Functional properties of submucosal venules in the rat stomach

Stress-induced impairment of parasympathetic NO-mediated inhibition of sympathetic vasoconstriction in submucosal arteriole of rat rectum

In the gastrointestinal tract, nitrergic inhibition of the arteriolar contractility has not been demonstrated. Here, we explored whether neurally-released nitric oxide (NO) inhibits sympathetic vasoconstrictions in the rat rectal arterioles. Changes in sympathetic vasoconstrictions and their nitrergic modulation in rats exposed to water avoidance stress (WAS, 10 days, 1 h per day) were also examined. In rectal submucosal preparations, changes in arteriolar diameter were monitored using video microscopy. In control or sham-treated rats, electrical field stimulation (EFS)-induced sympathetic vasoconstrictions were increased by the neuronal nitric oxide synthase (nNOS) inhibitor L-NPA (1 μM) and diminished by the cyclic guanosine monophosphate-specific phosphodiesterase 5 (PDE5) inhibitor tadalafil (10 nM). In phenylephrine-constricted, guanethidine-treated arterioles, EFS-induced vasodilatations were inhibited by the calcitonin gene-related peptide (CGRP) receptor antagonist BIBN-4096 (1 μM) but not L-NPA. Perivascular nNOS-immunoreactive nitrergic fibres co-expressing the parasympathetic marker vesicular acetylcholine transporter (VAChT) were intermingled with tyrosine hydroxylase (TH)-immunoreactive sympathetic fibres expressing soluble guanylate cyclase (sGC), a receptor for NO. In WAS rats in which augmented sympathetic vasoconstrictions were developed, L-NPA failed to further increase the vasoconstrictions, while tadalafil-induced inhibition of the vasoconstrictions was attenuated. Phenylephrine- or α,β-methylene ATP-induced vasoconstrictions and acetylcholine-induced vasodilatations were unaltered by WAS. Thus, in arterioles of the rat rectal submucosa, NO released from parasympathetic nerves appears to inhibit sympathetic vasoconstrictions presumably by reducing sympathetic transmitter release. In WAS rats, sympathetic vasoconstrictions are augmented at least partly due to the diminished pre-junctional nitrergic inhibition of transmitter release without changing α-adrenoceptor or P2X-purinoctor mediated vasoconstriction and endothelium-dependent vasodilatation.

Synchrony of spontaneous Ca2+ activity in microvascular mural cells

Spontaneous rhythmic constrictions known as vasomotion are developed in several microvascular beds in vivo. Vasomotion in arterioles is considered to facilitate blood flow, while venular vasomotion would facilitate tissue metabolite drainage. Mechanisms underlying vasomotion periodically generate synchronous Ca²⁺ transients in vascular smooth muscle cells (VSMCs). In visceral organs, mural cells (pericytes and VSMCs) in arterioles, capillaries and venules exhibit synchronous spontaneous Ca²⁺ transients. Since sympathetic regulation is rather limited in the intra-organ microvessels, spontaneous activity of mural cells may play an essential role in maintaining tissue perfusion. Synchronous spontaneous Ca²⁺ transients in precapillary arterioles (PCAs)/capillaries appear to propagate to upstream arterioles to drive their vasomotion, while venules develop their own synchronous Ca²⁺ transients and associated vasomotion. Spontaneous Ca²⁺ transients of mural cells primarily arise from IP₃ and/or ryanodine receptor-mediated Ca²⁺ release from sarcoendoplasmic reticulum (SR/ER) Ca²⁺ stores. The resultant opening of Ca²⁺-activated Cl^- channels (CaCCs) causes a membrane depolarisation that triggers Ca²⁺ influx via T-type and/or L-type voltage-dependent Ca²⁺ channels (VDCCs). Mural cells are electrically coupled with each other via gap junctions, and thus allow the sequential spread of CaCC or VDCC-dependent depolarisations to develop the synchrony of Ca²⁺ transients within their network. Importantly, the synchrony of spontaneous Ca²⁺ transients also requires a certain range of the resting membrane potential that is maintained by the opening of K_v7 voltage-dependent K⁺ (K_v7) and inward rectifier K⁺ (K_ir) channels. Thus, a depolarised membrane would evoke asynchronous, 'premature' spontaneous Ca²⁺ transients, while a hyperpolarised membrane prevents any spontaneous activity.

Role of Pericytes in the Initiation and Propagation of Spontaneous Activity in the Microvasculature

The microvasculature is composed of arterioles, capillaries and venules. Spontaneous arteriolar constrictions reduce effective vascular resistance to enhance tissue perfusion, while spontaneous venular constrictions facilitate the drainage of tissue metabolites by pumping blood. In the venules of visceral organs, mural cells, i.e. smooth muscle cells (SMCs) or pericytes, periodically generate spontaneous phasic constrictions, Ca²⁺ transients and transient depolarisations. These events arise from spontaneous Ca²⁺ release from the sarco-endoplasmic reticulum (SR/ER) and the subsequent opening of Ca²⁺-activated chloride channels (CaCCs). CaCC-dependent depolarisation further activates L-type voltage-dependent Ca²⁺ channels (LVDCCs) that play a critical role in maintaining the synchrony amongst mural cells. Mural cells in arterioles or capillaries are also capable of developing spontaneous activity. Non-contractile capillary pericytes generate spontaneous Ca²⁺ transients primarily relying on SR/ER Ca²⁺ release. Synchrony amongst capillary pericytes depends on gap junction-mediated spread of depolarisations resulting from the opening of either CaCCs or T-type VDCCs (TVDCCs) in a microvascular bed-dependent manner. The propagation of capillary Ca²⁺ transients into arterioles requires the opening of either L- or TVDCCs again depending on the microvascular bed. Since the blockade of gap junctions or CaCCs prevents spontaneous Ca²⁺ transients in arterioles and venules but not capillaries, capillary pericytes appear to play a primary role in generating spontaneous activity of the microvasculature unit. Pericytes in capillaries where the interchange of substances between tissues and the circulation takes place may provide the fundamental drive for upstream arterioles and downstream venules so that the microvasculature network functions as an integrated unit.

Ca2+ Signalling in Pericytes

Microcirculation is the generic name for the finest level of the circulatory system and consists of arteriolar and venular networks located upstream and downstream of capillaries, respectively. Anatomically arterioles are surrounded by a monolayer of spindle-shaped smooth muscle cells (myocytes), while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are surrounded by a monolayer of morphologically different perivascular cells (pericytes). Pericytes are essential components of the microvascular vessel wall. Wrapped around endothelial cells, they occupy a strategic position at the interface between the circulating blood and the interstitial space. There are physiological differences in the responses of pericytes and myocytes to vasoactive molecules, which suggest that these two types of vascular cells could have different functional roles in the regulation of local blood flow within the same microvascular bed. Also, pericytes may play different roles in different microcirculatory beds to meet the characteristics of individual organs. Contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca²⁺concentration. In this chapter, we attempt to summarize the results in the field of Ca²⁺ signalling in pericytes especially in light of their contractile roles in different tissues and organs. We investigate the literature and describe our results regarding sources of Ca²⁺, relative importance and mechanisms of Ca²⁺ release and Ca²⁺ entry in control of the spatio-temporal characteristics of the Ca²⁺ signals in pericytes, where possible Ca²⁺ signalling and contractile responses in pericytes are compared to those of myocytes.

Role of K+ channels in maintaining the synchrony of spontaneous Ca2+ transients in the mural cells of rat rectal submucosal arterioles

Mural cells in precapillary arterioles (PCAs) generate spontaneous Ca²⁺ transients primarily arising from the periodic release of Ca²⁺ from sarcoendoplasmic reticulum (SR/ER). The Ca²⁺ release induces Ca²⁺-activated chloride channel (CaCC)-dependent depolarisations that spread to neighbouring mural cells to develop the synchrony of their Ca²⁺ transients. Here, we explored the roles of K⁺ channels in maintaining the synchrony of spontaneous Ca²⁺ transients. Intracellular Ca²⁺ dynamics in mural cells were visualised by Cal-520 fluorescence Ca²⁺ imaging in the submucosal PCAs of rat rectum. Increasing extracellular K⁺ concentration ([K⁺]_o) from 5.9 to 29.7 mM converted synchronous spontaneous Ca²⁺ transients into asynchronous, high-frequency Ca²⁺ transients. Similarly, the blockade of inward rectifier K⁺ (K_ir) channels with Ba²⁺ (50 μM) or K_v7 voltage-dependent K⁺ (K_v7) channels with XE 991 (10 μM) disrupted the synchrony of spontaneous Ca²⁺ transients, while the blockers for large-, intermediate- or small-conductance Ca²⁺-activated K⁺ channels had no effect. K_ir2.1 immunoreactivity was detected in the arteriolar endothelium but not mural cells. In the PCAs that had been pretreated with XE 991 or Ba²⁺, nifedipine (1 μM) attenuated the asynchronous Ca²⁺ transients but failed to restore their synchrony. In contrast, levcromakalim, an ATP-sensitive K⁺ channel opener, restored the synchronous Ca²⁺ transients. Thus, constitutively active K_v7 and K_ir channels appear to be involved in maintaining the relatively hyperpolarised membrane of mural cells. The hyperpolarised membrane prevents depolarisation-induced 'premature' Ca²⁺ transients to ensure sufficient SR/ER Ca²⁺ refilling that is required for regenerative Ca²⁺ release resulting in synchronous Ca²⁺ transients amongst the mural cells.

Venous Vasomotion

Veins exhibit spontaneous contractile activity, a phenomenon generally termed vasomotion. This is mediated by spontaneous rhythmical contractions of mural cells (i.e. smooth muscle cells (SMCs) or pericytes) in the wall of the vessel. Vasomotion occurs through interconnected oscillators within and between mural cells, entraining their cycles. Pharmacological studies indicate that a key oscillator underlying vasomotion is the rhythmical calcium ion (Ca²⁺) release-refill cycle of Ca²⁺ stores. This occurs through opening of inositol 1,4,5-trisphosphate receptor (IP₃R)- and/or ryanodine receptor (RyR)-operated Ca²⁺ release channels in the sarcoplasmic/endoplasmic (SR/ER) reticulum and refilling by the SR/ER reticulum Ca²⁺ATPase (SERCA). Released Ca²⁺ from stores near the plasma membrane diffuse through the cytosol to open Ca²⁺-activated chloride (Cl^-) channels, this generating inward current through an efflux of Cl^-. The resultant depolarisation leads to the opening of voltage-dependent Ca²⁺ channels and possibly increased production of IP₃, which through Ca²⁺-induced Ca²⁺ release (CICR) of IP₃Rs and/or RyRs and IP₃R-mediated Ca²⁺ release provide a means by which store oscillators entrain their activity. Intercellular entrainment normally involves current flow through gap junctions that interconnect mural cells and in many cases this is aided by additional connectivity through the endothelium. Once entrainment has occurred the substantial Ca²⁺ entry that results from the near-synchronous depolarisations leads to rhythmical contractions of the mural cells, this often leading to vessel constriction. The basis for venous/venular vasomotion has yet to be fully delineated but could improve both venous drainage and capillary/venular absorption of blood plasma-associated fluids.

Role of capillary pericytes in the integration of spontaneous Ca2+ transients in the suburothelial microvasculature in situ of the mouse bladder

KEY POINTS

In the bladder suburothelial microvasculature, pericytes in different microvascular segments develop spontaneous Ca²⁺ transients with or without associated constrictions. Spontaneous Ca²⁺ transients in pericytes of all microvascular segments primarily rely on the cycles of Ca²⁺ uptake and release by the sarco- and endoplasmic reticulum. The synchrony of spontaneous Ca²⁺ transients in capillary pericytes exclusively relies on the spread of depolarizations resulting from the opening of Ca²⁺ -activated chloride channels (CaCCs) via gap junctions. CaCC-dependent depolarizations further activate L-type voltage-dependent Ca²⁺ channels as required for the synchrony of Ca²⁺ transients in pericytes of pre-capillary arterioles, post-capillary venules and venules. Capillary pericytes may drive spontaneous Ca²⁺ transients in pericytes within the suburothelial microvascular network by sending CaCC-dependent depolarizations via gap junctions.

ABSTRACT

Mural cells in the microvasculature of visceral organs develop spontaneous Ca²⁺ transients. However, the mechanisms underlying the integration of these Ca²⁺ transients within a microvascular unit remain to be clarified. In the present study, the origin of spontaneous Ca²⁺ transients and their propagation in the bladder suburothelial microvasculature were explored. Cal-520 fluorescence Ca²⁺ imaging and immunohistochemistry were carried out on mural cells using mice expressing red fluorescent protein (DsRed) under control of the NG2 promotor. NG2(+) pericytes in both pre-capillary arterioles (PCAs) and capillaries developed synchronous spontaneous Ca²⁺ transients. By contrast, although NG2-DsRed also labelled arteriolar smooth muscle cells, these cells remained quiescent. Both NG2(+) pericytes in post-capillary venules (PCVs) and NG2(-) venular pericytes exhibited propagated Ca²⁺ transients. L-type voltage-dependent Ca²⁺ channel (LVDCC) blockade with nifedipine prevented Ca²⁺ transients or disrupted their synchrony in PCA, PCV and venular pericytes without dis-synchronizing Ca²⁺ transients in capillary pericytes. Blockade of gap junctions with carbenoxolone or Ca²⁺ -activated chloride channels (CaCCs) with 4,4'-diisothiocyanato-2,2'-stilbenedisulphonic acid disodium salt prevented Ca²⁺ transients in PCA and venular pericytes and disrupted the synchrony of Ca²⁺ transients in capillary and PCV pericytes. Spontaneous Ca²⁺ transients in pericytes of all microvascular segments were abolished or suppressed by cyclopiazonic acid, caffeine or tetracaine. The synchrony of Ca²⁺ transients in capillary pericytes arising from spontaneous Ca²⁺ release from the sarco- and endoplasmic reticulum appears to rely exclusively on CaCC activation, whereas subsequent LVDCC activation is required for the synchrony of Ca²⁺ transients in pericytes of other microvascular segments. Capillary pericytes may drive spontaneous activity in the suburothelial microvascular unit to facilitate capillary perfusion.

Properties of synchronous spontaneous Ca2+ transients in the mural cells of rat rectal arterioles

Synchrony of spontaneous Ca²⁺ transients among venular mural cells (smooth muscle cells and pericytes) in visceral organs relies on the intercellular spread of L-type voltage-dependent Ca²⁺ channel (LVDCC)-dependent depolarisations. However, the mechanisms underlying the synchrony of spontaneous Ca²⁺ transients between arteriolar mural cells are less understood. The spontaneous intracellular Ca²⁺ dynamics of arteriolar mural cells in the rat rectal submucosa were visualised by Cal-520 Ca²⁺ imaging to analyse their synchrony. The mural cells in fine arterioles that had a rounded cell body with several extended processes developed spontaneous 'synchronous' Ca²⁺ transients arising from Ca²⁺ released from sarcoendoplasmic reticulum Ca²⁺ stores. Gap junction blockers (3 μM carbenoxolone, 10 μM 18β-glycyrrhetinic acid), a Ca²⁺-activated Cl^- channel (CaCC) blocker (100 μM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) or lowering extracellular Cl^- concentration (from 134.4 to 12.4 mM) disrupted the synchrony of Ca²⁺ transients between arteriolar mural cells. Blockers of T-type voltage-dependent Ca²⁺ channels (TVDCCs, 1 μM mibefradil or ML218) or LVDCCs (1 μM nifedipine) reduced the Ca²⁺ transient frequency or their area under curve (AUC), respectively. However, neither TVDCC nor LVDCC blockers disrupted the synchrony of Ca²⁺ transients among arteriolar mural cells. This is in contrast with rectal venules in which nifedipine disrupted the synchrony of spontaneous Ca²⁺ transients. Thus, spontaneous transient depolarisations arising from the opening of CaCCs may effectively spread to neighbouring arteriolar mural cells via gap junctions to maintain the Ca²⁺ transient synchrony. Activation of TVDCCs appears to accelerate spontaneous Ca²⁺ transients, while LVDCCs predominantly contribute to the duration of Ca²⁺ transients.

Spontaneous activity in the microvasculature of visceral organs: role of pericytes and voltage-dependent Ca(2+) channels

The microvasculature plays a primary role in the interchange of substances between tissues and the circulation. In visceral organs that undergo considerable distension upon filling, the microvasculature appears to display intrinsic contractile properties to maintain their flow. Submucosal venules in the bladder or gastrointestinal tract generate rhythmic spontaneous phasic constrictions and associated Ca(2+) transients. These events are initiated within either venular pericytes or smooth muscle cells (SMCs) arising from spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) and the opening of Ca(2+) -activated chloride channels (CaCCs) that trigger Ca(2+) influx through L-type voltage-dependent Ca(2+) channels (VDCCs). L-type VDCCs also play a critical role in maintaining synchrony within the contractile mural cells. In the stomach myenteric layer, spontaneous Ca(2+) transients originating in capillary pericytes appear to spread to their neighbouring arteriolar SMCs. Capillary Ca(2+) transients primarily rely on SR Ca(2+) release, but also require Ca(2+) influx through T-type VDCCs for their synchrony. The opening of T-type VDCCs also contribute to the propagation of Ca(2+) transients into SMCs. In visceral microvasculature, pericytes act as either spontaneously active contractile machinery of the venules or as pacemaker cells generating synchronous Ca(2+) transients that drive spontaneous contractions in upstream arterioles. Thus pericytes play different roles in different vascular beds in a manner that may well depend on the selective expression of T-type and L-type Ca(2+) channels.

Mechanisms underlying spontaneous constrictions of postcapillary venules in the rat stomach

Postcapillary venules (PCVs) play a critical role in regulating capillary hydrostatic pressure, but their contractile mechanisms are not well understood. We examined the properties of spontaneous vasomotion and corresponding Ca(2+) transients in gastric PCV. In the rat gastric submucosa, changes in PCV diameter and intracellular Ca(2+) dynamics were visualised by video tracking system and fluorescent Ca(2+) imaging, respectively, while PCV morphology was examined by immunohistochemistry. Stellate-shaped PCV mural cells expressing α-smooth muscle actin exhibited synchronised spontaneous Ca(2+) transients to develop vasomotion which was abolished by nifedipine (1 μM), cyclopiazonic acid (10 μM), or Ca(2+)-activated Cl(-) channel inhibitors (100 μM niflumic acid, 1 μM T16Ainh-A01). A gap junction blocker (3 μM carbenoxolone) disrupted the synchrony of spontaneous Ca(2+) transients amongst PCV mural cells and attenuated spontaneous vasomotion. Low chloride solution ([Cl(-)]0 = 12.4 mM) also disrupted the synchrony of spontaneous Ca(2+) transients and abolished vasomotion. Na(+)-K(+)-Cl(-) co-transporter inhibitors (10 μM bumetanide, 30 μM furosemide) suppressed spontaneous Ca(2+) transients and vasoconstrictions. A phosphodiesterase type 5 (PDE5) inhibitor (1 μM tadalafil) disrupted the spontaneous Ca(2+) transient synchrony and abolished vasomotion in a nitric oxide (NO)-dependent manner. Thus, gastric PCVs exhibit spontaneous vasomotion, resulting from synchronised spontaneous Ca(2+) transients within a network of stellate-shaped PCV mural cells. An active Cl(-) accumulation partly via Na(+)-K(+)-Cl(-) co-transport appears to be fundamental in maintaining depolarisation upon the opening of Ca(2+)-activated Cl(-) channels that triggers Ca(2+) influx via voltage-dependent L-type Ca(2+) channels. Basal PDE5 activity may continuously counteract vaso-relaxing effects of endothelial NO to maintain spontaneous vasomotion.

Pacemaker role of pericytes in generating synchronized spontaneous Ca2+ transients in the myenteric microvasculature of the guinea-pig gastric antrum

Properties of spontaneous Ca(2+) transients in the myenteric microvasculature of the guinea-pig stomach were investigated. Specifically, we explored the spatio-temporal origin of Ca(2+) transients and the role of voltage-dependent Ca(2+) channels (VDCCs) in their intercellular synchrony using fluorescence Ca(2+) imaging and immunohistochemistry. The microvasculature generated spontaneous Ca(2+) transients that were independent of both Ca(2+) transients in interstitial cells of Cajal (ICC) and neural activity. Spontaneous Ca(2+) transients were highly synchronous along the length of microvasculature, and appeared to be initiated in pericytes and spread to arteriolar smooth muscle cells (SMCs). In most cases, the generation or synchrony of Ca(2+) transients was not affected by blockers of L-type VDCCs. In nifedipine-treated preparations, synchronous spontaneous Ca(2+) transients were readily blocked by Ni(2+), mibefradil or ML216, blockers for T-type VDCCs. These blockers also suppressed the known T-type VDCC dependent component of ICC Ca(2+) transients or slow waves. Spontaneous Ca(2+) transients were also suppressed by caffeine, tetracaine or cyclopiazonic acid (CPA). After the blockade of both L- and T-type VDCCs, asynchronous Ca(2+) transients were generated in pericytes on precapillary arterioles and/or capillaries but not in arteriolar SMCs, and were abolished by CPA or nominally Ca(2+) free solution. Together these data indicate that pericytes in the myenteric microvasculature may act as the origin of synchronous spontaneous Ca(2+) transients. Pericyte Ca(2+) transients arise from Ca(2+) release from the sarco-endoplasmic reticulum and the opening of T-type Ca(2+) VDCCs is required for their synchrony and propagation to arteriolar SMCs.