The role of sarcoplasmic reticulum in endothelium-dependent and endothelium-independent rhythmic contractions in the rabbit mesenteric artery

Is Vasomotion in Cerebral Arteries Impaired in Alzheimer's Disease?

A substantial body of evidence supports the hypothesis of a vascular component in the pathogenesis of Alzheimer’s disease (AD). Cerebral hypoperfusion and blood-brain barrier dysfunction have been indicated as key elements of this pathway. Cerebral amyloid angiopathy (CAA) is a cerebrovascular disorder, frequent in AD, characterized by the accumulation of amyloid-β (Aβ) peptide in cerebral blood vessel walls. CAA is associated with loss of vascular integrity, resulting in impaired regulation of cerebral circulation, and increased susceptibility to cerebral ischemia, microhemorrhages, and white matter damage. Vasomotion— the spontaneous rhythmic modulation of arterial diameter, typically observed in arteries/arterioles in various vascular beds including the brain— is thought to participate in tissue perfusion and oxygen delivery regulation. Vasomotion is impaired in adverse conditions such as hypoperfusion and hypoxia. The perivascular and glymphatic pathways of Aβ clearance are thought to be driven by the systolic pulse. Vasomotion produces diameter changes of comparable amplitude, however at lower rates, and could contribute to these mechanisms of Aβ clearance. In spite of potential clinical interest, studies addressing cerebral vasomotion in the context of AD/CAA are limited. This study reviews the current literature on vasomotion, and hypothesizes potential paths implicating impaired cerebral vasomotion in AD/CAA. Aβ and oxidative stress cause vascular tone dysregulation through direct effects on vascular cells, and indirect effects mediated by impaired neurovascular coupling. Vascular tone dysregulation is further aggravated by cholinergic deficit and results in depressed cerebrovascular reactivity and (possibly) impaired vasomotion, aggravating regional hypoperfusion and promoting further Aβ and oxidative stress accumulation.

Effect of phenylephrine and endothelium on vasomotion in rat aorta involves potassium uptake

Vasomotion is defined as the rhythmic contractions in blood vessels, consisting of two components: vasoconstriction and oscillations of the plasma membrane potential. To determine whether vasomotion is associated with changes in K(+) uptake, we measured the effect of phenylephrine (PE) and acetylcholine (ACh) on the K(+) uptake and vascular reactivity in rat aortic rings. We found that the incubation of aortic rings with 10(-7) M PE (210 ± 28 mg maximum amplitude), and 10(-6) M ACh (177 ± 6 mg maximum amplitude) produced the highest rhythmic contractions. Both 10(-7) M PE and 10(-6) M ACh significantly increased K(+) uptake in endothelium-intact aorta versus control (121 % PE, 117 % ACh). Removal of the endothelium blunted rhythmic contractions and decreased K(+) uptake in presence of vasoactive substances (88 % PE, 81 % ACh). The inhibition of nitric oxide synthase with 10(-4) M L-NNA significantly reduced the rhythmic contractions, and it was reversed in the presence of 10(-8) M sodium nitroprusside (SNP; a nitric oxide donor). Also, we found that 10(-4) M L-NNA significantly decreased the effect of 10(-7) M PE on K(+) uptake in aortic rings (104 % PE + L-NNA vs. control). The incubation of endothelium-denuded rings with 10(-8) M SNP significantly increased the K(+) uptake (116 % SNP vs. control), similar to those observed in the presence of 10(-6) M ACh. The inhibition of protein kinase G with KT-5823 blocked SNP-mediated increase in K(+) uptake. In conclusion, these data suggest that a certain range of K(+) uptake is necessary to induce the rhythmic contractions in response to vasoactive substances.

Vasomotion - what is currently thought?

This minireview discusses vasomotion, which is the oscillation in tone of blood vessels leading to flowmotion. We will briefly discuss the prevalence of vasomotion and its potential physiological and pathophysiological relevance. We will also discuss the models that have been suggested to explain how a coordinated oscillatory activity of the smooth muscle tone can occur and emphasize the role of the endothelium, the handling of intracellular Ca(2+) and the role of smooth muscle cell ion conductances. It is concluded that vasomotion is likely to enhance tissue dialysis, although this concept still requires more experimental verification, and that an understanding at the molecular level for the pathways leading to vasomotion is beginning to emerge.

Effects of K+ channel modulators on oscillatory contractions in sinoaortic denervated rat aortas

Sinoaortic denervated (SAD) rats present arterial pressure lability without sustained hypertension. We investigated the relation between sinoaortic denervation and the occurrence of oscillatory contractions in SAD rat aortas, as well as the effect of various K(+) channel modulators on these oscillations. Aortas were removed and concentration-effect curves to phenylephrine (0.01 to 10 muM) were constructed in arteries from SAD and Sham-operated rats in order to verify the occurrence of oscillations. We also evaluated the effects of various K(+) channel modulators on these oscillations. Only SAD rat aortas exhibited oscillatory contractions. Tetraethylammonium increased the frequency (28.5+/-3.5 to 41.5+/-4.5 counts/5 min) and amplitude (0.435+/-0.07 to 0.630+/-0.09 g) of the oscillations. Apamin and 4-aminopyridine did not alter the oscillations. Barium chloride converted the oscillatory contractions to a tonic contraction. Pinacidil rapidly blocked the oscillatory contractions and glibenclamide evoked reduction in amplitude from 0.410+/-0.07 to 0.180+/-0.06 g. Iberiotoxin increased the frequency of oscillatory contractions (from 28.0+/-3.5 to 51.5+/-7.5 counts/5 min) but decreased the amplitude (from 0.410+/-0.08 to 0.195+/-0.2 g). Our results demonstrate that SAD rat aortas exhibit oscillatory contractions and K(+) channels, mainly K(ATP) and BK(Ca), play a dominant role in these oscillations.

Antiphase oscillations of endothelium and smooth muscle [Ca2+]i in vasomotion of rat mesenteric small arteries

The mechanisms leading to vasomotion in the presence of noradrenaline and inhibitors of the sarcoplasmic/endoplasmic reticulum calcium ATPase were investigated in isolated rat mesenteric small arteries. Isobaric diameter and isometric force were measured together with membrane potential in endothelial cells and smooth muscle cells (SMC). Calcium in the endothelial cells and SMC was imaged with confocal microscopy. In the presence of noradrenaline and cyclopiazonic acid, ryanodine-insensitive oscillations in tone were produced. The frequency was about 1 min(-1) and amplitude about 70% of the maximal tone. The amplitude was reduced by indomethacin and increased with L-NAME. Vasomotion was inhibited by nifedipine and by 40 mM potassium. The frequency was increased and amplitude decreased by removal of the endothelium and by application of charybdotoxin and apamin. The vasomotion was associated with in-phase oscillations of membrane potential in endothelial cells and SMC and oscillations of [Ca2+]i that were in near anti-phase. We suggest a working model for the generation of oscillation based on a membrane oscillator where ion channels in both endothelial cells and SMC interact via a current running between the two cell types through myoendothelial gap junctions, which sets up a near anti-phase oscillation of [Ca2+]i in the two cell types.

Vasomotion in Human Umbilical and Placental Veins: Role of Gap Junctions and Intracellular Calcium Reservoirs in Their Synchronous Propagation

Vasomotion was characterized using human placentae vessel rings; force displacement transducers recorded isometric contractions. Umbilical vein rings display rhythmic contractions occurring with a frequency of 1.47+/-0.01 min(-1) and 274+/-2.2 mg (n=211) of amplitude, which corresponds to 11.1+/-0.4% of the maximal KCl contracture. Vasomotion waves were recorded for up to 8 h; their amplitude and duration was larger in umbilical veins than arteries or chorionic vessels (p<0.001), vasomotion frequency was indistinguishable among these vessels. Segments of the umbilical vein closer to the fetus showed larger amplitudes and longer-lasting waves. Gap junction blockers, including peptide Gap 27, 18alpha-glycyrrhetinic acid, hexanol, heptanol or octanol, reduced the amplitude but not the frequency of vasomotion; all these drugs, in addition, decreased tissue basal tension. The role of intracellular calcium stores was evidenced using calcium-free buffer, which reduced oscillation amplitude and tissue basal tension. Cyclopiazonic acid increased wave amplitude and tissue basal tension, reducing oscillatory frequency. We propose that biological oscillators localized in the smooth muscle layer of the umbilical cord, trigger vasomotion waves, which are synchronized and propagated via gap junctions; internal calcium reservoirs are essential for their maintenance. These myogenic oscillations may be relevant for maternal-fetus blood flow and contribute to fetal nutrition and development.

Effect of cyclic guanosine-monophosphate on porcine retinal vasomotion

PURPOSE

Vasomotion refers to periodic oscillations in vascular tone that ensure the intermittent supply of blood to adjacent microvascular units. Previous evidence from vessels outside the eye suggests that cyclic guanosine-monophosphate (cGMP) is involved in the regulation of vasomotion, but it is unknown whether this compound has an effect on vasomotion in retinal vessels.

METHODS

Retinal arterioles from porcine eyes were studied in a wire myograph. After initiation of vasomotion, the vessels were stimulated with increasing concentrations of the cGMP agonist 8-Br-cGMP (n = 6), the phosphodiesterase inhibitor zaprinast (n = 6) and the cGMP synthesis inhibitor L-NAME (n = 6). High concentrations of L-NAME blocked vasomotion, and control experiments (n = 20) using 8-Br-cGMP, S-nitroso-N-acetylpenicillamine (SNAP), adenosine and pinacidil were carried out to elucidate whether this effect was related to changes in the general tone of the vessel. Additionally, the relationship between oscillations in vascular tone and intracellular calcium concentration was studied.

RESULTS

Induction of cGMP agonistic activity with either 8-Br-cGMP or zaprinast lowered the vasomotion frequency significantly, whereas L-NAME-induced inhibition of cGMP increased this frequency. Neither of the agents affected the amplitude of the oscillations. The control experiments indicated that the effect of cGMP on vasomotion frequency was independent of the accompanying increase in tone. The oscillations in tone during vasomotion were accompanied by similar oscillations in intracellular calcium concentration.

CONCLUSION

Cyclic GMP lowers the frequency without affecting the amplitude of vasomotion in isolated porcine retinal arterioles.

Rhythmicity in arterial smooth muscle

Many arteries and arterioles exhibit rhythmical contractions which are synchronous over considerable distances. This vasomotion is likely to assist in tissue perfusion especially during periods of altered metabolism or perfusion pressure. While the mechanism underlying vascular rhythmicity has been investigated for many years, it has only been recently, with the advent of imaging techniques for visualizing intracellular calcium release, that significant advances have been made. These methods, when combined with mechanical and electrophysiological recordings, have demonstrated that the rhythm depends critically on calcium released from intracellular stores within the smooth muscle cells and on cell coupling via gap junctions to synchronize oscillations in calcium release amongst adjacent cells. While these factors are common to all vessels studied to date, the contribution of voltage-dependent channels and the endothelium varies amongst different vessels. The basic mechanism for rhythmical activity in arteries thus differs from its counterpart in non-vascular smooth muscle, where specific networks of pacemaker cells generate electrical potentials which drive activity within the otherwise quiescent muscle cells.

Vasomotion: cellular background for the oscillator and for the synchronization of smooth muscle cells

1. Vasomotion is the oscillation of vascular tone with frequencies in the range from 1 to 20 min(-1) seen in most vascular beds. The oscillation originates in the vessel wall and is seen both in vivo and in vitro. 2. Recently, our ideas on the cellular mechanisms responsible for vasomotion have improved. Three different types of cellular oscillations have been suggested. One model has suggested that oscillatory release of Ca2+ from intracellular stores is important (the oscillation is based on a cytosolic oscillator). A second proposed mechanism is an oscillation originating in the sarcolemma (a membrane oscillator). A third mechanism is based on an oscillation of glycolysis (metabolic oscillator). For the two latter mechanisms, only limited experimental evidence is available. 3. To understand vasomotion, it is important to understand how the cells synchronize. For the cytosolic oscillators synchronization may occur via activation of Ca2+-sensitive ion channels by oscillatory Ca2+ release. The ensuing membrane potential oscillation feeds back on the intracellular Ca2+ stores and causes synchronization of the Ca2+ release. While membrane oscillators in adjacent smooth muscle cells could be synchronized through the same mechanism that sets up the oscillation in the individual cells, a mechanism to synchronize the metabolic-based oscillators has not been suggested. 4. The interpretation of the experimental observations is supported by theoretical modelling of smooth muscle cells behaviour, and the new insight into the mechanisms of vasomotion has the potential to provide tools to investigate the physiological role of vasomotion.

A theoretical model of nitric oxide transport in arterioles: frequency- vs. amplitude-dependent control of cGMP formation

Nitric oxide (NO) plays many important physiological roles, including the regulation of vascular smooth muscle tone. In response to hemodynamic or agonist stimuli, endothelial cells produce NO, which can diffuse to smooth muscle where it activates soluble guanylate cyclase (sGC), leading to cGMP formation and smooth muscle relaxation. The close proximity of red blood cells suggests, however, that a significant amount of NO released will be scavenged by blood, and thus the issue of bioavailability of endothelium-derived NO to smooth muscle has been investigated experimentally and theoretically. We formulated a mathematical model for NO transport in an arteriole to test the hypothesis that transient, burst-like NO production can facilitate efficient NO delivery to smooth muscle and reduce NO scavenging by blood. The model simulations predict that 1) the endothelium can maintain a physiologically significant amount of NO in smooth muscle despite the presence of NO scavengers such as hemoglobin and myoglobin; 2) under certain conditions, transient NO release presents a more efficient way for activating sGC and it can increase cGMP formation severalfold; and 3) frequency-rather than amplitude-dependent control of cGMP formation is possible. This suggests that it is the frequency of NO bursts and perhaps the frequency of Ca(2+) oscillations in endothelial cells that may limit cGMP formation and regulate vascular tone. The proposed hypothesis suggests a new functional role for Ca(2+) oscillations in endothelial cells. Further experimentation is needed to test whether and under what conditions in silico predictions occur in vivo.

Amine metabolism: a novel path to coronary artery vasospasm

We hypothesized that allylamine (AA) induces subendocardial necrosis in mammals via coronary artery (CA) vasospasm. Additionally, AA toxicity is likely dependent on the enzyme semicarbazide-sensitive amine oxidase (SSAO), which is highly expressed in the aorta of rats and humans. We tested whether AA or acrolein (1, 10, 100, and 1000 microM), a highly reactive product of AA metabolism by SSAO, could contract CA or thoracic aorta (TA) in vitro and if the AA effects involved SSAO. AA or acrolein produced a similar pattern of responses in both CA and TA rings at 100 and 1000 microM, including (1) increased basal tension, (2) enhanced agonist-induced contraction (hypercontractility or vasospasm), (3) remarkable, agonist-induced slow wave vasomotion (vasospasm), and (4) irreversible reduction in vessel contractility after 1 mM exposure. Endothelium-dependent acetylcholine-induced relaxation was not altered during vasospasm in either vessel. Pretreatment with the SSAO inhibitor semicarbazide (1 mM; 10 min) prevented or significantly reduced the majority of AA's effects in both CA and TA rings and inhibited 100% of the SSAO activity present in rat TA and human CA and TA. We propose a two-step model for AA induction of CA vasospasm and resultant myocardial necrosis: (1) metabolism of AA to acrolein by coronary arterial SSAO activity and (2) acrolein induction of CA vasospasm independent of endothelial injury-a novel path.

Contribution of vasomotion to vascular resistance: a comparison of arteries from virgin and pregnant rats

Intrinsic oscillatory activity, or vasomotion, within the microcirculation has many potential functions, including modulation of vascular resistance. Alterations in oscillatory activity during pregnancy may contribute to the marked reduction in vascular resistance. The purpose of this study was 1) to mathematically model the oscillatory changes in vessel diameter and determine the effect on vascular resistance and 2) to characterize the vasomotion in resistance arteries of pregnant and nonpregnant (virgin) rats. Mesenteric arteries were isolated from Sprague-Dawley rats and studied in a pressurized arteriograph. Mathematical modeling demonstrated that the resistance in a vessel with vasomotion was greater than that in a static vessel with the same mean radius. During constriction with the alpha1-adrenergic agonist phenylephrine, the amplitude of oscillation was less in the arteries from pregnant rats. We conclude that vasomotor activity may provide a mechanism to regulate vascular resistance and blood flow independent of static changes in arterial diameter. During pregnancy the decrease in vasomotor activity in resistance arteries may contribute to the reduction in peripheral vascular resistance.

Endothelium-dependent rhythmic contractions induced by cyclopiazonic acid in rat mesenteric artery

The action of cyclopiazonic acid, the putative inhibitor of the Ca(2+)-ATPase of endoplasmic reticulum, on phenylephrine-evoked-isometric contractions in rat isolated mesenteric arteries were investigated. Cyclopiazonic acid (3 microM) induced an initial relaxation followed by rhythmic contractions of the phenylephrine-precontracted arteries with intact endothelium. Removal of endothelium abolished the effect of cyclopiazonic acid. Pretreatment of tissues with NG-nitro-L-arginine (100 microM) abolished the initial relaxation but not the rhythmic contractions. Indomethacin and glibenclamide did not affect the cyclopiazonic acid-induced response. Charybdotoxin (100 nM) converted the cyclopiazonic acid-induced rhythmic contractions to the sustained tension in the absence or presence of NG-nitro-L-arginine (100 microM). Pretreatment of charybdotoxin (100 nM) abolished cyclopiazonic acid-induced rhythmic activity but not the initial relaxation. Nifedipine (10 nM) abolished the effect of cyclopiazonic acid. Moderate increase of extracellular K+ (20 mM) reduced the initial relaxation but completely abolished rhythmic contractions induced by cyclopiazonic acid. The remaining relaxation was reversed or prevented by NG-nitro-L-arginine (100 microM). The results of the present investigation indicate that cyclopiazonic acid caused endothelium-dependent response in rat isolated mesenteric arteries probably by releasing nitric oxide responsible for the initial relaxation, and probably by releasing endothelium-derived hyperpolarizing factors primarily responsible for activation of charybdotoxin-sensitive K+ channels and induction of rhythmic contractile activity.

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.

Endothelium-dependent rhythmic contractions induced by cyclopiazonic acid, a sarcoplasmic reticulum Ca(2+)-pump inhibitor, in the rabbit femoral artery

The vascular responses to cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase in the sarcoplasmic reticulum, were investigated in the rabbit femoral artery, suspended in an organ chamber for isometric tension recordings. CPA produced rhythmic contractions in the femoral artery which had been contracted with phenylephrine. CPA, however, did not induce the rhythmic responses in endothelium-denuded arteries. NG-nitro-L-arginine methyl ester and methylene blue, inhibitors of the formation and the action of nitric oxide, respectively, failed to antagonize the CPA-induced rhythmic contractions in the phenylephrine-contracted artery. In contrast, the CPA-induced rhythmic contractions were abolished by charybdotoxin, a Ca(2+)-activated K+ channel antagonist, but not by glibenclamide, a blocker of the ATP-sensitive K+ channel. Nifedipine also inhibited the CPA-induced rhythmic contractions in the endothelium-intact artery and relaxed the endothelium-denuded artery treated with CPA. These results indicate that the CPA-induced rhythmic contractions in the phenylephrine-contracted rabbit femoral artery may be attributed to the periodic inactivation of the voltage-dependent Ca2+ channel, presumably regulated by the Ca(2+)-activated K+ channel. The activation of the K+ channel by CPA might occur only when the endothelium is present.