The conducted vasomotor response and the principles of electrical communication in resistance arteries

Abstract

Biological tissues are fed by arterial networks whose task is to set blood flow delivery in accordance with energetic demand. Coordinating vasomotor activity among hundreds of neighboring segments is an essential process, one dependent upon electrical information spreading among smooth muscle and endothelial cells. The "conducted vasomotor response" is a functional expression of electrical spread, and it is this process that lies at the heart of this critical review. Written in a narrative format, this review first highlights historical manuscripts and then characterizes the conducted response across a range of preparations. Trends are highlighted and used to guide subsequent sections, focused on cellular foundations, biophysical underpinnings, and regulation in health and disease. Key information has been tabulated; figures reinforce grounding concepts and reveal a framework within which theoretical and experimental work can be rationalized. This summative review highlights that despite 30 years of concerted experimentation, key aspects of the conducted response remain ill defined. Of note is the need to rationalize the regulation and deterioration of conduction in pathobiological settings. New quantitative tools, along with transgenic technology, are discussed as a means of propelling this investigative field forward.

Keywords: arteries; blood flow; endothelial cells; gap junctions; ion channels; smooth muscle cells.

Graphical abstract

FIGURE 1.

A: textbook explanation of blood flow autoregulation with constrictor influences (blood pressure and sympathetic nerve activity) setting the arterial tone while vasodilatory metabolites tune perfusion according to energetic demand. B: in living tissues, vasodilatory metabolites are discretely rather than homogeneously presented to arterial networks, the latter being represented as resistors linked in series. If metabolites only dilate that portion of the network they directly contact, blood flow cannot increase, as the upstream resistors are limiting (right). In contrast, if metabolites produce a dilatory signal that spreads, blood flow magnitude will markedly rise.

FIGURE 2.

Typical protocols used to study conducted vasomotor responses. Vasoactive agents are pressure ejected via a pipette onto a small region of a resistance artery in vivo (top and middle) and in vitro (bottom) while vasomotor and electrical response are monitored at sites close to or distal from the site of application. Conduction experiments characteristically employ agents that dilate/hyperpolarize or constrict/depolarize vascular cells. V_M, membrane potential.

FIGURE 3.

Illustration of the 4 manuscripts that defined the origin and foundation of conducted vasomotor response. These microcirculatory manuscripts span a 60-year time frame from August Krogh in 1921 (from Ref. , with permission from Journal of Physiology) to Hilton in 1959 (from Ref. , with permission from Journal of Physiology), Hirst and Neild in 1978 (from Ref. , with permission from Journal of Physiology), and, finally, Segal and Duling in 1986 (from Ref. , with permission from Science). See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 4.

Illustrative diagram highlighting how capillary flow is tuned to tissue metabolic demand. Increasing tissue metabolic demand results in a drop of Po₂ that triggers 1) metabolite production (adenosine) and release (e.g., K⁺) in skeletal muscle or brain or 2) ATP releases from red blood cells (RBCs). These and other presumptive stimuli activate capillary K⁺ channels directly or through G_s-coupled receptors (GCPRs), the resulting hyperpolarization spreading upstream into transitional vessels and arterioles. The ensuing dilation drives a rise in capillary RBC supply. Ado, adenosine; ATP, adenosine trisphosphate; Cx, connexin; K_ATP, ATP-sensitive K⁺ channels; K_IR, inwardly rectifying K⁺ channels.

FIGURE 5.

Conducted hyperpolarization and dilation in 2 vascular preparations. A: in the hamster cheek pouch preparation, vasomotor and electrical measurements were simultaneously attained with the stimulus pipette positioned near or distal to the measurement site (top). Discrete application of acetylcholine (middle) or high KCl (bottom) elicited local and conducted hyperpolarization/dilation and depolarization/constriction, respectively. Smooth muscle and endothelial electrical responses paralleled one another. Insets: photomicrographs illustrate how Lucifer yellow dye in the recording pipette identifies endothelial and smooth muscle cells; note the labeling pattern running parallel and perpendicular to the vessel axis. Image from Ref. , with permission from American Physiological Society. B: a pressurized myogenically responsive, penetrating arteriole-capillary preparation was isolated from mouse brain slices (top). In the photomicrograph below, a K⁺ stimulus (10 or 60 mM) was then applied via pipette to capillaries to elicit a conducted vasomotor response as measured in 2 zones in the upstream penetrating arteriole. To round out this work, conducted hyperpolarization and dilation were measured simultaneously as 10 mM K⁺ was applied onto a capillary. On right, whole cell electrophysiology performed on endothelial cells isolated from capillaries (photomicrograph) confirmed the presence of the Ba²⁺-sensitive inwardly rectifying K⁺ (K_IR) current, the conductance activated by elevated K⁺. Image from Ref. , with permission from Nature Neuroscience. C: a live mouse cranial window preparation and 2-photon laser-scanning microscopy were subsequently used to extend the preceding isolated vessel/cell work. With a pipette (solid red lines) backfilled with 10 mM K⁺ brain capillaries were stimulated (dashed red circle) and the upstream vasomotor response (penetrating and precapillary arterioles) was monitored. Note the dilation of the upstream penetrating arteriole. Note the white line of identity that connects the capillaries to the precapillary-penetrating arteriole. FITC-dextran was injected a priori into the mouse to aid vessel visualization. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 6.

Neurovascular coupling along the cerebral arterial network. A penetrating arteriole branches from a surface pial artery and ramifies through the brain tissue into smaller arterioles that transition into capillaries. Astrocytic endfoot encases much of the cerebral microvasculature, contributing to the blood-brain barrier. Larger arterioles are endowed with perivascular nerves, whereas smaller arterioles and capillaries are in sporadic contact with neural processes. TABLE 4 highlights the stimulus origin and target of vasomotor stimuli.

FIGURE 7.

Structural properties of myoendothelial projections in resistance arteries. A and B: electron photomicrographs (Aa, Ba, and Bc) and 3-dimensional (3-D) reconstructions (Ab, Bb, and Bd) of myoendothelial projections from hamster retractor muscle (from Ref. , with permission from American Physiological Society) and hamster mesenteric arteries (from Ref. , with permission from Microcirculation). Note the intimate contact between the endothelial projection and the overlying smooth muscle cell (SMC) as it penetrates through the internal elastic lamina (IEL). Intracellular organelles, including the endoplasmic reticulum (ER; denoted by arrowheads in Aa), the mitochondria (Mito), the caveolae, and the nucleus can be observed. Ac and Ad and Ae and Af are electron photomicrographs of immunogold labeling for inositol triphosphate receptor (IP₃R)1 and intermediate-conduction Ca²⁺-activated K⁺ channels (IK_Ca), respectively (179). The presence of both IP₃R1 and IK_Ca is notable in the endothelial projections. Ag and Ah and Ai and Aj are immunohistochemical photomicrographs of hamster retractor muscle feed arteries labeled for IP₃R1 and IK_Ca, respectively. Hoechst 33342 was used to label nuclei yellow; the red fluorescence channel was decreased, so concentrated areas of IP₃R1 and IK_Ca labeling could be observed between the 2 sets of nuclei (endothelial and smooth muscle) that penetrated the IEL layer (green) (179). EC, endothelial cells. C: immunohistochemical analysis of connexin (Cx)40 expression at myoendothelial projections from mouse cremaster and mesenteric arteries (from Ref. , with permission from Journal of Physiology). In each dataset, IEL holes (denoted by arrowheads) are visible as dark spots through the connective tissue layer. Cx40 maps overtop of the IEL holes consistent with their expression at the tip of endothelial projections. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 8.

Illustrative diagram of myoendothelial projections. Aspects of endothelial cells (EC) project through internal elastic lamina (IEL), coming in contact with smooth muscle cells (SMC). Myoendothelial projections are rich in gap junctions, ion channels, and an endoplasmic reticulum (ER) signaling complex that generates localized Ca²⁺ events (i.e., wavelets and pulsars). A complete list of channels, receptors, and signaling proteins observed in myoendothelial projects can be found in TABLE 7. Cx, connexin; IK_Ca, intermediate-conduction Ca²⁺-activated K⁺ channels; IP₃R, inositol triphosphate receptor; K_IR, inwardly rectifying K⁺ channels; SK_Ca, small-conduction Ca²⁺-activated K⁺ channels; SMC, smooth muscle cells; TRPV, vanilloid transient receptor potential channels.

FIGURE 9.

The conducted response and the evolution of computational modeling. A, top: a simplified virtual artery is constructed of 1 layer of endothelium and smooth muscle (from Ref. , with permission from Journal of Physiology). Each arterial segment was comprised of endothelial (EC) and smooth muscle (SMC) cells with defined dimensions, gap junctional coupling, and ionic conductance. Neighboring smooth muscle cells were coupled, neighboring endothelial cells were coupled, and every smooth muscle cell was randomly coupled to 2 endothelial cells (red dot). The equivalent circuit representation notes that each cell is treated as a capacitor coupled in parallel with a nonlinear resistor representing ionic conductance (C) and ohmic resistors (R) representing gap junctions. The current-voltage properties of the nonlinear resistors are in the lower image. Bottom: endothelial or smooth muscle cells within 1 arterial segment were voltage clamped (arrowhead) 15 mV negative or positive to resting membrane potential (V_M; −40 mV), respectively. The ensuing electrical responses are color mapped along the vessel and note the stark difference in spread depending on the cells stimulated. B: a detailed electrical model of capillary endothelium (cEC) incorporates currents through Kir (I_Kir), TRPV4 (I_TRPV4), nonselective cation (NSC), and chloride (Cl) channels, sodium-potassium (NaK) and plasma membrane-calcium ATPase (PMCA) pumps, sodium-calcium (NCX) exchanger, and the NaKCl cotransport (from Ref. , with permission from Proceedings of the National Academy of Sciences USA). A minimal representation of the cEC includes explicit descriptions for I_Kir and I_TRPV4, while the rest of the transmembrane currents are lumped into a nonspecific background current (I_bg). Simulations of electrical signal propagation are performed in realistic angioarchitectures from the mouse primary sensory cortex. Capillary endothelial cells (cECs) are coupled in series through gap junctions to create multicellular capillary segments and networks. Conduction in arterioles accounts for the number of endothelial (EC) and coupled smooth muscle (SMC) cells at each longitudinal position. Each endothelium-smooth muscle unit is modeled by including a background current (I_bg,PA) based on an effective membrane conductance of the 2-cell system and a net Kir current (I_Kir,PA). Gap junctional currents between neighboring cells in capillaries (I_gj) and in arterioles (I_gj,PA) are based on V_M gradients. CSQN, calsequestrin; IP₃, inositol triphosphate; IP₃R, receptor; PIP₂, phosphatidylinositol 4,5-bisphosphate. C: K_IR-mediated control of electrical conduction in microvascular networks constructed of ∼25,000 cells within a 500-nL volume. 10 mM K⁺ stimulation of cECs deep within the cortex (∼200) results in local excitability or in regenerative electrical conduction toward the surface vessels. Higher gap junctional conductance (G_gj) and preferential distribution of Kir channels in the vicinity of penetrating arterioles [represented by changes in K_IR conductance (G_Kir)] promote regenerative conduction. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 10.

Electrical signals conduct along the vascular wall. Activated K⁺ channels trigger a hyperpolarizing signal that spreads longitudinally along endothelial cells (EC); connexins are densely expressed at the interendothelial contacts, providing low resistance to ionic movement. This charge spreads radially from the endothelium to smooth muscle cells (SMC) with greater resistance and is propagated along the smooth muscle layer with intermediate resistance. For reference, TABLE 9 highlights the measured gap junctional resistivity of interconnected vascular cells. V_M, membrane potential.

FIGURE 11.

Impact of connexin deletion/modification on conducted dilation. A: summary data highlighting the enhanced decay of the conducted vasodilation in situ in mouse cremaster arterioles (from Ref. , with permission from American Physiological Society). Top: vasomotor responses were monitored at the local site (0 µm) and 1 conducted site (2,000 µm) in wild-type and connexin (Cx)40 knockout (KO) mice. Bottom: the % maximal response to acetylcholine was plotted at the local and all conducted sites in 5 studies employing global and endothelium-specific Cx40 KO and 1 transgenic strain (, –72, 114). Note the diminished response in the Cx40 deletion/transgenic strains at the conducted (1,500 µm) but not the local (0 µm) site, a finding consistent with reduced endothelial cell (EC)-EC conductance (from Ref. , with permission from American Physiological Society). B, top: a virtual artery comprised of 1 EC layer circumscribed by a single smooth muscle cell (SMC) layer was constructed as described previously (33). Hyperpolarizing current (−35 pA) was injected into a small number of endothelial cells, and voltage resolved along the artery as EC-EC and EC-SMC conductance was systematically altered relative to its starting point of 1× conductance. The 3-dimensional plot (middle) highlights the electrical conduction profile [change in membrane potential (ΔV_M) vs. vessel length] as a function of coupling conductance among smooth muscle and endothelial cells. The 2-dimensional plot (bottom) highlights the local (0 µm) and remote (1,500 µm) V_M response in SMCs (green and blue, respectively). Note that as EC-EC but not EC-SMC coupling conductance decreased, the voltage separation among the local and remote smooth muscle cells markedly rose, findings indicative of enhanced electronic decay (47, 104,105, 337,338). TABLE 11 is a summation of other notable conducted vasomotor observations performed in mouse KO models. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 12.

Physiological regulation of intercellular conduction. A: illustrative diagram of the arterial wall and key physiological stimuli that could alter the conduction of vasodilation. Note the potential role of neural activation, intravascular pressure, angiotensin II (Ang II), blood flow, and nitric oxide (NO) production. CGRP, calcitonin gene-related peptide; eNOS, endothelial nitric oxide synthase. B: in the hamster retractor muscle preparation (left), feed arteries were locally stimulated with acetylcholine while vasomotor responses were monitored upstream along the vessel (400–1,600 µm from the point of stimulation) (from Ref. , with permission from Journal of Physiology). Measurements were performed under control conditions, during sympathetic nerve activation, and with the application of norepinephrine ± phentolamine (α-adrenoreceptor blocker). Summary data (center and right) illustrate the ability of sympathetic nerves and the release of norepinephrine to attenuate conducted vasodilation to acetylcholine. TABLE 12 summates the impact of potential physiological stimuli on conducted dilation. Original figures have been simplified for clarity and to foster consistency among the datasets. See original manuscripts for enhanced methodological, analytical, and interpretational depth. *Significant difference from control (P < 0.05).

FIGURE 13.

Illustrative examples of diminished intercellular conduction with the development of hypertension. A: in isolated retractor muscle arteries isolated from normotensive and hypertensive hamsters local and conducted vasomotor responses to acetylcholine were assessed (top; from Ref. , with permission from American Physiological Society). Vessels were preconstricted with pressurization ± phenylephrine or U46619. *Significant differences among groups. B, top: illustrative diagram of the rat retinal microvasculature and the sealing of perforated-patch pipettes onto smooth muscle cells on secondary arterioles, myocytes on tertiary arterioles, or pericytes on capillaries (from Ref. , with permission from Journal of Physiology). Middle: an example of a dual recording confirming electrotonic transmission within a tertiary arteriole from the stimulated to the recording myocyte (200-µm distance); note the effect of angiotensin (500 nM) on electrotonic transmission, plotted over time as the ρV_responder-to-ρV_stimulator ratio. The depolarizing current was injected via the pipette sealed onto the capillary pericyte. On right are voltage traces from a current-injected capillary pericyte and a myocyte on the tertiary arteriole under control conditions and in the presence of angiotensin. Bottom: predicted voltage decay-based experimental measurements. C, top: in a mouse cranial preparation, a laser Doppler flow probe assessed whisker stimulation-induced functional hyperemia (from Ref. , with permission from Journal of Clinical Investigation). Experiments were performed in normotensive and hypertensive animals (1–8 mo) in the absence and presence of Ba²⁺, a inwardly rectifying K⁺ channel (K_IR) inhibitor dissolved in the artificial cerebrospinal fluid (aCSF). a.u., Arbitrary units; CBF, cerebral blood flow. Hypertension-induced reductions in K_IR activity were confirmed below by whole cell electrophysiology. Bottom: the experimental scheme showing the impact of focal capillary or arteriolar stimulation (10 mM K⁺) on the diameter (green box) of a parenchymal arteriole. Representative dilatory traces are presented for preparations isolated from normotensive and hypertensive mice. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth.

FIGURE 14.

Illustrative examples of diminished capillary-to-arteriole electrical signaling after brain disease and injury. A: cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), a small-vessel disease caused by mutations of the NOTCH3 receptor, presumptively reduces functional hyperemia through matrix metalloproteinase inhibitor TIMP3 accumulation, the suppression of ADAM17 activity, and the impairment of the epidermal growth factor receptor (EGFR) shedding. B, left: pipette positions for arteriole (left, orange arrow) and capillary (right, purple arrow) stimulation in penetrating arteriole-capillary preparations. Right: traces of the arteriolar diameter response to 10 mM K⁺ applied discretely to arterioles (P1, orange circle) or capillaries (P2, purple triangle) in wild-type mice or a CADASIL model of small-vessel disease. C, left: live cranial window preparation and 2-photon laser-scanning microscopy: a micropipette containing 10 mM K⁺ and TRITC-dextran (red) was placed in close apposition to capillaries (vessels labeled with FITC-dextran, green). Right: traces of the red blood cell (RBC) flux response to 10 mM K⁺ applied to capillaries in wild-type mice or a CADASIL model of small-vessel disease. D, left: live cranial window preparation and laser Doppler flow (LDF) measurements during whisker stimulation. Right: representative traces of whisker-induced changes in cerebral blood flow (CBF) in wild-type mice or a CADASIL model of small-vessel disease. Measurements were performed in the presence and absence of Ba²⁺ (100 μM), inwardly rectifying K⁺ (K_IR) channel activity inhibitor. E, top: with a cranial window preparation and 2-photon laser-scanning microscopy, 10 mM K⁺ was briefly spritzed via pipette onto capillaries. Bottom: capillary RBC flux was measured over time in control and traumatic brain-injured mice. Note the rise in RBC flux in control mice due to conducted arteriolar dilation. F, top: micrographs illustrating pipette placement next to capillaries for in vivo monitoring of the upstream arteriolar diameter response; note the dilation to 10 mM K⁺. Bottom: summary data before and after 10 mM K⁺ application; note the impairment in traumatic brain-injured mice. *Significant differences among groups. Original figures have been simplified for clarity and to foster consistency among the datasets. See original articles for enhanced methodological, analytical, and interpretational depth. A–D from Ref. , with permission from Proceedings of National Academy of Sciences USA; E and F from Ref. , with permission from Journal of Cerebral Blood Flow and Metabolism.