]c.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ inhibition of inositol trisphosphate-induced Ca2+ release in single smooth muscle cells of guinea-pig small intestine

1. Single smooth muscle cells from the longitudinal muscle layer of guinea-pig small intestine were voltage clamped using patch pipettes in the whole-cell mode. 2. When D-myo-inositol 1,4,5-trisphosphate (InsP3) was released at intervals, by photolysis of 'caged' InsP3 within the cell, increases in [Ca2+]i in many cells, as judged from Ca(2+)-activated K(+)-current, were all-or-none; release of InsP3 before a critical interval had elapsed, which was quite stable for an individual cell, resulted in no response. After Ca(2+)-induced Ca2+ release had been evoked by depolarization, the InsP3 response was inhibited. Oscillations in [Ca2+]i evoked by muscarinic receptor activation were unaffected by Ruthenium Red; during these oscillations exogenous InsP3 was not effective close to, or shortly after, peak [Ca2+]i but was effective at other times. 3. Reproducible release of Ca2+ and elevation of [Ca2+]i could be produced by brief (up to 0.5 s) pressure applications of 10 mM caffeine at intervals of 10 s or greater but caffeine itself rarely evoked oscillations in [Ca2+]i. Responses to flash release of InsP3 were reduced after caffeine-induced responses and recovery of caffeine-induced Ca2+ release was faster than recovery of InsP3-induced Ca2+ release. 4. The results support the idea that InsP3-induced Ca(2+)-store release can be inhibited by a certain level of [Ca2+]i at a time when Ca2+ stores have refilled and can be released by caffeine; they also support the suggestion that during oscillations of [Ca2+]i evoked by muscarinic receptor activation, Ca2+ inhibition of InsP3-induced Ca2+ release at some critical level of [Ca2+]i allows Ca2+ stores to refill and leads to a fall in [Ca2+]i so contributing to the oscillations which are observed.

A developmental study of the dose-response curve of the respiratory sensory reflex

We have shown previously that inhalation of high concentration of CO2 (about 8%) inhibits breathing in preterm infants, presumably through an upper airway sensory reflex. To study the developmental aspects and the dose-response curve of this reflex, we studied eight preterm infants (body weight, 1.6 +/- 0.1 kg mean +/- SE; gestational age, 31 +/- 1 wk; postnatal age, 22 +/- 5 days) and eight term infants (body weight, 3.2 +/- 0.1 kg; gestational age, 39 +/- 1 wk; postnatal age, 8 +/- 6 days) using a flow-through system; eight adult subjects (weight, 67 +/- 5 kg; age, 30 +/- 4 yr) were studied during quiet sleep using a nasal mask. We gave 2, 4, 6, and 8% CO2 in 21+ O2 randomly for 20 to 30 s. A clear inhibition of breathing typically occurred during inhalation of 8% CO2 only in preterm infants, as reflected by the presence of an apnea of 11 +/- 1 s occurring at 7 +/- 2 s after the beginning of CO2 inhalation. Short apneas were occasionally observed with lower concentrations of CO2, but they were significantly fewer and shorter than with 8% CO2. No clear inhibition was observed in term infants or adult subjects, but pauses of 4 and 6 s were observed in the former group and a pause of 7 s was observed in the latter one. The associated changes in minute ventilation during inhalation of 2, 4, and 6% CO2 were not significantly different between the three groups. During inhalation of 8% CO2, minute ventilation decreased only in preterm infants (-26 +/- 10 compared with +32 +/- 10 in term infants and to +17 +/- 5% in adult subjects; p = 0.003 between groups).(ABSTRACT TRUNCATED AT 250 WORDS)