An electron-microscopic study of smooth muscle cell dye coupling in the pig coronary arteries. Role of gap junctions

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Morphological and pharmacological characterization of the porcine popliteal artery: A novel model for study of lower limb arterial disease

OBJECTIVE

This study was undertaken to characterize structural and pharmacological properties of the pig popliteal artery in order to develop a novel system for the examination of lower limb blood flow regulation in a variety of cardiovascular pathologies, such as diabetes-induced peripheral artery disease.

METHODS

Popliteal arteries were isolated from streptozocin-induced diabetic pigs or age-matched saline-injected control pigs for morphological study using transmission electron microscopy and for examination of vasoreactivity to pharmacological agents using wire myography.

RESULTS

Transmission electron microscopy of the porcine popliteal artery wall revealed the presence of endothelial cell-smooth muscle cell interactions (myoendothelial junctions) and smooth muscle cell-smooth muscle cell interactions, for which we have coined the term "myo-myo junctions." These myo-myo junctions were shown to feature plaques indicative of connexin expression. Further, the pig popliteal artery was highly responsive to a variety of vasoconstrictors including norepinephrine, phenylephrine, and U46619, and vasodilators including acetylcholine, adenosine 5'-[β-thio] diphosphate, and bradykinin. Finally, 2 weeks after streptozocin-induced diabetes, the normalized vasoconstriction of the pig popliteal artery to norepinephrine was unaltered compared to control.

CONCLUSIONS

The pig popliteal artery displays structural and pharmacological properties that might prove useful in future studies of diabetes-associated peripheral artery disease and other lower limb cardiovascular diseases.

Smooth muscle gap-junctions allow propagation of intercellular Ca2+ waves and vasoconstriction due to Ca2+ based action potentials in rat mesenteric resistance arteries

The role of vascular gap junctions in the conduction of intercellular Ca2+ and vasoconstriction along small resistance arteries is not entirely understood. Some depolarizing agents trigger conducted vasoconstriction while others only evoke a local depolarization. Here we use a novel technique to investigate the temporal and spatial relationship between intercellular Ca2+ signals generated by smooth muscle action potentials (APs) and vasoconstriction in mesenteric resistance arteries (MA). Pulses of exogenous KCl to depolarize the downstream end (T1) of a 3 mm long artery increased intracellular Ca2+ associated with vasoconstriction. The spatial spread and amplitude of both depended on the duration of the pulse, with only a restricted non-conducting vasoconstriction to a 1 s pulse. While blocking smooth muscle cell (SMC) K+ channels with TEA and activating L-type voltage-gated Ca2+ channels (VGCCs) with BayK 8644 spread was dramatically facilitated, so the 1 s pulse evoked intercellular Ca2+ waves and vasoconstriction that spread along an entire artery segment 3000 μm long. Ca2+ waves spread as nifedipine-sensitive Ca2+ spikes due to SMC action potentials, and evoked vasoconstriction. Both intercellular Ca2+ and vasoconstriction spread at circa 3 mm s-1 and were independent of the endothelium. The spread but not the generation of Ca2+ spikes was reversibly blocked by the gap junction inhibitor 18β-GA. Thus, smooth muscle gap junctions enable depolarization to spread along resistance arteries, and once regenerative Ca2+-based APs occur, spread along the entire length of an artery followed by widespread vasoconstriction.

Cellular mechanisms of human atherosclerosis: Role of cell-to-cell communications in subendothelial cell functions

The present study was undertaken in order to extend of our earlier work, focusing on the analysis of roles of cell-to-cell communications in the regulation of the subendothelial cell function. In present study, we have found that the expression of connexin43 (Cx43) is dramatically reduced in human atherosclerotic lesions, compared with undiseased intima. In atherosclerotic lesions, the number of so-called 'connexin plaques' was found to be lower in lipid-laden cells than in cells which were free from lipid inclusions. In primary cell culture, subendothelial intimal cells tended to create multicellular structures in the form of clusters. Cluster creation was accompanied by the formation of gap junctions between cells; the degree of gap junctional communication correlated with the density of cells in culture. We found that atherosclerosis-related processes such as DNA synthesis, protein synthesis and accumulation of intracellular cholesterol correlated with the degree of cell-to-cell communication. The relation of DNA and protein synthesis with cell-to-cell communication could be described as "bell-shaped". We further incubated cells, cultured from undiseased subendothelial intima, with various forms of modified LDL causing intracellular cholesterol accumulation. After the incubation of intimal cells with modified LDL, intercellular communication has "dropped" considerably. The findings indicate that intracellular lipid accumulation might be a reason for a decrease of the number of gap junctions. The findings also suggest that the disintegration of cellular network is associated with foam cell formation, the process known as a key event of atherogenesis.

Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte–non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

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Electrical coupling of cardiomyocytes and fibroblasts is well-established in vitro

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Whether such hetero-cellular coupling exists in vivo has been a matter of debate

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We review the development of experimental and conceptual insight into the topic

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Conclusion 1: hetero-cellular coupling in heart tissue has been shown in principle

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Conclusion 2: extent, regulation, context, and relevance remain to be established

Control of vascular smooth muscle cell growth by connexin 43

Connexin 43 (Cx43), the principal gap junction protein in vascular smooth muscle cells (VSMCs), regulates movement of ions and other signaling molecules through gap junction intercellular communication (GJIC) and plays important roles in maintaining normal vessel function; however, many of the signaling mechanisms controlling Cx43 in VSMCs are not clearly described. The goal of this study was to investigate mechanisms of Cx43 regulation with respect to VSMC proliferation. Treatment of rat primary VSMCs with the cAMP analog 8Br-cAMP, the soluble guanylate cyclase (sGC) stimulator BAY 41-2272 (BAY), or the Cx inducer diallyl disulfide (DADS) significantly reduced proliferation after 72 h compared with vehicle controls. Bromodeoxyuridine uptake revealed reduction (p < 0.05) in DNA synthesis after 6 h and flow cytometry showed reduced (40%) S-phase cell numbers after 16 h in DADS-treated cells compared with vehicle controls. Cx43 expression significantly increased after 270 min treatment with 8Br-cAMP, 8Br-cGMP, BAY or DADS. Inhibition of PKA, PKG or PKC reversed 8Br-cAMP-stimulated increases in Cx43 expression, whereas only PKG or PKC inhibition reversed 8Br-cGMP- and BAY-stimulated increases in total Cx43. Interestingly, stimulation of Cx43 expression by DADS was not dependent on PKA, PKG or PKC. Using fluorescence recovery after photobleaching, only 8Br-cAMP or DADS increased GJIC with 8Br-cAMP mediated by PKC and DADS mediated by PKG. Further, DADS significantly increased phosphorylation at MAPK-sensitive Serine (Ser)255 and Ser279, the cell cycle regulatory kinase-sensitive Ser262 and PKC-sensitive Ser368 after 30 min while 8Br-cAMP significantly increased phosphorylation only at Ser279 compared with controls. This study demonstrates that 8Br-cAMP- and DADS-enhanced GJIC rather than Cx43 expression and/or phosphorylation plays important roles in the regulation of VSMC proliferation and provides new insights into the growth-regulatory capacities of Cx43 in VSM.

Lucifer yellow - an angel rather than the devil

The fluorescent dye Lucifer yellow (LY) was introduced in 1978, and has been extremely useful in studying cell structure and communications. This dye has been used mostly for labelling cells by intracellular injection from microelectrodes. This review describes the numerous applications of LY, with emphasis on the enteric nervous system and interstitial cells of Cajal. Of particular importance is the dye coupling method, which enables the detection of cell coupling by gap junctions.

Biological and biophysical properties of vascular connexin channels

Intercellular channels formed by connexin proteins play a pivotal role in the direct movement of ions and larger cytoplasmic solutes between vascular endothelial cells, between vascular smooth muscle cells, and between endothelial and smooth muscle cells. Multiple genetic and epigenetic factors modulate connexin expression levels and/or channel function, including cell-type-independent and cell-type-specific transcription factors, posttranslational modifications, and localized membrane targeting. Additionally, differences in protein-protein interactions, including those between connexins, significantly contribute to both vascular homeostasis and disease progression. The biophysical properties of the connexin channels identified in the vasculature, those formed by Cx37, Cx40, Cx43 and/or Cx45 proteins, are discussed in this chapter in the physiological and pathophysiological context of vessel function.

Connexin isoform expression in smooth muscle cells and endothelial cells of hamster cheek pouch arterioles and retractor feed arteries

OBJECTIVE

Gap junction channels formed by connexin (Cx) protein subunits enable cell-to-cell conduction of vasoactive signals. Given the lack of quantitative measurements of Cx expression in microvascular endothelial cells (EC) and smooth muscle cells (SMC), the objective was to determine whether Cx expression differed between EC and SMC of resistance microvessels for which conduction is well-characterized.

METHODS

Cheek pouch arterioles (CPA) and retractor feed arteries (RFA) were hand-dissected and dissociated to obtain SMC or endothelial tubes. In complementary experiments, small intestine was dissociated to obtain SMC. Following reverse transcription, quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) was performed by using specific primers and fluorescent probes for Cx37, Cx40, and Cx43. Smooth muscle alpha-actin (SMAA) and platelet endothelial cell adhesion molecule-1 (PECAM-1) served as respective reference genes.

RESULTS

Transcript copy numbers were similar for each Cx isoform in EC from CPA and RFA (approximately 0.5 Cx/PECAM-1). For SMC, Cx43 transcript in CPA and RFA (< 0.1 Cx/SMAA) was less (p < 0.05) than that in small intestine (approximately 0.4 Cx/SMAA). Transcripts for Cx37 and Cx40 were also detected in SMC. Punctate immunolabeling for each Cx isoform was pronounced at EC borders and that for Cx43 was pronounced in SMC of small intestine. In contrast, Cx immunolabeling was not detected in SMC of CPA or RFA.

CONCLUSIONS

Connexin expression occurs primarily within the endothelium of arterioles and feed arteries, supporting a highly effective pathway for conducting vasoactive signals along resistance networks. The apparent paucity of Cx expression within SMC underscores discrete homocellular coupling and focal localization of myoendothelial gap junctions.

Role of membrane potential in endothelium-dependent relaxation of isolated mouse main pulmonary artery

The physiology of smooth muscle and endothelial cells of a particular vascular bed and from different species differs from each other. Acetylcholine causes an endothelium-dependent relaxation of preconstricted pulmonary arteries from the rat. This relaxation is mediated by nitric oxide (NO) plus a yet-unidentified endothelium-derived hyperpolarizing factor, which relaxes the smooth muscles by hyperpolarizing them. Our aim is to test whether these observations could be generalized to the smooth muscle cells from the mouse pulmonary artery. Smooth muscle or endothelial cell membrane potential of strips of murine pulmonary artery were measured simultaneously with the force developed by the strip. Acetylcholine hyperpolarized the endothelial cells. However, acetylcholine did not induce an endothelium-dependent hyperpolarization of the smooth muscle, while it relaxed the strip in an endothelium-dependent manner. This relaxation was abolished by an inhibitor of NO synthesis, nitro-L-arginine. Moreover, nitroglycerin relaxed the strips without changing the membrane potential of the smooth muscle cells. Injection of Lucifer yellow into the endothelial cells and the smooth muscle cells did not show heterocellular dye coupling. Furthermore, electron microscopy did not show gap junction plate at the myoendothelial junctions. We conclude that in the mouse main pulmonary artery, NO alone is responsible for the acetylcholine-induced endothelium-dependent vasodilatation, whereas the phenomenon called endothelium-derived hyperpolizing factor is not present. Therefore, caution should be taken when comparing different animal models to study pulmonary circulation and its reactivity.

Evidence for the involvement of the cyclooxygenase-metabolic pathway in diclofenac-induced inhibition of spontaneous contraction of rat portal vein smooth muscle cells

The effects of diclofenac, a cyclooxygenase (COX) inhibitor, were investigated on spontaneous phasic contractions of longitudinal preparations of the rat portal vein. Diclofenac produced a concentration-dependent decrease in the amplitude of these spontaneous phasic contractions. Diclofenac (30 microM) decreased the amplitude of the spontaneous phasic increase in the F340/F380 ratio of Fura PE3, an indicator of intracellular Ca2+ concentration. It also reduced the number of action potentials in each burst discharge without changing the resting membrane potential of longitudinal smooth muscle cells. The extent of the distribution of Lucifer Yellow injected into a smooth muscle cell was decreased in the presence of diclofenac (30 microM). Both AH6809, a prostanoid EP receptor antagonist, and SQ22536, an adenylate cyclase inhibitor, decreased the amplitude of the spontaneous contractions. On the other hand, neither ozagrel, a thromboxane synthase inhibitor, nor SQ29548, a prostanoid TP receptor antagonist, significantly affected spontaneous contractions. These results indicate that diclofenac inhibits the amplitude of spontaneous contractions of the rat portal vein through inhibition of electrical activity, which may be related to an inhibition of the cyclooxygenase pathway.

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.

Angiotensin-converting enzyme inhibition restores endothelial but not medial connexin expression in hypertensive rats

OBJECTIVE AND DESIGN

Remodelling in the media and decreases in connexin (Cx) expression and size of endothelial cells occur in the caudal artery of spontaneously hypertensive rats (SHR). The objective of this study was to determine whether similar changes are found in the aorta and whether effects in both aorta and caudal artery are present in the pre-hypertensive period or can be reversed by antihypertensive treatment.

METHODS AND RESULTS

In the aorta of SHR, there was no difference in endothelial cell size although Cxs 37 and 40 were decreased, compared with normotensive Wistar-Kyoto rats. Cxs 37 and 43 were also reduced in the media. These differences were not apparent in pre-hypertensive SHR. Inhibition of angiotensin-converting enzyme (ACE) in SHR decreased blood pressure and restored Cx expression in the endothelium of both aorta and caudal artery. The decreased endothelial cell size in the caudal artery or the reduced Cxs in the media of the aorta of SHR were unaffected by ACE inhibition.

CONCLUSION

We conclude that cellular coupling is reduced in the endothelium of arteries of SHR, but this can be restored by inhibition of the renin-angiotensin system. Decreased cellular coupling in the media or decreased endothelial size in SHR were not reversed by this antihypertensive treatment.

Connexins: gaps in our knowledge of vascular function

Gap junctions are common features in the vasculature, long thought to provide a pathway for cell-cell signaling. Emerging understanding of the gap-junctional proteins (connexins) and new tools for their investigation now offer the opportunity to explore the vital role that the gap junctions may play in cardiovascular homeostasis and pathophysiology.

Spreading dilatation in rat mesenteric arteries associated with calcium-independent endothelial cell hyperpolarization

Both ACh and levcromakalim evoke smooth muscle cell hyperpolarization and associated relaxation in rat mesenteric resistance arteries. We investigated if they could evoke conducted vasodilatation along isolated arteries, whether this reflected spreading hyperpolarization and the possible mechanism involved. Focal micropipette application of either ACh, to stimulate endothelial cell muscarinic receptors, or levcromakalim, to activate smooth muscle K(ATP) channels, each evoked a local dilatation (88 +/- 14%, n= 6 and 92 +/- 6% reversal of phenylephrine-induced tone, n= 11, respectively) that rapidly spread upstream (at 1.5 mm 46 +/- 19%, n= 6 and 57 +/- 13%, n= 9) to dilate the entire isolated artery. The local dilatation to ACh was associated with a rise in endothelial cell [Ca(2+)](i) (F/F(t = 0)= 1.22 +/- 0.33, n= 14) which did not spread beyond 0.5 mm (F/F(t = 0)= 1.01 +/- 0.01, n= 14), while the local dilatation to levcromakalim was not associated with any change in endothelial cell [Ca(2+)](i). In contrast, ACh and levcromakalim both stimulated local (12.7 +/- 1.2 mV, n= 10 and 13.5 +/- 4.7 mV, n= 10) and spreading (at 2 mm: 3.0 +/- 1.1 mV, n= 5 and 4.1 +/- 0.7 mV, n= 5) smooth muscle hyperpolarization. The spread of hyperpolarization could be prevented by cutting the artery, so was not due to a diffusible agent. Both the spreading dilatation and hyperpolarization were endothelium dependent. The injection of propidium iodide into either endothelial or smooth muscle cells revealed extensive dye coupling between the endothelial cells, but limited coupling between the smooth muscle cells. Some evidence for heterocellular spread of dye was also evident. Together, these data show that vasodilatation can spread over significant distances in mesenteric resistance arteries, and suggest this reflects an effective coupling between the endothelial cells to facilitate [Ca(2+)](i)-independent spread of hyperpolarization.

Heterogeneity in the distribution of vascular gap junctions and connexins: implications for function

1. Gap junctions, which are comprised of members of a family of membrane proteins called connexins (Cx), permit the transfer of electrical and chemical information between adjacent cells in a wide variety of tissues. The aim of the present study was to compare the expression of Cx37, 40 and 43 in the smooth muscle and endothelium of a large elastic artery and two smaller muscular arteries of the rat. Serial section electron microscopy was also used to determine the presence of pentalaminar gap junctions in the smooth muscle and the incidence of myoendothelial gap junctions between the smooth muscle and endothelial cells in muscular arteries of different size. 2. Using immunohistochemistry, Cx37, 40 and 43 were found in the endothelium of the aorta, caudal and basilar arteries, with Cx43 being the least abundant. Connexin 43 was readily observed throughout the muscle layers of the aorta, but was not detected in the media of the caudal or basilar arteries. Connexin 40 was not detected in the media of any of the arteries, while very fine punctate staining was observed with Cx37 antibodies in the media of the caudal and basilar arteries, but not in the aorta. 3. Real-time polymerase chain reaction showed that the expression of mRNA for Cx43 was 15-fold greater in the aorta than in the caudal artery of the rat. 4. At the ultrastructural level, small pentalaminar gap junctions (< 100 nm) were found between the fine processes of adjacent smooth muscle cells and also between the smooth muscle and endothelial cells. The incidence of myoendothelial gap junctions in the mesenteric vascular bed and in the caudal artery increased as vessel size decreased. 5. In summary, heterogeneity exists within the vascular system with regard to the distribution of gap junctions and their constituent Cx. Such variation will have important consequences for the coordination and propagation of vascular responses. In muscular arteries, in comparison with elastic arteries, Cx37 may be more important than Cx43 for cell coupling within the smooth muscle layers. The correlation between the incidence of myoendothelial gap junctions and the role of endothelium-derived hyperpolarizing factor, relative to nitric oxide, in vasodilatory responses suggests that myoendothelial gap junctions play an important physiological role in the regulation of vascular tone.

Decreased endothelial size and connexin expression in rat caudal arteries during hypertension

OBJECTIVES

Hypertension is accompanied by endothelial dysfunction. The present study has investigated endothelial cell morphology and connexin expression in the caudal artery of the rat during the development of hypertension.

METHODS

A significant increase in systolic blood pressure was detected from 9 weeks of age in spontaneously hypertensive male rats (SHR) compared to normotensive Wistar-Kyoto (WKY) rats, reaching a maximum by 11-12 weeks of age. Immunohistochemistry was used to quantify cell size and expression of connexins (Cxs) 37, 40 and 43 in the endothelium of prehypertensive (3-week-old) and hypertensive (12-week-old) rats.

RESULTS

At 12 weeks, the size of endothelial cells and the expression of all three Cxs per endothelial cell were significantly less in SHR than WKY rats. At 3 weeks, there was no significant difference in cell size nor in the expression of Cxs 37 or 43; however, expression of Cx40 was significantly lower in SHR than in WKY rats. Between 3 and 12 weeks in WKY rats, there was no change in endothelial cell size, nor in the expression of Cxs 37, 40 and 43. In SHR, both cell size and Cx expression per endothelial cell were significantly decreased during the same developmental period, with a significant decrease in the density of Cx40 plaques.

CONCLUSION

The development of hypertension in the SHR is accompanied by significant decreases in endothelial cell size and expression of Cx40, which may contribute to the endothelial dysfunction present in hypertension.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Intercellular electrical communication among smooth muscle and endothelial cells in guinea-pig mesenteric arterioles

1. Current clamp studies using two patch electrodes and morphological observations have been performed in guinea-pig mesenteric arterioles to evaluate intercellular electrical couplings. 2. In electron micrographs, preparations were found to have a single layer of smooth muscle cells. Typical gap junctions were readily observed between endothelial cells only. 3. While immunoreactivity to connexin 40 was strongly expressed on the membranes of endothelial cells only, that to connexin 43 was expressed on both smooth muscle and endothelial cell membranes. 4. Neurobiotin injected into a smooth muscle cell diffused into several neighbouring smooth muscle cells while that injected into an endothelial cell diffused into many endothelial cells. 5. Acetylcholine-induced hyperpolarizations were conducted from endothelial cells to smooth muscle cells with a relative amplitude of 80.1 %. Ba(2+)-induced action potentials were conducted in the opposite direction with a relative amplitude of 92.4 %. 6. An electrotonic potential produced in a smooth muscle cell by current injection diminished steeply with distance as it spread along the muscle layer, plateauing at distances beyond 25 microm. An electrotonic potential produced in an endothelial cell spread within the intima with virtually no reduction. Electrotonic potentials could conduct through myoendothelial couplings, which seemed to behave as ohmic resistors without rectification. 7. The coupling resistance between adjacent smooth muscle cells was estimated to be at least 90 MOhms and that between a smooth muscle cell and the whole endothelial layer to be 0.9 GOhms. 8. The results indicate that although the resistance of myoendothelial couplings is appreciable, the endothelium may be important as a low resistance path connecting many smooth muscle cells.

Connexin37, not Cx40 and Cx43, is induced in vascular smooth muscle cells during coronary arteriogenesis

W.-J. Cai, S. Koltai, E. Kocsis, D. Scholz, W. Schaper and J. Schaper. Connexin37, not Cx40 and Cx43, is Induced in Vascular Smooth Muscle Cells During Coronary Arteriogenesis. Journal of Molecular and Cellular Cardiology (2001) 33, 957-967. The hypothesis that an altered expression of gap junction (GJ) proteins, connexin37 (Cx37), Cx40 and Cx43 will contribute to adaptive arteriogenesis was tested in growing coronary collateral vessels (CV) of the dog heart by immunoconfocal microscopy and transmission electron microscopy (TEM). We found that: (1) in the normal coronary system Cx37 and Cx40 were only expressed in endothelial cells (EC) from artery to capillary; (2) during collateral growth Cx37 was significantly induced in smooth muscle cells (SMC) from small-large arteries to precapillary arterioles (Ø=15 microm), while Cx40 was still only present in EC; (3) both homogeneous and heterogeneous distribution of Cx37 was observed in normal vessels (NV) and growing vessels (GV); (4) in mature vessels (MV), Cx37 was downregulated, similar to NV; (5) dual immunostaining revealed an inverse correlation between expression of Cx37 and desmin in GV occurring prior to downregulation of alpha-smooth actin and calponin; (6) Cx43 was undetectable in any vascular cells, both in NV and GV; (7) GJ were not found in SMC by TEM. Our data for the first time show the profile of connexin expression in the coronary system and provide evidence for existence of GJ proteins in capillaries. It is a novel finding that an altered expression of Cx37 is characteristic of adaptive arteriogenesis in the dog heart and may be used as a marker of vascular growth. Induced Cx37 may be an early signal indicating that SMC are responding to haemodynamic changes, i.e. increased shear stress.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Diastolic (dys-)function and electrophysiology

Cross-talk between cardiac electrical and mechanical function is a bidirectional process: The origin and spread of electric excitation govern cardiac contraction and relaxation, while the mechanic environment provides feedback information to the heart's electric behavior. The latter tends to be unduly disregarded by the medical community. This article reviews experimental findings on the effects of diastolic mechanics on cardiac electrophysiology, and describes physiological correlates, clinical manifestations, and therapeutic utility of cardiac mechanic stimulation in humans.

Role of gap junctions and EETs in endothelium-dependent hyperpolarization of porcine coronary artery

1. The effects of endothelium-derived hyperpolarizing factor (EDHF: elicited using substance P or bradykinin) were compared with those of 11,12-EET in pig coronary artery. Smooth muscle cells were usually impaled with microelectrodes through the adventitial surface. 2. Substance P (100 nM) and 11,12-EET (11,12-epoxyeicosatrienoic acid; 3 microM) hyperpolarized endothelial cells in intact arteries. These actions were unaffected by 100 nM iberiotoxin but were abolished by charybdotoxin plus apamin (each 100 nM). 3. Substance P (100 nM) and bradykinin (30 nM) hyperpolarized intact artery smooth muscle; Substance P had no effect after endothelium removal. 11,12-EET hyperpolarized de-endothelialized vessels by 12.6+/-0.3 mV, an effect abolished by 100 nM iberiotoxin. 4. 11,12-EET hyperpolarized intact arteries by 18.6+/-0.8 mV, an action reduced by iberiotoxin, which was ineffective against substance P. Hyperpolarizations to 11, 12-EET and substance P were partially inhibited by 100 nM charybdotoxin and abolished by further addition of 100 nM apamin. 5. 30 microM barium plus 500 nM ouabain depolarized intact artery smooth muscle but responses to substance P and bradykinin were unchanged. 500 microM gap 27 markedly reduced hyperpolarizations to substance P and bradykinin which were abolished in the additional presence of barium plus ouabain. 6. Substance P-induced hyperpolarizations of smooth muscle cells immediately below the internal elastic lamina were unaffected by gap 27, even in the presence of barium plus ouabain. 7. In pig coronary artery, 11,12-EET is not EDHF. Smooth muscle hyperpolarizations attributed to 'EDHF' are initiated by endothelial cell hyperpolarization involving charybdotoxin- (but not iberiotoxin) and apamin-sensitive K(+) channels. This may spread electrotonically via myoendothelial gap junctions but the involvement of an unknown endothelial factor cannot be excluded.

Inverse relationship between connexin43 and desmin expression in cultured porcine aortic smooth muscle cells

Our previous work has shown that in vascular tissues the elastic medial regions express high levels of the gap junctional protein, connexin43, but low levels of desmin, while the muscular medial regions express low levels of connexin43 but high levels of desmin. It is uncertain, however, whether this regional difference at the tissue level extends down to the level of the individual cell, or reflects an averaged relationship of groups of cells of different connexin43 and desmin expression. The present study has addressed this question using cultured porcine aortic smooth muscle cells. Immunoconfocal microscopic analysis of single-labeled cells showed that while smooth muscle alpha-actin, calponin and vimentin were positively labeled in the majority of medial smooth muscle cells both in intact porcine aorta and corresponding cultured cells, desmin and connexin43 labeling was highly heterogeneous. In the cultured cells, 0.3-0.5% of cells were found to be desmin-positive, and quantitative analysis after double labeling for desmin and connexin43 revealed that the desmin-positive cells were smaller, and contained significantly lower numbers and smaller sizes of connexin43 gap-junctional spots than did desmin-negative cells. Our findings demonstrate that an inverse expression pattern of connexin43 and desmin holds true at the level of the individual cell. This suggests a close relationship between intrinsic phenotypic control and the regulation of connexin43 expression in the arterial smooth muscle cell.

Information Networks in the Arterial Wall

The main task of the arterial system is to secure an adequate supply of oxygen to organs. This fact implies the integration of multiple signals in the vascular wall. This review deals with the exchange of information between and among smooth muscle and endothelial cells through gap junctions in the vessel walls of arteries and arterioles.

Stretch-induced changes in heart rate and rhythm: clinical observations, experiments and mathematical models

Clinical and research data indicate that active and passive changes in the mechanical environment of the heart are capable of influencing both the initiation and the spread of cardiac excitation via pathways that are intrinsic to the heart. This direction of the cross-talk between cardiac electrical and mechanical activity is referred to as mechano-electric feedback (MEF). MEF is thought to be involved in the adjustment of heart rate to changes in mechanical load and would help to explain the precise beat-to-beat regulation of cardiac performance as it occurs even in the recently transplanted (and, thus, denervated) heart. Furthermore, there is clinical evidence that MEF may be involved in mechanical initiation of arrhythmias and fibrillation, as well as in the re-setting of disturbed heart rhythm by 'mechanical' first aid procedures. This review will outline the clinical relevance of cardiac MEF, describe cellular correlates to the responses observed in situ, and discuss the role that quantitative mathematical models may play in identifying the involvement of cardiac MEF in the regulation of heart rate and rhythm.

Identification of rhythmically active cells in guinea-pig stomach

1. When intracellular recordings were made from the antral region of guinea-pig stomach, cells with different patterns of electrical activity were detected. 2. One group of cells, slow-wave cells, generated slow waves which consisted of initial and secondary components. When filled with either Lucifer Yellow or neurobiotin, the cells identified as smooth muscle cells lying in the circular muscle layer. 3. A second group of cells, driving cells, generated large, rapidly rising, potential changes, driving potentials. They had small cell bodies with several processes. With neurobiotin, a network of cells was visualized that resembled c-kit positive interstitial cells of the myenteric region. 4. A third group of cells generated sequences of potential changes which resembled driving potentials but had smaller amplitudes and slow rates of rise. These cells resembled smooth muscle cells lying in the longitudinal muscle layer. 5. When simultaneous recordings were made from the driving and slow-wave cells, driving potentials and slow waves occurred synchronously. Current injections indicated that both cell types were part of a common electrical syncytium. 6. The initial component of slow waves persisted in low concentrations of caffeine, but the secondary component was abolished; higher concentrations shortened the duration of the residual initial component. Driving potentials continued in the presence of low concentrations of caffeine; moderate concentrations of caffeine shortened their duration. 7. Hence three different types of cells were distinguished on the basis of their electrical activity, their responses to caffeine and their structure. These were smooth muscle cells, lying in the longitudinal and circular layers, and interstitial cells in the myenteric region. The observations suggest that interstitial cells initiate slow waves.

Blockade by 18beta-glycyrrhetinic acid of intercellular electrical coupling in guinea-pig arterioles

1. Intercellular electrical communication between smooth muscle and endothelial cells was examined in guinea-pig mesenteric arterioles using the whole-cell patch-clamp method. The time course of the current required to impose a 10 mV voltage clamp step was used to determine the extent of electrical coupling between them. Currents recorded from both smooth muscle and endothelial cells relaxed in a multi-exponential manner, indicating the existence of electrical coupling between cells. 2. 18beta-Glycyrrhetinic acid, a gap junction blocker, quickly blocked electrical communication at 40 microM, while neither heptanol nor octanol did so at concentrations of up to 1 mM. 3. In the current clamp mode, repetitive spikes, induced by 10 mM Ba2+ solutions, could be recorded from both kinds of cells. After blocking gap junctions, spikes could only be recorded from the smooth muscle cell layer, indicating that they had been conducted through myoendothelial junctions. 4. In endothelial cells, acetylcholine (ACh, 3 microM) induced hyperpolarizing responses, which had two phases (an initial fast and a second slower phase) in the current clamp condition. This ACh response persisted in the presence of 18beta-glycyrrhetinic acid, although this compound seemed to make the membrane slightly leaky. 5. After blocking gap junctions, the membrane potential of a single cell in a multicellular preparation could be well clamped. Thus, 18beta-glycyrrhetinic acid may be useful in studying the function of both arteriolar smooth muscle and endothelial cells while they remain located within a multicellular preparation.

Hints of a functional connection between the neuropeptidergic innervation of arteriovenous anastomoses and the appearance of epithelioid cells in the rabbit ear

Peripheral blood flow can be regulated by specialized vessel segments, the arteriovenous anastomoses. Their wall consists of a relatively thick layer of smooth muscle cells and so-called epithelioid cells. The epithelioid cell is a specialized myogenic cell phenotype expressing nitric oxide synthase. We studied the innervation of the different segments of arteriovenous anastomoses in the rabbit ear using antisera against neuropeptide Y, tyrosine hydroxylase, calcitonin gene-related peptide and substance P, as well as neuron-specific enolase, calbindin D and neurotubulin. The participation was especially examined of neuropeptidergic innervation and a possible morphological connection to the occurrence of epithelioid cells and a paracrine function. The NADPH diaphorase reaction and alpha-smooth muscle actin immunoelectron microscopy served to distinguish epithelioid cells from smooth muscle cells. Using conventional fluorescence microscopy and confocal laser scanning microscopy, we found the most dense innervation pattern of pan-neuronal markers (neurotubulin, neuron-specific enolase), tyrosine hydroxylase-immunoreactive nerve fibres and neuropeptidergic nerve fibres (neuropeptide Y, calcitonin gene-related peptide, substance P) around the intermediate segment in arteriovenous anastomoses, whereas the venous segment was barely marked. Single nerve fibres penetrated into the medial layer and reached the epithelioid cells. Using immunoelectron microscopy, we found intercellular contacts between epithelioid cells, but not the gap junction protein connexin 43. Here, we report for the first time a correlation of the innervation pattern with epithelioid cell type in arteriovenous anastomoses. Our findings suggest that epithelioid cells of the arteriovenous anastomoses are controlled by a dense network of neuropeptidergic nerve fibres in functional connection to their paracrine role as a nitric oxide producer.

Restricted expression of the gap junctional protein connexin 43 in the arterial system of the rat

Connexin 43 (Cx43) has been reported to be expressed in vascular smooth muscle cells and endothelial cells. Evidence for possible variations in Cx43 distribution within different parts of the vascular system is limited. We have therefore investigated the expression of Cx43 in the endothelia and media of 11 vessels of different size and function in the rat, using immunofluorescence and confocal laser scanning microscopy. The results showed that punctate Cx43 staining was abundant in the endothelia and media of all of the 5 elastic arteries examined. In the media, the amount of Cx43 staining decreased as the size of the elastic arteries became smaller. In the 6 muscular arteries examined, 2 different patterns of Cx43 staining were observed. In the first type, Cx43 expression was high in the endothelium but virtually absent from the media. Mesenteric resistance, hepatic and tail arteries were examples. In the second type, Cx43 staining was absent from both the media and the endothelia. The coronary, basilar, and middle cerebral arteries showed this appearance. The results suggest that expression of Cx43 is largely restricted to elastic arteries in the arterial system of the rat. The lack of immunodetectable Cx43 from the media of all muscular arteries examined, and from the endothelia of some of these arteries, raises the possibility of significant differences in the form of expression of Cx43 in these vessels or the presence of other connexins.

Endothelium-independent relaxation and hyperpolarization to C-type natriuretic peptide in porcine coronary arteries

Endothelial cells produce C-type natriuretic peptide (CNP), which has been proposed as an endothelium-derived hyperpolarizing factor. In porcine coronary arteries, we investigated the vasodilatory effects of CNP and compared them with endothelium-dependent relaxations and hyperpolarizations to bradykinin. Isolated epicardial porcine coronary arteries were studied in organ chambers, and concentration-response curves to CNP and bradykinin were obtained. Membrane potential was measured in endothelial cells and smooth muscle of intact porcine coronary arteries during stimulation with CNP or bradykinin. In precontracted porcine coronary arteries with or without endothelium, CNP (10[-10]-10[-6] M) evoked relaxations (maximum, 42 +/- 4%) smaller than those evoked by bradykinin (100 +/- 1%), blunted in preparations contracted by KCl instead of U46619 (9,11-dideoxy-11a,9a-epoxymethano-prostaglandin F2alpha; p < 0.05) and unaffected by inhibition of NO synthase (NS). CNP evoked hyperpolarization of vascular smooth muscle of similar magnitude in endothelium-intact (-4.4 +/- 1 mV) and endothelium-denuded (-4.6 +/- 1 mV) porcine coronary arteries. Bradykinin (10[-10]-10[-6] M) evoked concentration-dependent relaxations in preparations with endothelium only. Although atrial natriuretic peptide-receptor antagonist HS-142-1 (25 microM) slightly reduced the sensitivity to bradykinin (log shift at IC50, twofold; p < 0.05), it had no effect on the maximal response to bradykinin. Inhibition of NO synthase partially attenuated, whereas high potassium chloride (30 mM) markedly inhibited relaxations to bradykinin (p < 0.05). Hyperpolarization to bradykinin was much more pronounced than that to CNP (-17 +/- 3 mV; p < 0.05 vs. CNP) and was observed in endothelium-intact preparations only and unaffected by HS-142-1. In conclusion, in contrast to bradykinin, CNP induces endothelium-independent and weaker relaxation and hyperpolarization of coronary artery vascular smooth muscle, suggesting that CNP is an unlikely mediator of endothelium-dependent hyperpolarization of porcine coronary arteries.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Differential regulation of gap junctions by proinflammatory mediators in vitro

The development of inflammation is an important component of host defense against infection. The cellular and molecular processes underlying inflammation are well-studied, and it is known that cells of the blood vessel wall, such as endothelial cells and smooth muscle cells, play pivotal roles. Additionally, a wide variety of proinflammatory mediators have been defined, which coordinate the multicellular processes of inflammation. Knowledge of the potential role of blood vessel gap junctional intercellular communication (GJIC) in coordinating the inflammation process, however, is limited. In this study, we report that bacterial lipopolysaccharide (LPS), as well as the proinflammatory cytokines TNF-alpha and IL-1beta, selectively inhibit human myoendothelial GJIC in vitro without affecting GJIC between the respective homologous cell populations. This finding may represent a physiologically relevant component of the inflammatory response to infection. The work also provides some of the first clear evidence suggesting that a single eukaryotic cell can differentially regulate its GJIC between homologous and heterologous cell types in a simultaneous manner.

Gap junctions in human umbilical cord endothelial cells contain multiple connexins

We investigated the expression pattern of gap junctional proteins (connexins, Cx) in situ and in vitro and their functional characteristics in cultured human umbilical vein endothelial cells (HUVEC) and cultured human umbilical artery endothelial cells (HUAEC). In both arteries and veins, Cx37, Cx40, and Cx43 could be detected in situ and in vitro (passages 2-4). Distribution patterns of Cx40 and Cx43 were homogeneous in situ but more heterogeneous in vitro. Cx37 is heterogeneously expressed both in situ and in vitro. Among most cells, no Cx37 staining could be detected; when present, it was found as bright spots between some clusters of cells. Cx40 was more abundant in cultured arterial endothelium than in cultured venous endothelium. Dye-coupling experiments with Lucifer yellow CH revealed extensive dye spread in HUVEC (15.2 +/- 0.4, mean +/- SE, n = 110) but was significantly restricted in HUAEC (9.8 +/- 0.3, n = 110). Electrophysiological gap junctional characteristics were determined in cultured HUVEC and HUAEC pairs by use of the dual voltage-clamp technique. In contrast to the dye-coupling experiments, mean macroscopic electrical conductance was significantly larger for HUAEC pairs (31.4 +/- 6.0 nS, n = 12) than for HUVEC pairs (16.6 +/- 2.8, n = 18). In HUVEC, we measured multiple single gap junctional channel conductances in the range of 19-75 pS. Interestingly, additional conductances of 80-200 pS were measured in HUAEC, possibly partially reflecting activity of channels formed of Cx40, which are more abundant in the cultured arterial endothelial cells.

Connections with connexins: the molecular basis of direct intercellular signaling

Adjacent cells share ions, second messengers and small metabolites through intercellular channels which are present in gap junctions. This type of intercellular communication permits coordinated cellular activity, a critical feature for organ homeostasis during development and adult life of multicellular organisms. Intercellular channels are structurally more complex than other ion channels, because a complete cell-to-cell channel spans two plasma membranes and results from the association of two half channels, or connexons, contributed separately by each of the two participating cells. Each connexon, in turn, is a multimeric assembly of protein subunits. The structural proteins comprising these channels, collectively called connexins, are members of a highly related multigene family consisting of at least 13 members. Since the cloning of the first connexin in 1986, considerable progress has been made in our understanding of the complex molecular switches that control the formation and permeability of intercellular channels. Analysis of the mechanisms of channel assembly has revealed the selectivity of inter-connexin interactions and uncovered novel characteristics of the channel permeability and gating behavior. Structure/function studies have begun to provide a molecular understanding of the significance of connexin diversity and demonstrated the unique regulation of connexins by tyrosine kinases and oncogenes. Finally, mutations in two connexin genes have been linked to human diseases. The development of more specific approaches (dominant negative mutants, knockouts, transgenes) to study the functional role of connexins in organ homeostasis is providing a new perception about the significance of connexin diversity and the regulation of intercellular communication.

Langhans cells of human arterial intima: uniform by stellate appearance but different by nature

The stellate cells in human arterial intima known as Langhans cells were investigated. Arterial specimens were obtained during carotid endarterectomy and aortic reconstruction and included atherosclerotic lesions as well as areas of the adjacent normal appearing arterial wall. Following immunohistochemical and electron microscopic analysis, most of the stellate cells were found to inhabit the elastic-hyperplastic layer of the intima in the normal arterial wall but in atherosclerotic lesions, stellate cells were distributed throughout all intimal layers. Immunohistochemical examination revealed that different types of intimal cells, including smooth muscle cells (HHF-35; smooth muscle alpha-actin +) and vascular dendritic cells (CD1a+, S-100+), exhibited a typical stellate appearance but the cell processes of macrophages (HAM56+, CD68+) were too short for macrophages to be considered as stellate. No other intimal cells formed processes which could be detected under immunohistochemical examination. In atherosclerotic lesions, some smooth muscle cells transforming to foam cells retained their stellate shape. Smooth muscle cells interacted with each other through gap junctions while other intimal cells including vascular dendritic cells contacted each other without forming any specialized structures. We conclude that Langhans cells comprise two histological types of intimal cells, namely, smooth muscle cells and vascular dendritic cells.

Endothelium-dependent vasodilators do not cause propagated intercellular Ca2+ waves in vascular endothelial monolayers

Local application of a number of vasoactive agents affects vasomotor tone not only downstream to the point of application but also upstream. The mechanism(s) of upstream propagation is unknown. In endothelial cell monolayers, mechanical stimulation of one cell leads to intercellular propagation of increases in endothelial cell (EC) [Ca2+]i. In this study, we tested whether increases in EC [Ca2+]i induced by the local application of the endothelium-dependent vasodilators ATP, bradykinin and acetylcholine could spread across the monolayer. We demonstrate that unlike the response seen to a mechanical stimulus, there was no significant propagation of increases in EC [Ca2+]i levels in response to localized application of these agents. These findings suggest that upstream vasodilation in response to endothelium-dependent vasodilators is not mediated by propagation of EC [Ca2+]i waves and suggest that other electrical or chemical signals are responsible.

Control of skeletal muscle blood flow during dynamic exercise: contribution of endothelium-derived nitric oxide

Traditional explanations for the hyperaemia which accompanies exercise have invoked the 'metabolic theory' of vasodilation, whereby contractile activity in the active muscle gives rise to metabolic by-products which dilate vessels bathed in interstitial fluid. Whilst metabolites with vasodilator properties have been identified, this theory does not adequately explain the magnitude of hyperaemia observed in active skeletal muscle, principally because large increases in flow are dependent on dilation of 'feed' arteries which lie outside the tissue parenchyma and are not subjected to changes in the interstitial milieu. Coordinated resistance vessel dilation during exercise is therefore dependent on a signal which 'ascends' from the microvessels to the feed arteries located upstream. Recent studies of ascending vasodilation have concentrated on the possible contribution of the endothelium, a monolayer of flattened squamous cells which lie at the interface between the circulating blood and vascular wall. These cells are uniquely positioned to respond to changes in rheological and humoral conditions within the cardiovascular system, and to transduce these changes into vasoactive signals which regulate blood flow, vascular tone and arterial pressure. Endothelial cells produce nitric oxide (NO), a rapidly diffusing labile substance which relaxes adjacent vascular smooth muscle. NO is released basally and contributes to the regulation of vascular tone by acting as a functional antagonist to sympathetic neural constriction. In addition, NO is spontaneously released in response to deformation of the endothelial cell membrane, indicating that changes in pulsatile flow and wall shear stress are likely physiological stimuli. Since the dilation of microvessels in response to exercise increases blood flow through the upstream feed arteries, which subsequently dilate, one explanation for ascending vasodilation is that NO release is stimulated by flow-induced shear stress. Evidence that NO contributes to ascending vasodilation is reviewed, along with studies which indicate that NO mediates exercise hyperaemia, that physical conditioning upregulates NO production and that NO controls blood flow by modifying other physiological mechanisms.

Gap junctional communication in primary culture of cells derived from human aortic intima

Intercellular communication via gap junctions plays an important role in the regulation and homeostasis. The presence of gap junctions and the efficiency of their function directly correlates with the degree of cell differentiation in a tissue. In the present study, gap junctional communication has been investigated in a primary culture of highly differentiated mesenchymal cells (subendothelial smooth muscle cells isolated from grossly normal and atherosclerotic areas of human aorta) and in poorly differentiated cells of mesenchymal origin (adult human skin fibroblasts as well as skin fibroblasts and aortic smooth muscle cells, derived from human fetus). The fluorescent dye transfer technique was used in this study. In cell cultures isolated from grossly normal and atherosclerotic aorta, the number of cells coupled via gap junctions increased with cell density and reached a plateau at a cell density of 50 to 70 cells/mm2. In cultures of normal aortic cells the number of coupled cells was 23.0 +/- 4.1 per injected cell and was significantly higher than in cultures of atherosclerotic cells (16.4 +/- 2.1, p < 0.05). Gap junctional communication between cells loaded with lipid inclusions was 2.4-fold lower than between cells free of excess intracellular lipids. In cultures of human skin fibroblasts the rate of intercellular communication was lower than in cultures of normal aortic cells and was comparable to that in cultures of atherosclerotic cells. There was practically no cell-to-cell communication in cultures of fetal cells. It is hypothesized that the reduced gap junctional communication in atherosclerotic human aorta is associated with the alterations in the degree of smooth muscle cell differentiation and impairs the function of the intima in atherosclerosis.

Intercellular metabolic coupling in canine colon musculature

Intercellular communication within the musculature of the canine colon was studied by examining the results of neurobiotin diffusion after injection of the tracer into smooth muscle cells at different locations within the muscle layer. Circular muscle at the submucosal surface, circular muscle adjacent to the myenteric plexus, and longitudinal muscle demonstrated different degrees of time-dependent tracer spread. At the submucosal surface, tracer spread was rapid, extensive, and unimpeded by connective tissue septa. At the myenteric side, tracer spread was also extensive but was much slower and confined to bundles of cells bordered by septa. In contrast to previous studies that suggest an absence of gap junctions at the myenteric side of the circular muscle, the neurobiotin spread indicates full metabolic coupling of all circular smooth muscle cells. Furthermore, in contrast to the belief that longitudinal muscle is completely devoid of gap junctions, tracer spread occurred between cells in this layer, although neurobiotin diffusion was very limited, nonuniform, and slow. In each area of the musculature studied, tracer spread was inhibited by octanol. When very long injection and wait times were implemented at the submucosal surface of the circular muscle, neurobiotin was observed to cross septa through the network of interstitial cells of Cajal, indicating that it is this network that provides communication between lamellae.

Effect of 5-hydroxytryptamine on the membrane potential of endothelial and smooth muscle cells in the pig coronary artery

1. Many endothelium-dependent vasodilators hyperpolarize the endothelial cells in blood vessels. It is not known whether these hyperpolarizations are linked to nitric oxide synthesis or to an endothelium-derived hyperpolarizing phenomenon, since most of the vasodilators release both factors. In this context, we first verified that the endothelium-dependent relaxations induced by 5-hydroxytryptamine (5-HT) on pig coronary arteries are due only to the activation of the nitric oxide pathway. Then we studied the effects of 5-HT on membrane potential of endothelial and smooth muscle cells. 2. In the absence of endothelium, 5-HT caused a concentration-dependent contraction of coronary artery strips. No change of the smooth muscle cell membrane potential was observed during contraction to 1 microM 5-HT. 3. In the presence of 1 microM ketanserin to suppress the contractile effect of 5-HT, 5-HT induced concentration-dependent relaxation of endothelium-intact strips precontracted by 10 microM prostaglandin F2 alpha (PGF2 alpha). These relaxations were suppressed by 1 microM NG-nitro-L-arginine, an inhibitor of nitric oxide synthesis, showing that they were produced predominantly by nitric oxide. 4. In the presence of 1 microM ketanserin, 1 microM 5-HT did not change the smooth muscle cell membrane potential of strips precontracted by either 10 microM PGF2 alpha or by 10 microM acetylcholine (ACh). In the same conditions, 1 microM 5-HT caused a weak 2.6 +/- 0.4 mV hyperpolarization, of the endothelial cells. 5. In conclusion, the fact that 5-HT did not change the membrane potential of smooth muscle cells and only weakly hyperpolarized the endothelial cells during relaxations, suggests that in both cell types no electrical events accompany activation of the nitric oxide pathway. This is in contrast to the hyperpolarizations observed in endothelial and smooth muscle cells when the endothelium-derived hyperpolarization factor (EDHF) pathway is activated.

Cell-to-cell communication coordinates blood flow control

The control of tissue blood flow is a dynamic process exemplified by the interaction among physical, chemical, and electrical events occurring within the vessel wall and between the vasculature and tissue parenchyma. The range of blood flow control achieved in vivo is illustrated by functional hyperemia in exercising skeletal muscle: maximal flow can exceed resting values by more than 50-fold. Blood flow control is integrated among many vessel segments, beginning with resistance arteries external to the muscle and encompassing the arteriolar network within the muscle. As metabolic demand increases, the locus of blood flow control shifts from distal arterioles, which control capillary perfusion and blood flow distribution within the tissue, to the proximal arterioles and resistance arteries, which control the total volume of flow into the muscle. A fundamental question centers on how this vasomotor activity is actually coordinated throughout the resistance network. The interaction within and among vascular segments can be explained by chemical and electrical signals to smooth muscle cells (SMCs) and endothelial cells (ECs) in response to changes in transmural pressure as well as luminal shear stress. Increasing pressure results in SMC contraction via the myogenic response. Increasing flow stimulates ECs to release autacoids (eg, nitric oxide), which relax SMCs. Pressure and flow thereby provide opposing mechanical stimuli that interact in the maintenance of vasomotor tone throughout the resistance network. Vasomotor signals are also conducted along arterioles through cell-to-cell coupling between ECs and SMCs, thereby coordinating vasomotor activity of cells within a branch and among branches.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of the membrane potential of endothelial and smooth muscle cells in the regulation of coronary blood flow

In the mammalian heart the supply of oxygen and energy-rich substrates through the coronary arterioles is continuously adapted to the variations of cardiac work. The coronary resistance arteries and the surrounding myocardium form a functional unit with multiple interactions between coronary endothelial cells, smooth muscle cells, perivascular nerves, and cardiac muscle cells. We describe the mechanisms underlying the electrical and chemical communication between the different cell types, the ionic channels contributing to the resting potential of endothelial and smooth muscle cells, and the mechanisms responsible for modulation of the resting potential. The main conclusion of our analysis is that the membrane potential of coronary endothelial and smooth muscle cells is one of the major determinants of coronary blood flow, and that modulation of the membrane potential provides a way to dilate or constrict coronary resistance arteries. It is proposed that the membrane potential of the myo-endothelial regulatory unit, i.e., of the endothelial cells and the underlying smooth muscle cells in the terminal arterioles, may function as an integrator of the numerous local and global vasodilator and constrictor signals that provide for the adaptation of coronary blood flow to the metabolic demands of the heart.