Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

Properties of gap junction blockers and their behavioural, cognitive and electrophysiological effects: animal and human studies

Gap junctions play an important role in brain physiology. They synchronize neuronal activity and connect glial cells participating in the regulation of brain metabolism and homeostasis. Gap junction blockers (GJBs) include various chemicals that impair gap junction communication, disrupt oscillatory neuronal activity over a wide range of frequencies, and decrease epileptic discharges. The behavioural and clinical effects of GJBs suggest that gap junctions can be involved in the regulation of locomotor activity, arousal, memory, and breathing. Severe neuropsychiatric side effects suggest the involvement of gap junctions in mechanisms of consciousness. Unfortunately, the available GJBs are not selective and can bind to targets other than gap junctions. Other problems in behavioural studies include the possible adverse effects of GJBs, for example, retinal toxicity and hearing disturbances, changes in blood-brain transport, and the metabolism of other drugs. Therefore, it is necessary to design experiments properly to avoid false, misleading or uninterpretable results. We review the pharmacological properties and electrophysiological, behavioural and cognitive effects of the available gap junction blockers, such as carbenoxolone, glycyrrhetinic acid, quinine, quinidine, mefloquine, heptanol, octanol, anandamide, fenamates, 2-APB, several anaesthetics, retinoic acid, oleamide, spermine, aminosulfonates, and sodium propionate. It is concluded that despite a number of different problems, the currently used gap junction blockers could be useful tools in pharmacology and neuroscience.

Review of pharmacological effects of Glycyrrhiza sp. and its bioactive compounds

The roots and rhizomes of licorice (Glycyrrhiza) species have long been used worldwide as a herbal medicine and natural sweetener. Licorice root is a traditional medicine used mainly for the treatment of peptic ulcer, hepatitis C, and pulmonary and skin diseases, although clinical and experimental studies suggest that it has several other useful pharmacological properties such as antiinflammatory, antiviral, antimicrobial, antioxidative, anticancer activities, immunomodulatory, hepatoprotective and cardioprotective effects. A large number of components have been isolated from licorice, including triterpene saponins, flavonoids, isoflavonoids and chalcones, with glycyrrhizic acid normally being considered to be the main biologically active component. This review summarizes the phytochemical, pharmacological and pharmacokinetics data, together with the clinical and adverse effects of licorice and its bioactive components.

Expression and role of connexins in the rat renal vasculature

Gap junctions are present in the juxtaglomerular apparatus enabling intercellular communication. Our study determined the location of different connexin subtypes within the juxtaglomerular apparatus of the rat, and the role of these subtypes in renal hemodynamics through the use of specific mimetic peptides. Immunohistochemical analysis showed connexins 37 and 40 expression in the endothelial and renin-secreting cells of the afferent arteriole, while connexin 40 was also found in extra- and intraglomerular mesangial cells. In contrast, connexin 43 was weakly expressed in endothelial cells of the afferent arteriole and within the glomerulus. Intra-renal infusion of the peptides (GAP) reported to block specific gap junctions ((Cx37,43)GAP27 or (Cx40)GAP27), elevated blood pressure, plasma renin activity, and angiotensin II levels, while decreasing renal plasma flow without a significant change in the glomerular filtration rate. Subsequent restoration of blood pressure reduced both renal plasma flow and glomerular filtration rate. In contrast, (Cx43)GAP26 reduced glomerular filtration rate without alterations in blood pressure, renal plasma flow, plasma renin activity, or angiotensin II levels. Hence, connexins 37 and 40 are expressed in the rat juxtaglomerular apparatus and these proteins control, in part, the renin-angiotensin system and renal autoregulation.

Connexins 37 and 40 transduce purinergic signals mediating renal autoregulation

Our previous data indicated that various subtypes of connexin (Cx) were expressed in the juxtaglomerular apparatus. Experiments were performed to characterize the effects on renal autoregulation of specific mimetic peptides that inhibit these Cx subtypes in Wistar-Kyoto rats. Intrarenal infusion of (Cx37,43)GAP27 increased autoregulatory index of renal plasma flow (0.06 +/- 0.05 to 0.47 +/- 0.06, n = 6, P < 0.05) and glomerular filtration rate (GFR; 0.01 +/- 0.07 to 0.49 +/- 0.07, P < 0.05). The additional administration of 8-cyclopentyl- 1,3-dipropylxanthine (CPX) produced a further elevation of autoregulatory index of RPF (0.86 +/- 0.07, P < 0.05) and GFR (0.88 +/- 0.09, P < 0.05), compared with (Cx37,43)GAP27 alone. However, the addition of pyridoxal-phosphate-6-azophenyl-2,4-disulfonic acid (PPADS) to (Cx37,43)GAP27 did not. Combined treatment with CPX and PPADS markedly worsened autoregulatory index of RPF (0.04 +/- 0.10 to 0.81 +/- 0.06, n = 6 P < 0.01) and GFR (0.05 +/- 0.08 to 0.79 +/- 0.05, P < 0.01). (Cx40)GAP27 induced similar changes to (Cx37,43)GAP27. Renal autoregulation was preserved in the presence of (Cx43)GAP26. Our results indicate that the inhibition of gap junction impaired renal autoregulation. Furthermore, the present data provide evidence that both adenosine and purinergic receptors contribute to glomerular autoregulation. Finally, our findings suggest that gap junctions, at least in part, transduce purinergic signals mediating renal autoregulation.

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.

Effects of heptanol and carbenoxolone on noradrenaline induced contractions in guinea pig vas deferens

We examined the effects of two putative gap junction blockers, heptanol and carbenoxolone, on noradrenaline-induced contractions in guinea pig vas deferens. The force generated due to the exogenously added noradrenaline (20 microM) consisted of two components: the tonic and the oscillatory. 2 mM heptanol abolished the oscillatory contractions and drastically suppressed both the maximum force (by 85.4 +/- 18.2%) as well as the tonic component (by 28.8 +/- 5.1%) (P<0.01, n=7). However, the effects of carbenoxolone (50 microM) were strikingly different, with the spikes of the oscillatory component being merged into a steady, "fused" contraction, without affecting the maximum force developed. The L-type Ca(2+) channel blocker nifedipine (2 microM) abolished the oscillatory component of the contractions and significantly reduced the maximum force and tonic component (by 82.4 +/- 6.8% and 19.7 +/- 6.4% respectively; P<0.01, n=4), in a manner similar to that elicited by heptanol. Our results indicate that (i) while carbenoxolone specifically blocks gap junctions, heptanol appears to exert its actions through non-gap junctional mechanisms, possibly by blocking VGCCs in smooth muscle; (ii) gap junctions play a significant modulatory role in the generation of noradrenaline-induced contractions in guinea pig vas deferens, particularly in the emergence of oscillatory contractions, while the maximum force developed may be independent of gap junctional contribution.

Blockade of gap junction coupling by glycyrrhetinic acids in guinea pig cochlear artery: a whole-cell voltage- and current-clamp study

BACKGROUND AND PURPOSE

Glycyrrhetinic acids (GAs) are widely used as gap junction blockers, but their efficacy and side effects have not been well determined.

EXPERIMENTAL APPROACH

Whole-cell electrical recordings were made from vascular smooth muscle cells (VSMCs) embedded in or dissociated from, guinea pig cochlear artery segments.

KEY RESULTS

18beta- & 18alpha-GA concentration-dependently increased membrane input resistance (R(in)) of in situ VSMCs, with a maximal input conductance (G(in)=1/R(in)) reduction of 92% & 77% and IC(50) of 2.0 & 4.4 microm, respectively. 18betaGA (30 microM) resulted in a R(in) of 2.2 GOmega and C(in) of 12 pF, comparable to those of freshly dissociated VSMCs (3.1 GOmega & 6.1 pF). The GAs (> or =30 microM) caused a depolarization in VSMCs in situ. In dispersed VSMCs, they both inhibited delayed rectifiers; 18betaGA also activated a non-selective cation conductance while 18alphaGA inactivated a voltage-independent K+-conductance. ACh induced an outward current in VSMCs in situ at -40 mV, with a positive slope I/V relation and a reversal potential near E(K). The ACh-induced current was attenuated by 18beta- & 18alphaGA with an IC(50) of 4.3 & 7.8 microM, respectively.

CONCLUSIONS AND IMPLICATIONS

18betaGA blocked the vascular gap junctions, achieving a complete electrical isolation of the recorded VSMC at > or =30 microM while causing a mild depolarization by a complex conductance alteration. 18betaGA suppressed the ACh-induced current in VSMC by blocking the myoendothelial gap junction and by a non-junctional action. 18alphaGA at 30-100 microM failed to fully block the gap junctions while exerting side actions.

Increasing Gap Junctional Coupling: A Tool for Dissecting the Role of Gap Junctions

Much of our current knowledge about the physiological and pathophysiological role of gap junctions is based on experiments where coupling has been reduced by either chemical agents or genetic modification. This has brought evidence that gap junctions are important in many physiological processes. In a number of cases, gap junctions have been implicated in the initiation and progress of disease, and experimental uncoupling has been used to investigate the exact role of coupling. The inverse approach, i.e., to increase coupling, has become possible in recent years and represents a new way of testing the role of gap junctions. The aim of this review is to summarize the current knowledge obtained with agents that selectively increase gap junctional intercellular coupling. Two approaches will be reviewed: increasing coupling by the use of antiarrhythmic peptide and its synthetic analogs and by interfering with the gating of gap junctional channels.

Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.

Connexin 43 mediates endothelium-derived hyperpolarizing factor-induced vasodilatation in subcutaneous resistance arteries from healthy pregnant women

The role of gap junctions in endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation of human arteries was assessed using connexin mimetic peptides (CMPs) designated (37,43)Gap27, (40)Gap27, and (43)Gap26 according to homology with the major vascular connexins (Cx37, Cx40, and Cx43). Resistance arteries were obtained from subcutaneous fat biopsies of healthy pregnant women undergoing elective cesarean section. Endothelium-dependent vasodilatation to bradykinin (BK) was assessed using wire myography. N(omega)-nitro-l-arginine methyl ester (l-NAME) and indomethacin (nitric oxide synthase and cyclooxygenase inhibitors, respectively) attenuated maximal relaxation to BK (R(max)) by approximately 50%. Coincubation with l-NAME, indomethacin, and the combined CMPs ((37,43)Gap27, (40)Gap27, and (43)Gap26) almost abolished relaxation to BK (R(max) = 12.2 +/- 3.7%). In arteries incubated with l-NAME and indomethacin, the addition of either (37,43)Gap27 or (40)Gap27 had no significant effect on R(max), whereas (43)Gap26 caused marked inhibition (R(max) = 21 +/- 6.4%, P = 0.005 vs. l-NAME plus indomethacin alone) that was similar to that of the triple combination. Endothelium-independent vasorelaxation was unaffected by CMPs, l-NAME, or indomethacin. Immunohistochemistry demonstrated Cx37, Cx40, and Cx43 expression in the endothelium and vascular smooth muscle. In pregnant women, EDHF-mediated vasorelaxation of subcutaneous resistance arteries is dependent on Cx43 and gap junctions.

C-type natriuretic peptide: a new endothelium-derived hyperpolarizing factor?

Vascular relaxation mediated by endothelium-derived hyperpolarizing factor (EDHF) is important for resistance artery function and is underpinned by hyperpolarization of the smooth muscle cells of the blood vessel wall. Debate surrounds the identity of EDHF and its mechanism of action, with the consensus being that there is no universal EDHF. Regional differences in vascular function reflect the complex mechanisms of EDHF. Two primary mechanistic pathways are implicated: (i) myoendothelial gap junctions mediating the spread of endothelial cell hyperpolarization or small signaling molecules (or both) to the smooth muscle; and (ii) diffusible mediators released from the endothelium, including K+ and epoxyeicosatrienoic acids. Here, we discuss the evidence for and against C-type natriuretic peptide (CNP), the latest candidate for a diffusible mediator.

Enteric nervous system and developmental abnormalities in childhood

ENS consists of a complex network of neurons, organised in several plexuses, which interact by means of numerous neurotransmitters. It is capable of modulating the intestinal motility, exocrine and endocrine secretions, microcirculation and immune and inflammatory responses within the gastrointestinal tract, independent of the central nervous system. Though the embryological development of various plexuses are completed by mid-way of gestation, the maturation of neurons and nerve plexuses appear to continue well after birth. Therefore, any histological or functional abnormalities related to the gastrointestinal function must be investigated with the ongoing maturational processes in mind.

Endothelial coordination of cerebral vasomotion via myoendothelial gap junctions containing connexins 37 and 40

Control of cerebral vasculature differs from that of systemic vessels outside the blood-brain barrier. The hypothesis that the endothelium modulates vasomotion via direct myoendothelial coupling was investigated in a small vessel of the cerebral circulation. In the primary branch of the rat basilar artery, membrane potential, diameter, and calcium dynamics associated with vasomotion were examined using selective inhibitors of endothelial function in intact and endothelium-denuded arteries. Vessel anatomy, protein, and mRNA expression were studied using conventional electron microscopy high-resolution ultrastructural and confocal immunohistochemistry and quantitative PCR. Membrane potential oscillations were present in both endothelial cells and smooth muscle cells (SMCs), and these preceded rhythmical contractions during which adjacent SMC intracellular calcium concentration ([Ca(2+)](i)) waves were synchronized. Endothelium removal abolished vasomotion and desynchronized adjacent smooth muscle cell [Ca(2+)](i) waves. N(G)-nitro-l-arginine methyl ester (10 microM) did not mimic this effect, and dibutyryl cGMP (300 muM) failed to resynchronize [Ca(2+)](i) waves in endothelium-denuded arteries. Combined charybdotoxin and apamin abolished vasomotion and depolarized and constricted vessels, even in absence of endothelium. Separately, (37,43)Gap27 and (40)Gap27 abolished vasomotion. Extensive myoendothelial gap junctions (3 per endothelial cell) composed of connexins 37 and 40 connected the endothelial cell and SMC layers. Synchronized vasomotion in rat basilar artery is endothelium dependent, with [Ca(2+)](i) waves generated within SMCs being coordinated by electrical coupling via myoendothelial gap junctions.

Effect of gap junction uncoupler heptanol on resistance arteries reactivity in experimental models of diabetes, hyperlipemia and hyperlipemia-diabetes

The understanding of the involvement of the gap junctions (GJ) in the vascular reactivity is an ongoing effort. In this study we questioned on impact of pathologies such as diabetes, hyperlipemia, and simultaneous hyperlipemia-diabetes on GJ involvement in the contractile/relaxant response of the mesenteric resistance arteries. To this purpose, four groups of Golden Syrian hamsters were used: (i) diabetics (D), injected by streptozotocin, (ii) hyperlipemics (H), fed the standard chow of the species supplemented with 3% cholesterol and 15% butter, (iii) simultaneously hyperlipemic-diabetics (HD), and (iv) controls (C), age-matched normal healthy animals. At 24 weeks after the beginning of the experiment, the vascular reactivity of the resistance arteries was measured by the myograph technique in the presence/absence of 1 mM Heptanol (Hep) and of vasoconstrictors and vasodilators. The results showed that: (i) in pathological conditions 1 mM Hep significantly impaired the constrictor response of the hamster resistance arteries to both 10(-5) M NA (noradrenaline, agonist of alpha(1)-adrenoceptors) and 64.1 mM K+ (potassium ion, the major intracellular cation). The impairment occur in the group range: HD < H < D < C being the highest at the simultaneous insult of hyperlipemia and diabetes; (ii) independently of the pathological condition, 1 mM Hep abolishes both endothelium-dependent and independent relaxation of the hamster resistance arteries. At 1 mM Hep we noticed a reversible effect on endothelium-dependent relaxation that may be partially restored (in normal) in the presence of L-arginine. It is hoped that these results may contribute to understanding of the involvement of GJ in vascular pathology/dysfunction.

Protective effect of gap junction uncouplers given during hypoxia against reoxygenation injury in isolated rat hearts

It has been shown that cell-to-cell chemical coupling may persist during severe myocardial hypoxia or ischemia. We aimed to analyze the effects of different, chemically unrelated gap junction uncouplers on the progression of ischemic injury in hypoxic myocardium. First, we analyzed the effects of heptanol, 18α-glycyrrhetinic acid, and palmitoleic acid on intracellular Ca2+ concentration during simulated hypoxia (2 mM NaCN) in isolated cardiomyocytes. Next, we analyzed their effects on developed and diastolic tension and electrical impedance in 47 isolated rat hearts submitted to 40 min of hypoxia and reoxygenation. All treatments were applied only during the hypoxic period. Cell injury was determined by lactate dehydrogenase (LDH) release. Heptanol, but not 18α-glycyrrhetinic acid nor palmitoleic acid, attenuated the increase in cytosolic Ca2+ concentration induced by simulated ischemia in cardiomyocytes and delayed rigor development (rigor onset at 7.31 ± 0.71 min in controls vs. 14.76 ± 1.44 in heptanol-treated hearts, P < 0.001) and the onset of the marked changes in electrical impedance (tissue resistivity: 4.02 ± 0.29 vs. 7.75 ± 1.84 min, P = 0.016) in hypoxic rat hearts. LDH release from hypoxic hearts was minimal and was not significantly modified by drugs. However, all gap junction uncouplers, given during hypoxia, attenuated LDH release during subsequent reoxygenation. Dose-response analysis showed that increasing heptanol concentration beyond the level associated with maximal effects on cell coupling resulted in further protection against hypoxic injury. In conclusion, gap junction uncoupling during hypoxia has a protective effect on cell death occurring upon subsequent reoxygenation, and heptanol has, in addition, a marked protective effect independent of its uncoupling actions.

Treatment With the Gap Junction Modifier Rotigaptide (ZP123) Reduces Infarct Size in Rats With Chronic Myocardial Infarction

Treatment with non-selective drugs (eg, long-chain alcohols, halothane) that reduce gap junction intercellular communication (GJIC) is associated with reduced infarct size after myocardial infarction (MI). Therefore, it has been suggested that gap junction intercellular communication stimulating compounds may increase infarct size. The antiarrhythmic peptide analogue rotigaptide (ZP123) increases cardiac gap junction intercellular communication and the purpose of the present study was to examine the effects of rotigaptide treatment on infarct size. Myocardial infarction was induced in male rats by ligation of the left anterior descending artery (LAD). Rats (n = 156) were treated with rotigaptide at three dose levels or vehicle from the onset of ischemia and for 3 weeks following LAD occlusion. Infarct size was determined using histomorphometry after 3 weeks treatment. Rotigaptide treatment producing steady state plasma levels of 0.8 +/- 0.1, 5.5 +/- 0.5, and 86 +/- 8 nmol/L had no effect on mortality, but reduced infarct size to 90 +/- 10% (P = 0.41), 67 +/- 7% (P = 0.005), and 82 +/- 7% (P = 0.13), respectively relative to vehicle-treated myocardial infarction rats (100 +/- 12%). In contrast to what was predicted, our data demonstrates that rotigaptide treatment was associated with a significant infarct size reduction. We conclude that whereas treatment with non-selective inhibitors of gap junction intercellular communication cause a reduction in infarct size, this information cannot be extrapolated to the effects of compounds that selectively increase gap junction intercellular communication.

Analysis of effects of connexin-mimetic peptides in rat mesenteric small arteries

Synthetic peptides homologous to the extracellular loops of the major vascular connexins represent a novel class of gap junction blockers that have been used to assess the role of direct cellular communication in arteries and veins. However, the specificity of action of such peptides on the coupling between smooth muscle cells (SMCs) has not yet been fully characterized. Isolated third-order rat mesenteric arteries were therefore studied with respect to isometric tension (myography), intracellular Ca2+ concentration ([Ca2+]i) (Ca2+ -sensitive dyes), membrane potential, and input resistance (sharp intracellular glass electrodes). Confocal imaging was used for visualization of [Ca2+]i events in individual SMCs in the arterial wall and membrane currents (patch clamp) measured in individual SMCs isolated from the same arteries. A triple peptide combination (37,43Gap 27 + 40Gap 27 + 43Gap 26) increased intercellular resistance (measured as input resistance) in intact arterial segments without affecting the membrane conductance of individual cells and also interrupted electrical coupling between pairs of rat aortic A7r5 myocytes. In intact arterial segments, the peptides desynchronized [Ca2+]i transients in individual SMCs and abolished vasomotion without suppressing Ca2+ transients in individual cells. They also depolarized SMCs, increased [Ca2+]i, and attenuated acetylcholine-induced, endothelium-dependent smooth muscle hyperpolarization. Experiments with endothelium-denuded arteries suggested that the depolarization produced by the peptides under basal conditions was in part secondary to electrical uncoupling of the endothelium from SMCs with loss of a tonic hyperpolarizing effect of the endothelium. Taken together, the results indicate that connexin-mimetic peptides block electrical signaling in rat mesenteric small arteries without exerting major nonjunctional effects.

Rotigaptide (ZP123) prevents spontaneous ventricular arrhythmias and reduces infarct size during myocardial ischemia/reperfusion injury in open-chest dogs

The antiarrhythmic and cardioprotective effect of increasing gap junction intercellular communication during ischemia/reperfusion injury has not been studied. The antiarrhythmic peptide rotigaptide (previously ZP123), which maintains gap junction intercellular communication, was tested in dogs subjected to a 60-min coronary artery occlusion and 4 h of reperfusion. Rotigaptide was administered i.v. 10 min before reperfusion as a bolus + i.v. infusion at doses of 1 ng/kg bolus + 10 ng/kg/h infusion (n = 6), 10 ng/kg bolus + 100 ng/kg/h infusion (n = 5), 100 ng/kg bolus + 1000 ng/kg/h infusion (n = 8), 1000 ng/kg bolus + 10 mug/kg/h infusion (n = 6), and vehicle control (n = 5). Premature ventricular complexes (PVCs) were quantified during reperfusion. A series of four or more consecutive PVCs was defined as ventricular tachycardia (VT). The total incidence of VT was reduced significantly with the two highest doses of rotigaptide (20.3 +/- 10.9 and 4.3 +/- 4.1 events; p < 0.05) compared with controls (48.7 +/- 6.0). Total PVCs were reduced significantly from 25.1 +/- 4.2% in control animals to 11.0 +/- 4.4 and 1.7 +/- 1.3% after the two highest doses of rotigaptide. Infarct size, expressed as a percentage of the left ventricle, was reduced significantly from 13.2 +/- 1.9 in controls to 7.1 +/- 1.0 (p < 0.05) at the highest dose of rotigaptide. Ultrastructural evaluation revealed no differences in myocardial injury in the infarct area, area at risk, border zone, or normal zone in vehicle and rotigaptide-treated animals. However, rotigaptide did increase the presence of gap junctions in the area at risk (p = 0.022, Fisher's exact test). Rotigaptide had no effect on heart rate, blood pressure, heart rate-corrected QT interval, or left ventricular end-diastolic pressure. In conclusion, these results demonstrate that rotigaptide is a potent antiarrhythmic compound with cardioprotective effects and desirable safety.

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.

Factors, fiction and endothelium-derived hyperpolarizing factor

1. The principal mediators of vascular tone are neural, endothelial and physical stimuli that result in the initiation of dilator and constrictor responses to facilitate the control of blood pressure. Two primary vasodilatory stimuli produced by the endothelium are nitric oxide (NO) and prostaglandins. An additional endothelium-dependent vasodilatory mechanism is characterized as the hyperpolarization-mediated relaxation that remains after the inhibition of the synthesis of NO and prostaglandins. This mechanism is due to the action of a so-called endothelium-derived hyperpolarizing factor (EDHF) and is dependent on either the release of diffusible factor(s) and/or to a direct contact-mediated mechanism. 2. Most evidence supports the concept that 'EDHF' activity is dependent on contact-mediated mechanisms. This involves the transfer of an endothelium-derived electrical current, as an endothelium-derived hyperpolarization (EDH), through direct heterocellular coupling of endothelial cells and smooth muscle cells via myoendothelial gap junctions (MEGJ). However, there is a lack of consensus with regard to the nature and mechanism of action of EDHF/EDH (EDH(F)), which has been shown to vary within and between vascular beds, as well as among species, strains, sex and during development, ageing and disease. 3. In addition to actual heterogeneity in EDH(F), further heterogeneity has resulted from the less-than-optimal design, analysis and interpretation of data in some key papers in the EDHF literature; with such views being perpetuated in the subsequent literature. 4. The focus of the present brief review is to examine what factors are proposed as EDH(F) and highlight the correlative structural and functional studies from our laboratory that demonstrate an integral role for MEGJ in the conduction of EDH, which account for the heterogeneity in EDH(F), while incorporating the reported diffusible mechanisms in the regulation of this activity. Furthermore, in addition to the reported heterogeneity in the nature and mechanism of action of EDH(F), the contribution of experimental design and technique to this heterogeneity will be examined.