Functionally and spatially distinct Ca2+ stores are revealed in cultured vascular smooth muscle cells

Diterpene from Baccharis trimera with a relaxant effect on rat vascular smooth muscle

A bioassay monitored fractionation of a chloroform extract from the aerial parts of Baccharis trimera yielded a mixture that blocked the Ca2+-induced contractions of KCl- depolarized rat portal vein preparations. Pharmacological tests of two pure compounds isolated from the mixture revealed the dilactonic clerodane diterpene as the active compound.

Source of calcium for contractile responses of large and small human intramyometrial arteries

The role of calcium (Ca(2+)) released from intracellular stores and the entry of extracellular Ca(2+) for vasopressin (AVP)-induced responses in large and small, human, intramyometrial arteries was investigated. There was no statistical difference as revealed by pD(2) values (-log EC(50)), in the sensitivity of large and small vessels to AVP. Nimodipine caused an inhibition of contractions induced by low concentrations (10(-10) mol/l) of AVP in both types of vessels but, at higher concentration (>10(-10) mol/l), whereas responses in small arteries were diminished, in large arteries they remained unchanged. In Ca(2+)-free solution, responses of large and small arteries to potassium and to 10(-10) mol/l AVP were abolished. With 10(-6) mol/l AVP, response in small arteries was completely inhibited, whereas in large arteries it was reduced by approximately 50%. Additional experiments were done on large arteries. Thapsigargin (TSG), which causes depletion of internal Ca(2+) stores, caused a significant reduction in responses. Following treatment with TSG, responses to AVP in Ca(2+)-free solution were almost completely inhibited but arteries responded again when incubated in normal physiological salt solution. The results indicate that in contrast to large arteries, small arteries are highly dependent on extracellular Ca(2+). Response of large arteries showed considerable dependence on Ca(2+) stored internally particularly, for maximum activation.

Cerebral artery sarcoplasmic reticulum Ca(2+) stores and contractility: changes with development

To test the hypothesis that sarcoplasmic reticulum (SR) Ca(2+) stores play a key role in norepinephrine (NE)-induced contraction of fetal and adult cerebral arteries and that Ca(2+) stores change with development, we performed the following study. In main branch middle cerebral arteries (MCA) from near-term fetal ( approximately 140 days) and nonpregnant adult sheep, we measured NE-induced contraction and intracellular Ca(2+) concentration ([Ca(2+)](i)) in the absence and presence of different blockers. In adult MCA, after thapsigargin (10(-6) M), the NE-induced responses of tension and [Ca(2+)](i) were 37 +/- 5 and 47 +/- 7%, respectively, of control values (P < 0.01 for each). In the fetal artery, in contrast, this treatment resulted in no significant changes from control. When this was repeated in the absence of extracellular Ca(2+), adult MCA increases in tension and [Ca(2+)](i) were 32 +/- 5 and 13 +/- 3%, respectively, of control. Fetal cerebral arteries, however, showed essentially no response. Ryanodine (RYN, 3 x 10(-6) to 10(-5) M) resulted in increases in tension and [Ca(2+)](i) in both fetal and adult MCA similar to that seen with NE. For both adult and fetal MCA, the increased tension and [Ca(2+)](i) responses to RYN were essentially eliminated in the presence of zero extracellular Ca(2+). These findings provide evidence that in fetal MCA, in contrast to those in the adult, SR Ca(2+) stores are of less importance in NE-induced contraction, with such contraction being almost wholly dependent on Ca(2+) flux via plasma membrane L-type Ca(2+) channels. In addition, they suggest that in both adult and fetal MCA, the RYN receptor is coupled to the plasma membrane Ca(2+)-activated K(+) channel and/or L-type Ca(2+) channel.

Seeing is believing: recent trends in the measurement of Ca2+ in subcellular domains and intracellular organelles

The role of Ca2+ in the regulation of the cell cycle has been investigated mostly in studies assessing global cytosolic free Ca2+. Recent studies, however, have used unique techniques to assess Ca2+ in subcellular organelles, such as mitochondria, and in discrete regions of the cytoplasm. These studies have used advanced fluorescence digital imaging techniques and Ca2+-sensitive fluorescence probes, and/or targeting of Ca2+-sensitive proteins to intracellular organelles. The present review describes the results of some of these studies and the techniques used. The novel techniques used to measure Ca2+ in microdomains and intracellular organelles are likely to be of great use in future investigations assessing Ca2+ homeostasis during the cell cycle.

Unloading and refilling of two classes of spatially resolved endoplasmic reticulum Ca(2+) stores in astrocytes

Signaling by two classes of endoplasmic reticulum (ER) Ca(2+) stores was studied in primary cultured rat astrocytes. Cytosolic and intra-ER Ca(2+) concentrations ([Ca(2+)](CYT) and [Ca(2+)](ER)) were measured with, respectively, Fura-2 and Furaptra, in separate experiments. The agonists, glutamate and ATP, released Ca(2+) primarily from cyclopiazonic acid (CPA)-sensitive ER Ca(2+) stores (CPA inhibits ER Ca(2+) pumps). Agonist-evoked release was abolished by prior treatment with CPA but was unaffected by prior depletion of caffeine/ryanodine (CAF/RY)-sensitive ER Ca(2+) stores. Conversely, prior depletion of the CPA-sensitive stores did not interfere with Ca(2+) release or reuptake in the CAF/RY-sensitive stores. Unloading of the CPA-sensitive stores, but not the CAF/RY-sensitive stores, promoted Ca(2+) entry through "store-operated channels." Resting [Ca(2+)](ER) averaged 153 microM (based on in situ calibration of Furaptra: K(D) = 76 microM, vs 53 microM in solution). The releasable Ca(2+) in both types of ER Ca(2+) stores was increased by Na(+) pump inhibition with 1 mM ouabain or K(+)-free medium. Using high spatial resolution imaging and image subtraction methods, we observed that some regions of the ER (45-58% of the total ER) unloaded and refilled when CPA was added and removed. Other regions of the ER (24-38%) unloaded and refilled when CAF was added and removed. The overlap between these two classes of ER was only 10-18%. These data indicate that there are two structurally separate, independent components of the ER and that they are responsible for the functional independence of the CPA-sensitive and CAF/RY-sensitive ER Ca(2+) stores.

Role of intracellular Ca(2+) release in histamine-induced depolarization in rabbit middle cerebral artery

The role of Ca(2+) mobilization from intracellular stores and Ca(2+)-activated Cl(-) channels in caffeine- and histamine-induced depolarization and contraction of the rabbit middle cerebral artery has been studied by recording membrane potential and isometric force. Caffeine induced a transient contraction and a transient followed by sustained depolarization. The transient depolarization was abolished by ryanodine, DIDS, and niflumic acid, suggesting involvement of Ca(2+)-activated Cl(-) channels. Histamine-evoked transient contraction in Ca(2+)-free solution was abolished by ryanodine or by caffeine-induced depletion of Ca(2+) stores. Ryanodine slowed the development of depolarization induced by histamine in Ca(2+)-containing solution but did not affect its magnitude. In arteries treated with 1 mM Co(2+), histamine elicited a transient depolarization and contraction, which was abolished by ryanodine. DIDS and niflumic acid reduced histamine-evoked depolarization and contraction. Histamine caused a sustained depolarization and contraction in low-Cl(-) solution. These results suggest that Ca(2+) mobilization from ryanodine-sensitive stores is involved in histamine-induced initial, but not sustained, depolarization and contraction. Ca(2+)-activated Cl(-) channels contribute mainly to histamine-induced initial depolarization and less importantly to sustained depolarization, which is most likely dependent on activation of nonselective cation channels.

P2Y receptors contribute to ATP-induced increases in intracellular calcium in differentiated but not undifferentiated PC12 cells

ATP-induced Ca2+ transients were examined in individual PC12 cells of a well defined clone, before and after treatment with nerve growth factor (NGF) to induce a neurone-like phenotype. Using reverse transcriptase PCR these cells were found to express mRNA for several P2 receptors. In undifferentiated cells the ATP-induced Ca2+ response was entirely dependent on Ca2+ influx, could not be mimicked by UTP, alpha,beta-methylene ATP or dibenzoyl ATP or be blocked by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). ATP had no significant effect on levels of cyclic AMP or inositol 1,4,5-trisphosphate (InsP3). These results suggest that in undifferentiated PC12 cells ATP mainly acts on a P2X receptor, possibly the P2X4 subtype. After treatment with NGF for 7 days the ATP response was increased and partially sensitive to PPADS. A component of the ATP-induced Ca2+ increase was due to mobilisation of intracellular Ca2+ stores and another to capacitative Ca2+ entry. UTP caused an increase in intracellular Ca2+, and InsP3 formation could be stimulated by ATP and UTP. ATP also caused a small increase in cyclic AMP, but this was abolished in the presence of indomethacin. Thus, after NGF treatment ATP acts partially via a P2Y receptor, possibly the P2Y2 subtype.

Mobilization of intracellular Ca(2+) by endothelin-1 in rat intrapulmonary arterial smooth muscle cells

Endothelin-1 (ET-1) increases intracellular Ca(2+) concentration ([Ca(2+)](i)) in pulmonary arterial smooth muscle cells (PASMCs); however, the mechanisms for Ca(2+) mobilization are not clear. We determined the contributions of extracellular influx and intracellular release to the ET-1-induced Ca(2+) response using Indo 1 fluorescence and electrophysiological techniques. Application of ET-1 (10(-10) to 10(-8) M) to transiently (24-48 h) cultured rat PASMCs caused concentration-dependent increases in [Ca(2+)](i). At 10(-8) M, ET-1 caused a large, transient increase in [Ca(2+)](i) (>1 microM) followed by a sustained elevation in [Ca(2+)](i) (<200 nM). The ET-1-induced increase in [Ca(2+)](i) was attenuated (<80%) by extracellular Ca(2+) removal; by verapamil, a voltage-gated Ca(2+)-channel antagonist; and by ryanodine, an inhibitor of Ca(2+) release from caffeine-sensitive stores. Depleting intracellular stores with thapsigargin abolished the peak in [Ca(2+)](i), but the sustained phase was unaffected. Simultaneously measuring membrane potential and [Ca(2+)](i) indicated that depolarization preceded the rise in [Ca(2+)](i). These results suggest that ET-1 initiates depolarization in PASMCs, leading to Ca(2+) influx through voltage-gated Ca(2+) channels and Ca(2+) release from ryanodine- and inositol 1,4,5-trisphosphate-sensitive stores.

Minimal model of arterial chaos generated by coupled intracellular and membrane Ca2+ oscillators

We have developed a mathematical model of arterial vasomotion in which irregular rhythmic activity is generated by the nonlinear interaction of intracellular and membrane oscillators that depend on cyclic release of Ca2+ from internal stores and cyclic influx of extracellular Ca2+, respectively. Four key control variables were selected on the basis of the pharmacological characteristics of histamine-induced vasomotion in rabbit ear arteries: Ca2+ concentration in the cytosol, Ca2+ concentration in ryanodine-sensitive stores, cell membrane potential, and the open state probability of Ca2+-activated K+ channels. Although not represented by independent dynamic variables, the model also incorporates Na+/Ca2+ exchange, the Na+-K+-ATPase, Cl- fluxes, and Ca2+ efflux via the extrusion ATPase. Simulations reproduce a wide spectrum of experimental observations, including 1) the effects of interventions that modulate the functionality of Ca2+ stores and membrane ion channels, 2) paradoxes such as the apparently unpredictable dual action of Ca2+ antagonists and low extracellular Na+ concentration, which can abolish vasomotion or promote the appearance of large-amplitude oscillations, and 3) period-doubling, quasiperiodic, and intermittent routes to chaos. Nonlinearity is essential to explain these diverse patterns of experimental vascular response.

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.

Magnolol induces cytosolic-free Ca2+ elevation in rat neutrophils primarily via inositol trisphosphate signalling pathway

In the present study, we describe the role of inositol trisphosphate in the signalling pathway that leads to the elevation of cytosolic-free Ca2+ in rat neutrophils stimulated with magnolol, a compound isolated from the cortex of Magnolia officinalis. Magnolol increased [Ca2+]i, by stimulating Ca2+ release from internal stores and Ca2+ influx across the plasma membrane, in a concentration-dependent manner. Ni2+ and [6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H -pyrrole-2,5-dione (U73122), but not pertussis toxin, inhibited the magnolol-induced Ca2+ influx. Measurement of cellular levels of inositol trisphosphate showed a clear increase upon exposure to magnolol. U73122 but not ryanodine suppressed the Ca2+ release from internal stores caused by magnolol. Pretreatment of cells with formyl-Met-Leu-Phe (fMLP) or cyclopiazonic acid greatly reduced the [Ca2+]i changes caused by the subsequent addition of magnolol. Collectively, these findings suggest that a pertussis toxin-insensitive inositol trisphosphate signalling pathway is involved in the magnolol-induced [Ca2+]i elevation in rat neutrophils.

Ryanodine- and cyclopiazonic acid-sensitive components in human vas deferens contractions to noradrenaline

1. The role of calcium stores in noradrenaline- (NA) and caffeine-induced contractions of human vas deferens were investigated using ryanodine and cyclopiazonic acid (CPA) in the presence of the calcium antagonist, nifedipine (1 microM) or in calcium-free/EGTA (1 mM) medium. 2. In either media, NA (100 microM) evoked biphasic contractions of longitudinal muscle and tonic circular muscle contractions. Caffeine (20 mM) evoked longitudinal but not circular muscle contractions. 3. Ryanodine (1-30 microM) or CPA (1-30 microM) inhibited contractions of circular muscle, and the initial but not secondary component of longitudinal muscle contraction to NA. 4. In the presence of nifedipine, pre-exposure to caffeine caused a potentiation of circular muscle, and the initial but not secondary longitudinal muscle contractions to NA. The presence of ryanodine or CPA during the caffeine pre-exposures effectively blocked the potentiation of the initial component and reduced the secondary component of the subsequent responses to NA in longitudinal muscle. 5. In calcium-free media, caffeine pre-exposures had little effect on subsequent NA-induced contractions in circular muscle, but reduced both components in longitudinal muscle. The presence of ryanodine or CPA during caffeine pre-exposures produced no further effects on either component of the subsequent NA-induced contraction in longitudinal muscle. 6 In the presence of nifedipine or in calcium-free media, repeated applications of caffeine evoked contractions in longitudinal muscle which were not blocked by either ryanodine or CPA. 7. These results suggest that circular muscle contraction by NA and the initial component of longitudinal muscle to NA both utilize an intracellular pool of calcium that is triggered via a ryanodine-sensitive mechanism and replenished via a CPA-sensitive Ca2+-ATPase. 8. In longitudinal muscle, both the secondary component of its response to NA and contraction to caffeine appear to involve an unusual but pharmacologically distinct (ryanodine- and CPA-insensitive) pathway. 9. The quiescence to caffeine of circular muscle may be caused by a relative absence of the ryanodine- and CPA-insensitive pathway.

Electromechanical coupling in human vas deferens: effects of agents that modulate intracellular release of calcium

1. The effects of ryanodine, cyclopiazonic acid (CPA) and caffeine on electromechanical coupling in human vas deferens were investigated. 2. High [K+]o (120 mM) evoked nifedipine-sensitive contractions of longitudinal and circular muscle which consisted of initial and secondary components. 3. Exposures to ryanodine (< or =10 microM) or CPA (< or = 3 microM) induced a change of basal tension, and higher doses (30 microM) induced intermittent rhythmic contractions of both muscle types in the quiescent tissue. In the presence of the drugs, contraction to high [K+]o was preceded by marked rhythmic activity. 4. In circular muscle, ryanodine (1-30 microM) or CPA (1-30 microM) reduced both components of contractions to high [K+]o. In longitudinal muscle, the drugs enhanced the initial component and prolonged the secondary component. High doses (> or = 10 microM) produced variable effects on the initial component. 5. Caffeine (20 mM) reliably contracted longitudinal, but not circular muscle. Pre-exposures to caffeine enhanced both components in the post-caffeine contractions of circular muscle to high [K+]o. In longitudinal muscle, only the initial component (post-caffeine) was enhanced. 6. Contractions evoked in longitudinal muscle by caffeine were not blocked by ryanodine (30 microM) or CPA (30 microM). However, the enhancement of post-caffeine contractions to high [K+]o was inhibited. 7. These results show that ryanodine and CPA produced comparable effects on the excitability of longitudinal and circular muscle in the quiescent tissue, but electromechanical coupling was affected differently. The findings suggest that the muscle types utilize different mechanisms to regulate elevations in cytosolic Ca2+ during stimulation. 8. Electromechanical coupling in both muscle types involves Ca2+ influx via nifedipine-sensitive voltage-operated calcium channels and activation of ryanodine-sensitive calcium-induced calcium release from the sarcoplasmic reticulum (SR). In longitudinal muscle, the SR also buffers increases in cytosolic Ca2+ via a pharmacologically distinct Ca2+ compartment (caffeine releasable but ryanodine/CPA-insensitive). In circular muscle, the SR (ryanodine/CPA-sensitive) serves mainly in the regulation of excitability of the quiescent tissue.

Variability in spontaneous subcellular calcium release in guinea-pig ileum smooth muscle cells

1. Spontaneous, localized transient increases in [Ca2+]i ('Ca2+ sparks') were observed in about 40 % of fluo-3-loaded myocytes examined using laser scanning confocal microscopy. Ca2+ sparks persisted after application of Cd2+ (200 microM), but were abolished by ryanodine (30 microM) or thapsigargin (0.1 microM), suggesting that they arise from the spontaneous activation of ryanodine receptors (RyR) in the sarcoplasmic reticulum (SR). 2. Ca2+ sparks occurred much more frequently at certain sites (or 'frequent discharge sites', FDSs) within any confocal plane of the cell and line-scan imaging revealed a wide variation in their spatial size, amplitude and time course. Some spontaneous local transients were very similar to 'Ca2+ sparks' observed in heart, i.e. lasting approximately 200 ms with a peak fluorescence ratio of 1.75 +/- 0.23 (mean +/- s.d., n = 33). Other events were faster and smaller, lasting only approximately 40 ms with a peak normalized fluorescence of 1.36 +/- 0.09 (mean +/- s.d., n = 28). 3. Spontaneous Ca2+ waves with a wide range of propagation velocities (between 30 and 260 micron s-1) were also observed. In about 60 % of records (n = 33), Ca2+ sparks could be detected at the sites of wave initiation. Waves of elevated [Ca2+]i propagated with non-constant velocity and in some cases terminated. These observations could be explained by heterogeneity in the distribution of subcellular release sites as well as variability in the contribution of each release site to the wave. 4. Spontaneous [Ca2+]i transients in single dispersed visceral smooth muscle cells have a wide spectrum of behaviour that is likely to be the result of spatio-temporal recruitment of smaller local events, probably via a calcium-induced calcium release (CICR) mechanism. The spatial non-uniformity of SR and RyR distribution within the cell may account for the existence of 'frequent discharge sites' firing the majority of the smooth muscle Ca2+ sparks and the wide variation in the Ca2+ wave propagation velocities observed.

Access to intracellular calcium during development in the feline gastric antrum

Circular smooth muscle cells from the feline newborn antrum, unlike the adult, are unable to respond to myogenic agonists in the absence of extracellular calcium or to exogenous inositol 1,4,5-trisphosphate (IP3). This study examined the reasons behind the relative inaccessibility of intracellular calcium stores in the newborn period. IP3 binding was determined in antral smooth muscle homogenates from adult cats and newborns by evaluating the competitive binding of D-myo-[3H]IP3 and unlabeled IP3. Receptor density (Bmax) (fmol/mg of protein) and binding affinity (Kd) were determined. The Kd was similar in adults (31 +/- 4 nM) and newborns (28 +/- 7 nM); however, the Bmax was markedly decreased in the newborn (647 +/- 181.0 fmol/mg) compared with the adult (1755 +/- 275 fmol/mg). In adult and newborn antral cells, thapsigargin, which causes a net release of Ca2+ from intracellular stores by inhibiting Ca(2+)-ATPase-dependent reuptake activity, caused an early contraction at 30 s that was maintained for at least 20 min. We conclude that, in the newborn, dynamic intracellular calcium stores are present in the smooth muscle of the feline antrum and that differences in accessibility of intracellular calcium stores may be related to changes in the release of calcium from IP3-sensitive stores.

Luminal Ca2+ protects against thapsigargin inhibition in neuronal endoplasmic reticulum

Thapsigargin is a specific and potent inhibitor of sarco/endoplasmic reticulum Ca2+-ATPases. However, in whole rat brain microsomes, 1 microM thapsigargin had no significant effect on the 10-min time course of ATP-dependent Ca2+ uptake in the absence of the luminal Ca2+ chelator oxalate. In contrast, 50 mM oxalate resolved a thapsigargin-sensitive Ca2+ uptake rate (IC50 approximately 1 nM thapsigargin) five times that of a thapsigargin-insensitive rate. This remaining approximately 20% of the total ATP-dependent accumulation was insensitive to thapsigargin (up to 10 microM), slightly less sensitive to vanadate inhibition, and unresponsive to 5 microM inositol 1,4,5-trisphosphate or 10 mM caffeine. Measuring both 12-min Ca2+ uptake and initial Ca2+ uptake rates, the apparent thapsigargin sensitivity increased as oxalate concentrations increased from 10 to 50 mM, corresponding to a range of luminal free Ca2+ concentrations of approximately 300 down to 60 nM. Addition of oxalate during steady-state 45Ca accumulation rapidly resolved the aforementioned thapsigargin sensitivity. These results strongly suggest that luminal Ca2+ may protect a large portion of neuronal endoplasmic reticulum Ca2+ pumps against thapsigargin inhibition. Although high [Ca2+] has been previously shown to protect against thapsigargin inhibition in several reticular membrane preparations, our results suggest that luminal Ca2+ alone is responsible for mediating this effect in neurons.

Farnesol inhibits L-type Ca2+ channels in vascular smooth muscle cells

Earlier experiments with animal and human arteries have shown that farnesol, a natural 15-carbon (C15) isoprenoid, is an inhibitor of vasoconstriction (Roullet, J.-B., Xue, H., Chapman, J., McDougal, P., Roullet, C. M., and McCarron, D. A. (1996) J. Clin. Invest. 97, 2384-2390). We report here that farnesol reduced KCl- and norepinephrine-dependent cytosolic Ca2+ transients in fura-2-loaded intact arteries. An effect on Ca2+ signaling was also observed in cultured aortic smooth muscle cells (A10 cells). In these cells, farnesol reduced KCl-induced [Ca2+]i transients and mimicked the inhibitory effect of Ca2+-free medium on the [Ca2+]i response to both 12,13-phorbol myristate acetate, a protein kinase C activator, and thapsigargin, a specific endoplasmic reticulum ATPase inhibitor. Perforated patch-clamp experiments further showed in two vascular smooth muscle cell lines (A10 and A7r5), a reversible, dose-dependent inhibitory effect of farnesol on L-type Ca2+ currents (IC50 = 2.2 microM). Shorter (C10, geraniol) and longer (C20, geranylgeraniol) isoprenols were inactive. L-type Ca2+ channel blockade also occurred under tight (gigaohm) seal configuration using cell-attached, single-channel analysis, thus suggesting a possible action of farnesol from within the intracellular space. We finally demonstrated that farnesol did not affect Ca2+-sensitive pathways implicated in smooth muscle contraction, as tested with alpha-toxin permeabilized arteries. Altogether, our results indicate that farnesol is an inhibitor of vascular smooth muscle Ca2+ signaling with plasma membrane Ca2+ channel blocker properties. The data have implications for the endogenous and pharmacological regulation of vascular tone by farnesol or farnesol analogues.

Regulation of intracellular calcium in human esophageal smooth muscles

We have investigated sources of Ca2+ contributing to excitation of human esophageal smooth muscle, using fura 2 to study cytosolic free Ca2+ concentration ([Ca2+]i) in dispersed cells and contraction of intact muscles. Acetylcholine (ACh) caused an initial peak rise of [Ca2+]i followed by a plateau accompanied by reversible contraction. Removal of extracellular Ca2+ or addition of dihydropyridine Ca2+ channel blockers reduced the plateau phase but did not prevent contraction. Caffeine also caused elevation of [Ca2+]i and blocked responses to ACh. Undershoots of [Ca2+]i were apparent after ACh or caffeine. Blockade of the sarcoplasmic reticular Ca(2+)-ATPase by cyclopiazonic acid (CPA) reduced the ACh-evoked increase of [Ca2+]i and abolished the undershoot, indicating involvement of Ca2+ stores. When contraction was studied in intact muscles, removal of Ca2+ or addition of nifedipine reduced, but did not abolish, carbachol (CCh)-induced contraction. Elevation of extracellular K+ caused contraction that was inhibited by nifedipine, although CCh still elicited contraction. CPA caused contraction and suppressed the CCh-induced contraction, whereas ryanodine reduced CCh-induced contraction. Our studies provide evidence that muscarinic excitation of human esophagus involves both release of Ca2+ from intracellular stores and influx of Ca2+.

Prominent role of intracellular Ca2+ release in hypoxic vasoconstriction of canine pulmonary artery

The possible role of sarcoplasmic reticulum (SR) Ca2+ stores in hypoxic pulmonary vasoconstriction (HPV) is not well understood. In order to assess the possible role of intracellular Ca2+ release from SR Ca2+ stores in HPV, we examined the effects of: (1) ryanodine (10 microM) depletion of intracellular Ca2+ stores, and (2) thapsigargin (THAPS, 2 microM) or cyclopiazonic acid (CPA, 10 microM) depletion of intracellular Ca2+ stores on HPV in canine pulmonary artery. 2 Isometric tension was measured from arterial ring suspended in Krebs-Henseliet solution (K-H) bubbled with 95%O2/5%CO2. Hypoxia was induced by bubbling phenylephrine (PE, 1 microM) precontracted rings with 95%N2/5%CO2. HPV was observed in both intact and endothelial-denuded arteries and expressed as % of maximal KCl contraction (% Tkmax) = 21.3 +/- 3.2%; n = 13 and 21.7 +/- 4%; n = 4 respectively. 3 When SR caffeine sensitive Ca2+ stores were depleted by pretreatment with ryanodine and brief caffeine (15 mM) exposure, the hypoxic response was significantly reduced to 19.1 +/- 9.2% of the control hypoxic contraction (n = 7; p < 0.001) with little or no effect on PE or KCl contractions. On the other hand, in normoxic rings pretreated with THAPS or CPA, the PE responses were significantly reduced (% Tkmax = 18.2 +/- 3.1% compared to 39.0 +/- 3.9% in control; n = 16; P < 0.001; %Tkmax = 3.4 +/- 1.6% compared to 49.9 +/- 7.9% in control; n = 6; P < 0.001; respectively) with no significant effect on caffeine-induced contractions, suggesting that both THAPS and CPA preferentially deplete InsP3-sensitive Ca2+ stores, without affecting the caffeine-sensitive Ca2+ store; consistent with the existence of separate and independent InsP3 and caffeine-sensitive Ca2+ stores in this preparation. 4 When hypoxia was induced in the presence of THAPS or CPA, developed tension was significantly larger than control (% Tkmax = 64.5 +/- 6.0%; n = 16; P < 0.05%; %Tkmax = 78.2 +/- 15%; n = 6; P < 0.05; respectively), was partially blocked by nisoldipine (10 microM) and ryanodine (% Tkmax = 20.3 +/- 3.7%; n = 6), and nearly completely blocked by SK&F 96365 (50 microM). However, the actions of SK&F 96365 appeared to be nonselective since this compound also significantly reduced contractions elicited by KCl, PE and caffeine. 5 Finally, evidence was obtained suggesting: (a) that at least some of the Ca2+ released from the caffeine- and ryanodine-sensitive Ca2+ stores by hypoxia may be taken up and buffered by the InsP3-sensitive Ca2+ stores, and (b) the apparent dependence of HPV on extracellular Ca2+ entry pathways may be partially due to the dependence of the Ca2+ content of intracellular SR Ca2+ stores on sarcolemmal Ca2+ entry pathways. 6 These data suggest that caffeine- and ryanodine-sensitive SR Ca2+ stores contribute significantly to HPV under normal conditions and, in the presence of THAPS or CPA, an additional nisoldipine- and ryanodine-insensitive Ca2+ entry pathway is evoked by hypoxia.

A realistic model of biphasic calcium transients in electrically nonexcitable cells

In many electrically nonexcitable cells, the release of calcium from internal stores is followed by a much slower phase in which the intracellular calcium concentration decreases gradually to a sustained value higher than the concentration before stimulation. This elevated calcium plateau has been shown to be the result of calcium influx. The model presented in this work describes a system consisting of a cytoplasmic calcium store and a plasma membrane calcium channel, both excitable by a membrane receptor; a fast cytoplasmic calcium buffer; and calcium pumps in both the calcium store and cellular membranes. Inherent difficulties in the numerical evaluation of the model, caused by very large calcium fluxes across the store membrane, were overcome by analytically separating the fast processes of calcium release from the slower processes of calcium cycling across the plasma membrane. This enabled the simulation of realistic biphasic calcium transients similar to those observed experimentally. The model predicted 1) a strong correlation between the rate of calcium cycling across the plasma membrane and the rate of calcium decay; and 2) a dependence on the level of cell excitation of the maximum rise in cytoplasmic calcium concentration, the level of the elevated calcium plateau, and the rate of calcium decay. Using the model, we simulated the washout of agonist from the bathing solution and the depletion of the calcium store by a pharmacological agent (such as thapsigargin) under several experimental conditions.

Release-activated Ca2+ transport in neurons of frog sympathetic ganglia

Frog sympathetic ganglion neurons exhibit a novel Ca2+ uptake mechanism, release-activated calcium transport or RACT, which is manifest in both cytosolic and store [Ca2+] signals as greatly accelerated Ca2+ uptake after Ca2+ release from internal stores. RACT is activated by Ca2+ release but not by Ca2+ entry and serves to selectively refill Ca2+ stores after release. RACT lowers cytosolic [Ca2+] with a rate constant about 1.6 times that of the SERCA pump with empty ER. RACT is thapsigargin-insensitive, was eliminated by ryanodine, but was not affected by blocking mitochondrial or plasma membrane Ca2+ transport. A Ca2+ flux model with RACT in the ER membrane reproduced the cytosolic and store [Ca2+] responses to all stimuli.

Intracellular free Ca2+ and basal Mn2+ influx in cultured aortic smooth muscle cells from spontaneously hypertensive and normotensive Wistar-Kyoto rats

Numerous studies investigating the possible role of altered Ca2+ homeostasis in hypertension have compared resting and agonist-stimulated intracellular free Ca2+ ([Ca2+]i) in cultured aortic smooth muscle cells from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. However, such studies have not given consistent results. Differences in the method used to load cells with the Ca(2+)-sensitive indicator fura-2 have been investigated here as a possible source of variability between studies. We also describe the adaptation of a fluorescence technique for the assessment of basal Ca2+ permeability in SHR and WKY through the measurement of Mn2+ influx. The results are consistent with the hypothesis that basal Ca2+ influx is elevated in cultured aortic smooth muscle cells from SHR compared to those from WKY. However, this was not reflected as a significant difference between the two strains in basal or angiotensin II (200 nmol/L)-stimulated [Ca2+]i. Furthermore, this result was not dependent on the protocol used to load cells with fura-2. Hence, measurement of bulk [Ca2+]i does not appear to be the most sensitive parameter for altered Ca2+ homeostasis in SHR. Other compartments of the cell may better reflect altered Ca2+ fluxes in hypertension and are discussed in this work.

Spatially and functionally distinct Ca2+ stores in sarcoplasmic and endoplasmic reticulum

The organization of calcium (Ca2+) stores in the sarcoplasmic and endoplasmic reticulum (S-ER) is poorly understood. The dynamics of the storage and release of calcium in the S-ER of intact, cultured astrocytes and arterial myocytes were studied with high-resolution imaging methods. The S-ER appeared to be a continuous tubular network; nevertheless, calcium stores in the S-ER were organized into small, spatially distinct compartments that functioned as discrete units. Cyclopiazonic acid (an inhibitor of the calcium pump in the S-ER membrane) and caffeine or ryanodine unloaded different, spatially separate compartments. Heterogeneity of calcium stores was also revealed in cells activated by physiological agonists. These results suggest that cells can generate spatially and temporally distinct calcium signals to control individual calcium-dependent processes.

Role of intracellular Ca2+ in the K channel opener action of CGRP in the guinea-pig ureter

1. The aim of this study was to assess the role of sarcoplasmic reticulum (SR) calcium (Ca2+) in the smooth muscle relaxant and hyperpolarizing actions of calcitonin gene-related peptide (CGRP) in the guinea-pig ureter. 2. CGRP (0.1 microM) rapidly and transiently reduced myogenic phasic contractions (twitches) produced by electrical field stimulation (EFS). Approximately 70% of the response to CGRP was antagonized by glibenclamide (1 microM). 3. Cyclopiazonic acid (CPA, 10 microM), ryanodine (100 microM) and thapsigargin (1 microM) reduced only the glibenclamide-sensitive component of the response to CGRP (0.1 microM) but did not modify the mechano-inhibitory effect of cromakalim (3 microM). A low concentration of CPA (1 microM), assumed to produce a limited impairment of Ca2+ uptake from the stores, prolonged the duration of the inhibitory response to CGRP. Pre-exposure to caffeine (5 mM) inhibited the suppression of twitches by CGRP or cromakalim. 4. When the frequency of EFS was increased, the suppression of twitches by CGRP was reduced. Under these conditions, CPA (1 microM) again prolonged the duration of the inhibitory response to CGRP. 5. CGRP (0.1 microM) and cromakalim (3 microM) markedly depressed the phasic component of contractions to 80 mM KCl. CPA (10 microM) antagonized the inhibitory effect of CGRP but not that of cromakalim. Inhibition of the tonic contraction to 80 mM KCl by CGRP was insensitive to CPA. 6. In sucrose gap experiments, a 5 min exposure to CGRP (0.1 microM) or cromakalim (3 microM) produced a sustained membrane hyperpolarization. Caffeine (5 mM) produced a glibenclamide-sensitive transient hyperpolarization followed by a sustained depolarization. When tested in a Ca(2+)-free medium the hyperpolarization produced by CGRP, cromakalim or caffeine was reduced. In normal Krebs, pre-exposure to CPA (10 microM, 60 min) only abolished the hyperpolarization induced by CGRP. In contrast, 5 min after a caffeine challenge (5 mM) the hyperpolarizations induced by CGRP or cromakalim were reduced. The CGRP-induced hyperpolarization was insensitive to apamin (0.1 microM) or charybdotoxin (0.1 microM). 7. We conclude that the K channel-opening action of CGRP in the guinea-pig ureter requires the mobilization of intracellular Ca2+ from a caffeine- and CPA-sensitive store, leading to transient activation of glibenclamide-sensitive K channels. The K channel-opening action of caffeine appears to involve Ca2+ mobilization from a store which is insensitive to depletion by CPA.

Calcium handling and purinoceptor subtypes involved in ATP-induced contraction in rat small mesenteric arteries

1. The relationship between the stimulation of ATP receptors, the increase in intracellular free calcium concentration ([Ca2+]i; measured using the fluorescent indicator fura-2), contraction and the subtypes of purinoceptors involved were investigated in the small mesenteric artery of the rat. 2. In normal physiological solution, ATP (0.001-3 mM) caused concentration-dependent increases in both [Ca2+]i and contraction. Both responses produced by ATP (1 mM) were inhibited by 50% in the presence of nitrendipine (1 microM) and were abolished in the presence of nitrendipine plus SK&F 96365 (30 microM). 3. In Ca(2+)-free medium, ATP (3 mM) elicited a transient increase in both [Ca2+]i and tension which were abolished by caffeine and decreased by 65% by thapsigargin (1 microM). Moreover, ATP (1 and 3 mM) produced increases in the [3H]D-myo-inositol 1,4,5-trisphosphate ([3H]IP3) content of vessels in a concentration-dependent manner. 4. Treatment of the vessels with Bordetella pertussis toxin (PTX) inhibited contractions to ATP linked to the influx of calcium through nitrendipine-sensitive mechanisms, but not those linked to the release of Ca2+ from intracellular stores nor the capacity of ATP in increasing IP3 content of the vessels. 5. The order of potency of ATP and its analogues in eliciting contraction was alpha, beta-methylene-ATP (alpha, beta-MeATP) > 2-methylthio-ATP (2-MeSATP) > ATP = ADP. The response to ATP was inhibited by suramin. Reactive Blue 2 (up to 100 microM) did not affect the contractile response to ATP. Pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium (PPADS) and alpha, beta-MeATP abolished the response to low concentrations of ATP and reduced contractions elicited by high concentrations of ATP. 6. After blockade of P2X-purinoceptors with PPADS, the order of potency of ATP and its analogues was 2-MeSATP > ATP = ADP. UTP produced concentration-dependent contractions which were not affected by suramin, Reactive Blue 2, PPADS or alpha, beta-MeATP, suggesting the presence of P2U-purinoceptors. 7. The results suggest that low concentrations of ATP activate P2X-purinoceptors and produce an influx of calcium through both voltage-dependent calcium channels sensitive to nitrendipine and through receptor-operated calcium channels sensitive to SK&F 96365. High concentrations of ATP activate P2Y-purinoceptors which promote firstly a nitrendipine-sensitive calcium influx via a PTX-sensitive G protein and secondly a release of Ca2+ from an internal source via the production of IP3.

Rhythmic relaxations of active tension in the rabbit large arteries induced by a combination of cyclopiazonic acid and Bay K 8644

1. We previously demonstrated that cyclopiazonic acid (CPA), an inhibitor of Ca(2+)-ATPase in the sarcoplasmic reticulum, induced rhythmic relaxations of active tension in the endothelium-denuded small arteries of the mesentery and the ear of the rabbit, but that this agent failed to induce rhythmic responses in the endothelium-denuded rabbit femoral artery. 2. In the present study, an attempt was made to induce rhythmic relaxations of active tension in the endothelium-denuded rabbit femoral artery and the thoracic aorta, both of which were suspended in organ chambers for isometric tension recordings, by using CPA plus Bay K 8644, an L-type Ca2+ channel agonist, to induce an excessive increase in cytosolic Ca2+. 3. CPA or Bay K 8644 alone failed to produce rhythmic relaxations in the femoral artery that had been contracted with phenylephrine. In contrast, rhythmic responses were induced by the sequential treatment of the femoral artery with CPA and Bay K 8644. 4. The rhythmic relaxations of active tension in the femoral artery induced by CPA plus Bay K 8644 were inhibited by charybdotoxin and by iberiotoxin, both of which are antagonists of the Ca(2+)-activated K+ channel, but not by glibenclamide, a blocker of the ATP-sensitive K+ channel. 5. The endothelium-denuded rabbit aorta also exhibited rhythmic responses by the sequential addition of CPA and Bay K 8644. These responses were sensitive to charybdotoxin. 6. These findings indicate that, like small arteries, the large femoral and aortic arteries of the rabbit are also capable of displaying rhythmic relaxations of active tension; these relaxations may be in part attributed to the activation of the Ca(2+)-activated K+ channel as a result of the Ca2+ overload caused by CPA and Bay K 8644.

Modulation of two functionally distinct Ca2+ stores in astrocytes: role of the plasmalemmal Na/Ca exchanger

Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura-2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca(2+)-ATPase, induced a transient, four-fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca(2+)-sensitive stores, induced a two-fold increase in [Ca2+]cyt. The CPA- and CAF-sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA- and CAF-induced Ca2+ transients. The amount of Ca2+ releasable from the IP3(CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage-dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3- (CPA-) sensitive, and the smaller, Ca(2+)- (CAF-) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.

Modulation of two functionally distinct Ca2+ stores in astrocytes: role of the plasmalemmal Na/Ca exchanger

Mechanisms that regulate the amount of releasable Ca2+ in intracellular stores of cultured mouse astrocytes were investigated using digital imaging of fura-2 loaded cells. At rest, the cytoplasmic Ca2+ concentration, [Ca2+]cyt, was about 110 nM. In the absence of extracellular Ca2+, cyclopiazonic acid (CPA), an inhibitor of the endoplasmic reticulum (ER) Ca(2+)-ATPase, induced a transient, four-fold increase in [Ca2+]cyt due to the release of Ca2+ from inositol triphosphate (IP3) sensitive stores. Caffeine (CAF), which releases Ca2+ from Ca(2+)-sensitive stores, induced a two-fold increase in [Ca2+]cyt. The CPA- and CAF-sensitive stores could be released independently. Changes in the amplitudes of the Ca2+ transients were taken as a measure of changes in store content. Removal of extracellular Na+ or addition of ouabain, which inhibit Ca2+ extrusion and promote Ca2+ entry across the plasmalemma via the Na/Ca exchanger, caused minimal increases in resting [Ca2+]cyt but greatly potentiated both CPA- and CAF-induced Ca2+ transients. The amount of Ca2+ releasable from the IP3(CPA) sensitive store was directly proportional to cytosolic Na+ concentration (i.e., inversely proportional to the transmembrane Na+ electrochemical gradient). Under these reduced Na+ gradient conditions, little, if any, Ca2+ destined for the ER stores enters the cells through voltage-dependent Ca2+ channels. These results demonstrate that mouse astrocytes contain two distinct ER Ca2+ stores, the larger, IP3- (CPA-) sensitive, and the smaller, Ca(2+)- (CAF-) sensitive. The Ca2+ content of both ER stores can be regulated by the Na/Ca exchanger. Thus, the magnitude of cellular responses to signals that are mediated by Ca2+ release induced by the two second messengers, IP3 and Ca2+, can be modulated by factors that affect the net transport of Ca2+ across the plasmalemma.

Dominant role of mitochondria in clearance of large Ca2+ loads from rat adrenal chromaffin cells

Cytosolic Ca2+ (Ca2+c) clearance from adrenal chromaffin cells was studied by whole-cell patch clamp and indo-1 Ca2+ photometry after influx of Ca2+ through voltage-dependent Ca2+ channels. We isolated the rates of Ca2+c clearance by several mechanisms using combinations of the following agents (with their expected targets): Li+ or TEA substituted for Na+ (Na(+)-Ca2+ exchange), 1 mM La3+ applied after the depolarization (Na(+)-Ca2+ exchange and plasma membrane Ca(2+)-ATPase), 1 microM thapsigargin (pumping into reticular stores), and 2 microM carbonyl cyanide m-chlorophenylhydrazone (uptake into mitochondria). Remarkably, whenever [Ca2+]c rose above approximately 500 nM, Ca2+c clearance by mitochondria exceeded clearance by either Na(+)-Ca2+ exchange or the Ca2+ pumps of the plasma and reticular membranes. As [Ca2+]c fell again, Ca2+ reemerged from mitochondria, prolonging the final return to basal levels.

Effects of 8-bromoguanosine 3':5'-cyclic monophosphate on phenylephrine-induced phosphatidylinositol hydrolysis and contraction in rat caudal artery

1. The effects of 8-bromoguanosine 3':5'-cyclic monophosphate (8-bromo-cyclic GMP) on phenylephrine-induced contractions and phosphatidylinositol (PI) hydrolysis were investigated in rat isolated caudal artery. The effects of the nucleotide were compared to those of felodipine, a dihydropyridine Ca2+ channel antagonist and ryanodine, a putative depletor of intracellular Ca2+ stores. The purpose of this investigation was to examine the regulatory effects of cyclic GMP on receptor-mediated signal transduction in vascular smooth muscle. 2. Phenylephrine induced a concentration-dependent increase in PI hydrolysis that reached a maximum at 10 microM phenylephrine. Pre-incubation with felodipine (10 nM) significantly reduced PI turnover, but did not affect basal hydrolysis. Similarly, removal of extracellular Ca2+ (2 mM ethylene glycol-bis(beta-amino-ethyl ether) N, N, N', N'-tetraacetic acid (EGTA)) blocked phenylephrine-induced PI hydrolysis, but did not affect basal turnover. In contrast, 8-bromo-cyclic GMP (10 microM) did not affect phenylephrine-induced PI hydrolysis, nor did it affect basal turnover. 3. Phenylephrine induced concentration-dependent contractions that were inhibited by each of 8-bromo-cyclic GMP (10 microM), felodipine (1 nM and 10 nM) and ryanodine (3 microM and 10 microM). In addition, removal of Ca2+ from the physiological salt solution (2 mM EGTA) completely abolished contractions elicited by phenylephrine. 4. Phenylephrine-induced contractions were not further affected by felodipine and 8-bromo-cyclic GMP applied concomitantly than by equivalent concentrations of felodipine alone. However, ryanodine and 8-bromo-cyclic GMP applied together significantly inhibited phenylephrine-induced contractions in comparison to ryanodine alone. 5 These results suggest that phospholipase C-activated PI hydrolysis in the rat caudal artery is dependent on extracellular Ca2+, mediated, in part, through dihydropyridine-sensitive Ca2+ channels.Inhibition of contraction by felodipine may be brought about through indirect inhibition of IP3 production and subsequent attenuation of intracellular Ca2+ release. 8-Bromo-cyclic GMP does not inhibit PI hydrolysis; it may regulate vascular smooth muscle contraction by inhibition of Ca2+ release from IP3-mediated intracellular stores, but it is unlikely that 8-bromo-cyclic GMP affects ryanodine-sensitive stores.