Topographical heterogeneity of K(IR) currents in pericyte-containing microvessels of the rat retina: effect of diabetes

Distinct potassium channel types in brain capillary pericytes

Capillaries, composed of electrically coupled endothelial cells and overlying pericytes, constitute the vast majority of blood vessels in the brain. The most arteriole-proximate three to four branches of the capillary bed are covered by α-actin-expressing, contractile pericytes. These mural cells have a distinctive morphology and express different markers compared with their smooth muscle cell (SMC) cousins but share similar excitation-coupling contraction machinery. Despite this similarity, pericytes are considerably more depolarized than SMCs at low intravascular pressures. We have recently shown that pericytes, such as SMCs, possess functional voltage-dependent Ca2+ channels and ATP-sensitive K+ channels. Here, we further investigate the complement of pericyte ion channels, focusing on members of the K+ channel superfamily. Using NG2-DsRed-transgenic mice and diverse configurations of the patch-clamp technique, we demonstrate that pericytes display robust inward-rectifier K+ currents that are primarily mediated by the Kir2 family, based on their unique biophysical characteristics and sensitivity to micromolar concentrations of Ba2+. Moreover, multiple lines of evidence, including characteristic kinetics, sensitivity to specific blockers, biophysical attributes, and distinctive single-channel properties, established the functional expression of two voltage-dependent K+ channels: KV1 and BKCa. Although these three types of channels are also present in SMCs, they exhibit distinctive current density and kinetics profiles in pericytes. Collectively, these findings underscore differences in the operation of shared molecular features between pericytes and SMCs and highlight the potential contribution of these three K+ ion channels in setting pericyte membrane potential, modulating capillary hemodynamics, and regulating cerebral blood flow.

Variability of Vascular Reactivity in the Retina and Choriocapillaris to Oxygen and Carbon Dioxide Using Optical Coherence Tomography Angiography

Purpose

To investigate the regional and layer-specific vascular reactivity of the healthy human retina and choriocapillaris to changes in systemic carbon dioxide or oxygen.

Methods

High-resolution 3 × 3-mm2 optical coherence tomography angiography (OCTA) images were acquired from the central macula, temporal macula, and peripapillary retina while participants were exposed to three gas breathing conditions-room air, 5%CO2, and 100% O2. OCTA from all three regions were extracted and the apparent skeletonized vessel density (VSD) was assessed. The mean flow deficit sizes (MFDSs) of the choriocapillaris were also assessed. Repeated-measures analysis of variance was used to compare the ratio of intrasubject VSD change induced by the gas conditions from baseline in the superficial retinal layer (SRL) and deep retinal layer (DRL) for each retinal region independently, as well as the MFDS of the choriocapillaris. We also compared the vessel reactivity between the retinal capillaries and the choriocapillaris.

Results

The cumulative intrasubject response to the gas conditions differed significantly among regions of the SRL (F(2, 7) = 28.22, P < 0.001), with the temporal macula showing the largest response (15%) compared to the macula (8%) and radial peripapillary capillaries (7%). A similar trend was found in the DRL. The choriocapillaris reactivity was similar between the macula (5.8%) and temporal macula (5.6%). There was also a significant heterogeneity in the layer-specific gas responses, with the DRL showing the largest response (28.2%) and the choriocapillaris showing the smallest response (2.8%).

Conclusions

Capillary reactivity to changes in inhaled O2 and CO2 is spatially heterogeneous across the retina but not choriocapillaris.

Inward rectifier potassium (Kir) channels in the retina: living our vision

Channel proteins are vital for conducting ions throughout the body and are especially relevant to retina physiology. Inward rectifier potassium (Kir) channels are a class of K+ channels responsible for maintaining membrane potential and extracellular K+ concentrations. Studies of the KCNJ gene (that encodes Kir protein) expression identified the presence of all of the subclasses (Kir 1-7) of Kir channels in the retina or retinal-pigmented epithelium (RPE). However, functional studies have established the involvement of the Kir4.1 homotetramer and Kir4.1/5.1 heterotetramer in Müller glial cells, Kir2.1 in bipolar cells, and Kir7.1 in the RPE cell physiology. Here, we propose the potential roles of Kir channels in the retina based on the physiological contributions to the brain, pancreatic, and cardiac tissue functions. There are several open questions regarding the expressed KCNJ genes in the retina and RPE. For example, why does not the Kir channel subtype gene expression correspond with protein expression? Catching up with multiomics or functional "omics" approaches might shed light on posttranscriptional changes that might influence Kir subunit mRNA translation within the retina that guides our vision.

Homogeneity or heterogeneity, the paradox of neurovascular pericytes in the brain

Pericytes are one of the main components of the neurovascular unit. They play a critical role in regulating blood flow, blood–brain barrier permeability, neuroinflammation, and neuronal activity. In the central nervous system (CNS), pericytes are classified into three subtypes, that is, ensheathing, mesh, and thin‐strand pericytes, based on their distinct morphologies and region‐specific distributions. However, whether these three types of pericytes exhibit heterogeneity or homogeneity with regard to membrane properties has been understudied to date. Here, we combined bulk RNA sequencing analysis with electrophysiological methods to demonstrate that the three subtypes of pericytes share similar electrical membrane properties in the CNS, suggesting a homogenous population of neurovascular pericytes in the brain. Furthermore, we identified an inwardly rectifying potassium channel subtype Kir4.1 functionally expressed in pericytes. Electrophysiological patch clamp recordings indicate that Kir4.1 channel currents in pericytes represent a small portion of the K⁺ macroscopic currents in physiological conditions. However, a significant augmentation of Kir4.1 currents in pericytes was induced when the extracellular K⁺ was elevated to pathological levels, suggesting pericytes Kir4.1 channels might play an important role as K⁺ sensors and contribute to K⁺ homeostasis in local neurovascular networks in pathology.

Diverse mechanisms regulating brain energy supply at the capillary level

Neural information processing depends critically on the brain's energy supply, which is provided in the form of glucose and oxygen in the blood. Regulation of this supply occurs by smooth muscle and contractile pericytes adjusting the diameter of arterioles and capillaries, respectively. Controversies exist over the relative importance of capillary and arteriolar level control, whether enzymatically generated signals or K⁺ ions are the dominant controller of cerebral blood flow, and the involvement of capillary endothelial cells. Here, we try to synthesise the relevant recent data into a coherent view of how brain energy supply is controlled and suggest approaches to answering key questions.

The Ion Channel and GPCR Toolkit of Brain Capillary Pericytes

Brain pericytes reside on the abluminal surface of capillaries, and their processes cover ~90% of the length of the capillary bed. These cells were first described almost 150 years ago (Eberth, 1871; Rouget, 1873) and have been the subject of intense experimental scrutiny in recent years, but their physiological roles remain uncertain and little is known of the complement of signaling elements that they employ to carry out their functions. In this review, we synthesize functional data with single-cell RNAseq screens to explore the ion channel and G protein-coupled receptor (GPCR) toolkit of mesh and thin-strand pericytes of the brain, with the aim of providing a framework for deeper explorations of the molecular mechanisms that govern pericyte physiology. We argue that their complement of channels and receptors ideally positions capillary pericytes to play a central role in adapting blood flow to meet the challenge of satisfying neuronal energy requirements from deep within the capillary bed, by enabling dynamic regulation of their membrane potential to influence the electrical output of the cell. In particular, we outline how genetic and functional evidence suggest an important role for G_s-coupled GPCRs and ATP-sensitive potassium (K_ATP) channels in this context. We put forth a predictive model for long-range hyperpolarizing electrical signaling from pericytes to upstream arterioles, and detail the TRP and Ca²⁺ channels and G_q, G_i/o, and G_12/13 signaling processes that counterbalance this. We underscore critical questions that need to be addressed to further advance our understanding of the signaling topology of capillary pericytes, and how this contributes to their physiological roles and their dysfunction in disease.

The role of pericytes in hyperemia-induced capillary de-recruitment following stenosis

Purpose

The microvascular capillary network is ensheathed by cells called pericytes - a heterogeneous population of mural cells derived from multiple lineages. Pericytes play a multifaceted role in the body, including in vascular structure and permeability, regulation of local blood flow, immune and wound healing functions, induction of angiogenesis, and generation of various progenitor cells. Here, we consider the role of pericytes in capillary de-recruitment, a pathophysiologic phenomenon that is observed following hyperemic stimuli in the presence of a stenosis and attenuates the hyperemic response.

Recent Findings

We discuss recent observations that conclusively demonstrate pericytes to be the cellular structures that contract in response to hyperemic stimuli when an upstream arterial stenosis is present. This response constricts capillaries, which is likely aimed at maintaining capillary hydrostatic pressure, an important factor in tissue homeostasis. Nonetheless, the ensuing attenuation of the hyperemic response can lead to a decrease in energy supply and negatively impact tissue health.

Summary

Therapeutics aimed at preventing pericyte-mediated capillary de-recruitment may prove beneficial in conditions such as coronary stenosis and peripheral arterial disease by reducing restriction in hyperemic flow. Identification of the pericyte subtypes involved in this de-recruitment and the underlying molecular mechanisms regulating this process will greatly assist this purpose.

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.

Ion channels and myogenic activity in retinal arterioles

Retinal pressure autoregulation is an important mechanism that protects the retina by stabilizing retinal blood flow during changes in arterial or intraocular pressure. Similar to other vascular beds, retinal pressure autoregulation is thought to be mediated largely through the myogenic response of small arteries and arterioles which constrict when transmural pressure increases or dilate when it decreases. Over recent years, we and others have investigated the signaling pathways underlying the myogenic response in retinal arterioles, with particular emphasis on the involvement of different ion channels expressed in the smooth muscle layer of these vessels. Here, we review and extend previous work on the expression and spatial distribution of the plasma membrane and sarcoplasmic reticulum ion channels present in retinal vascular smooth muscle cells (VSMCs) and discuss their contribution to pressure-induced myogenic tone in retinal arterioles. This includes new data demonstrating that several key players and modulators of the myogenic response show distinctively heterogeneous expression along the length of the retinal arteriolar network, suggesting differences in myogenic signaling between larger and smaller pre-capillary arterioles. Our immunohistochemical investigations have also highlighted the presence of actin-containing microstructures called myobridges that connect the retinal VSMCs to one another. Although further work is still needed, studies to date investigating myogenic mechanisms in the retina have contributed to a better understanding of how blood flow is regulated in this tissue. They also provide a basis to direct future research into retinal diseases where blood flow changes contribute to the pathology.

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.

Isolation of Retinal Arterioles for Ex Vivo Cell Physiology Studies

The retina is a highly metabolically active tissue that requires a substantial blood supply. The retinal circulation supports the inner retina, while the choroidal vessels supply the photoreceptors. Alterations in retinal perfusion contribute to numerous sight-threatening disorders, including diabetic retinopathy, glaucoma and retinal branch vein occlusions. Understanding the molecular mechanisms involved in the control of blood flow through the retina and how these are altered during ocular disease could lead to the identification of new targets for the treatment of these conditions. Retinal arterioles are the main resistance vessels of the retina, and consequently, play a key role in regulating retinal hemodynamics through changes in luminal diameter. In recent years, we have developed methods for isolating arterioles from the rat retina which are suitable for a wide range of applications including cell physiology studies. This preparation has already begun to yield new insights into how blood flow is controlled in the retina and has allowed us to identify some of the key changes that occur during ocular disease. In this article, we describe methods for the isolation of rat retinal arterioles and include protocols for their use in patch-clamp electrophysiology, calcium imaging and pressure myography studies. These vessels are also amenable for use in PCR-, western blotting- and immunohistochemistry-based studies.

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.

From in silico to in vitro: a trip to reveal flavonoid binding on the Rattus norvegicus Kir6.1 ATP-sensitive inward rectifier potassium channel

Background

ATP-sensitive inward rectifier potassium channels (Kir), are a potassium channel family involved in many physiological processes. K_ATP dysfunctions are observed in several diseases such as hypoglycaemia, hyperinsulinemia, Prinzmetal angina–like symptoms, cardiovascular diseases.

Methods

A broader view of the K_ATP mechanism is needed in order to operate on their regulation, and in this work we clarify the structure of the Rattus norvegicus ATP-sensitive inward rectifier potassium channel 8 (Kir6.1), which has been obtained through a homology modelling procedure. Due to the medical use of flavonoids, a considerable increase in studies on their influence on human health has recently been observed, therefore our aim is to study, through computational methods, the three-dimensional (3D) conformation together with mechanism of action of Kir6.1 with three flavonoids.

Results

Computational analysis by performing molecular dynamics (MD) and docking simulation on rat 3D modelled structure have been completed, in its closed and open conformation state and in complex with Quercetin, 5-Hydroxyflavone and Rutin flavonoids. Our study showed that only Quercetin and 5-Hydroxyflavone were responsible for a significant down-regulation of the Kir6.1 activity, stabilising it in a closed conformation. This hypothesis was supported by in vitro experiments demonstrating that Quercetin and 5-Hydroxyflavone were capable to inhibit K_ATP currents of rat tail main artery myocytes recorded by the patch-clamp technique.

Conclusion

Combined methodological approaches, such as molecular modelling, docking and MD simulations of Kir6.1 channel, used to elucidate flavonoids intrinsic mechanism of action, are introduced, revealing a new potential druggable protein site.

Role of ion channels in the functional response of conjunctival goblet cells to dry eye

Optimal vision requires an ocular surface with a stable tear film whose many critical tasks include providing >70% of the eye's refractive power. However, for millions, tear film instability produces uncomfortable sight-impairing dry eye. Despite the multitude of etiologies for dry eye, a universal hallmark is hyperosmolarity of the tear film. Presently, knowledge of how the ocular surface responds to hyperosmolarity remains incomplete with little understood about the role of ion channels. This bioelectric analysis focused on conjunctival goblet cells whose release of tear-stabilizing mucin is a key adaptive response to dry eye. In freshly excised rat conjunctiva, perforated-patch recordings demonstrated that a ≥10% rise in osmolarity triggers goblet cells to rapidly generate a ~15-mV hyperpolarization due to the oxidant-dependent activation of ATP-sensitive K⁺ (K_ATP) channels. High-resolution membrane capacitance measurements used to monitor exocytosis revealed that this hyperpolarization results in an approximately fourfold boost in exocytotic activity evoked by cholinergic input, which in vivo occurs via a neural reflex and depends chiefly on calcium influxing down its electro-gradient. We discovered that this adaptive response is transient. During 30-80 min of hyperosmolarity, development of a depolarizing nonspecific cation conductance fully counterbalances the K_ATP-driven hyperpolarization and thereby eliminates the exocytotic boost. We conclude that hyperosmotic-induced hyperpolarization is a previously unappreciated mechanism by which goblet cells respond to transient ocular dryness. Loss of this voltage increase during long-term dryness/hyperosmolarity may account for the clinical conundrum that goblet cells in chronically dry eyes can remain filled with mucin even though the tear film is hyperosmotic and mucin-deficient.

The physiological functions of central nervous system pericytes and a potential role in pain

Central nervous system (CNS) pericytes regulate critical functions of the neurovascular unit in health and disease. CNS pericytes are an attractive pharmacological target for their position within the neurovasculature and for their role in neuroinflammation. Whether the function of CNS pericytes also affects pain states and nociceptive mechanisms is currently not understood. Could it be that pericytes hold the key to pain associated with CNS blood vessel dysfunction? This article reviews recent findings on the important physiological functions of CNS pericytes and highlights how these neurovascular functions could be linked to pain states.

Boosting the signal: Endothelial inward rectifier K+ channels

Endothelial cells express a diverse array of ion channels including members of the strong inward rectifier family composed of K_IR 2 subunits. These two-membrane spanning domain channels are modulated by their lipid environment, and exist in macromolecular signaling complexes with receptors, protein kinases and other ion channels. Inward rectifier K⁺ channel (K_IR ) currents display a region of negative slope conductance at membrane potentials positive to the K⁺ equilibrium potential that allows outward current through the channels to be activated by membrane hyperpolarization, permitting K_IR to amplify hyperpolarization induced by other K⁺ channels and ion transporters. Increases in extracellular K⁺ concentration activate K_IR allowing them to sense extracellular K⁺ concentration and transduce this change into membrane hyperpolarization. These properties position K_IR to participate in the mechanism of action of hyperpolarizing vasodilators and contribute to cell-cell conduction of hyperpolarization along the wall of microvessels. The expression of K_IR in capillaries in electrically active tissues may allow K_IR to sense extracellular K⁺ , contributing to functional hyperemia. Understanding the regulation of expression and function of microvascular endothelial K_IR will improve our understanding of the control of blood flow in the microcirculation in health and disease and may provide new targets for the development of therapeutics in the future.

Capillary K+-sensing initiates retrograde hyperpolarization to increase local cerebral blood flow

Blood flow into the brain is dynamically regulated to satisfy the changing metabolic requirements of neurons, but how this is accomplished has remained unclear. Here, we demonstrate a central role for capillary endothelial cells in sensing neural activity and communicating it to upstream arterioles in the form of an electrical vasodilatory signal. We further demonstrate that this signal is initiated by extracellular potassium (K⁺)—a byproduct of neural activity—which activates capillary endothelial cell inward-rectifier K⁺ (K_IR2.1) channels to produce a rapidly propagating retrograde hyperpolarization that causes upstream arteriolar dilation, increasing blood flow into the capillary bed. Our results establish brain capillaries as an active sensory web that converts changes in external K⁺ into rapid, ‘inside-out’ electrical signaling to direct blood flow to active brain regions.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Bioelectric impact of pathological angiogenesis on vascular function

Pathological angiogenesis, as seen in many inflammatory, immune, malignant, and ischemic disorders, remains an immense health burden despite new molecular therapies. It is likely that further therapeutic progress requires a better understanding of neovascular pathophysiology. Surprisingly, even though transmembrane voltage is well known to regulate vascular function, no previous bioelectric analysis of pathological angiogenesis has been reported. Using the perforated-patch technique to measure vascular voltages in human retinal neovascular specimens and rodent models of retinal neovascularization, we discovered that pathological neovessels generate extraordinarily high voltage. Electrophysiological experiments demonstrated that voltage from aberrantly located preretinal neovascular complexes is transmitted into the intraretinal vascular network. With extensive neovascularization, this voltage input is substantial and boosts the membrane potential of intraretinal blood vessels to a suprahyperpolarized level. Coincident with this suprahyperpolarization, the vasomotor response to hypoxia is fundamentally altered. Instead of the compensatory dilation observed in the normal retina, arterioles constrict in response to an oxygen deficiency. This anomalous vasoconstriction, which would potentiate hypoxia, raises the possibility that the bioelectric impact of neovascularization on vascular function is a previously unappreciated pathophysiological mechanism to sustain hypoxia-driven angiogenesis.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Ion channel networks in the control of cerebral blood flow

One hundred and twenty five years ago, Roy and Sherrington made the seminal observation that neuronal stimulation evokes an increase in cerebral blood flow.(1) Since this discovery, researchers have attempted to uncover how the cells of the neurovascular unit-neurons, astrocytes, vascular smooth muscle cells, vascular endothelial cells and pericytes-coordinate their activity to control this phenomenon. Recent work has revealed that ionic fluxes through a diverse array of ion channel species allow the cells of the neurovascular unit to engage in multicellular signaling processes that dictate local hemodynamics.In this review we center our discussion on two major themes: (1) the roles of ion channels in the dynamic modulation of parenchymal arteriole smooth muscle membrane potential, which is central to the control of arteriolar diameter and therefore must be harnessed to permit changes in downstream cerebral blood flow, and (2) the striking similarities in the ion channel complements employed in astrocytic endfeet and endothelial cells, enabling dual control of smooth muscle from either side of the blood-brain barrier. We conclude with a discussion of the emerging roles of pericyte and capillary endothelial cell ion channels in neurovascular coupling, which will provide fertile ground for future breakthroughs in the field.

Vascular inward rectifier K+ channels as external K+ sensors in the control of cerebral blood flow

For decades it has been known that external K(+) ions are rapid and potent vasodilators that increase CBF. Recent studies have implicated the local release of K(+) from astrocytic endfeet-which encase the entirety of the parenchymal vasculature-in the dynamic regulation of local CBF during NVC. It has been proposed that the activation of KIR channels in the vascular wall by external K(+) is a central component of these hyperemic responses; however, a number of significant gaps in our knowledge remain. Here, we explore the concept that vascular KIR channels are the major extracellular K(+) sensors in the control of CBF. We propose that K(+) is an ideal mediator of NVC, and discuss KIR channels as effectors that produce rapid hyperpolarization and robust vasodilation of cerebral arterioles. We provide evidence that KIR channels, of the KIR 2 subtype in particular, are present in both the endothelial and SM cells of parenchymal arterioles and propose that this dual positioning of KIR 2 channels increases the robustness of the vasodilation to external K(+), enables the endothelium to be actively engaged in NVC, and permits electrical signaling through the endothelial syncytium to promote upstream vasodilation to modulate CBF.

Calcium signalling in pericytes

Recent advances in pericyte research have contributed to our understanding of the physiology and pathophysiology of microvessels. The microvasculature consists of arteriolar and venular networks located upstream and downstream of the capillaries. Arterioles are surrounded by a monolayer of spindle-shaped myocytes, while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are encircled by a monolayer of morphologically diverse pericytes. There are physiological differences in the response of pericytes and myocytes to vasoactive molecules, suggesting that these two vascular cell types could have different functional roles in the regulation of local blood flow. The contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca(2+) concentration. In this short review, we summarize our results and those of other authors on the contractility of pericytes and their Ca(2+) signalling. We describe results regarding sources of Ca(2+) and mechanisms of Ca(2+) release and Ca(2+) entry in control of the spatiotemporal characteristics of the Ca(2+) signals in pericytes.

Arginase in retinopathy

Ischemic retinopathies, such as diabetic retinopathy (DR), retinopathy of prematurity and retinal vein occlusion are a major cause of blindness in developed nations worldwide. Each of these conditions is associated with early neurovascular dysfunction. However, conventional therapies target clinically significant macula edema or neovascularization, which occur much later. Intra-ocular injections of anti-VEGF show promise in reducing retinal edema, but the effects are usually transient and the need for repeated injections increases the risk of intraocular infection. Laser photocoagulation can control pathological neovascularization, but may impair vision and in some patients the retinopathy continues to progress. Moreover, neither treatment targets early stage disease or promotes repair. This review examines the potential role of the ureahydrolase enzyme arginase as a therapeutic target for the treatment of ischemic retinopathy. Arginase metabolizes l-arginine to form proline, polyamines and glutamate. Excessive arginase activity reduces the l-arginine supply for nitric oxide synthase (NOS), causing it to become uncoupled and produce superoxide and less NO. Superoxide and NO react and form the toxic oxidant peroxynitrite. The catabolic products of polyamine oxidation and glutamate can induce more oxidative stress and DNA damage, both of which can cause cellular injury. Studies indicate that neurovascular injury during retinopathy is associated with increased arginase expression/activity, decreased NO, polyamine oxidation, formation of superoxide and peroxynitrite and dysfunction and injury of both vascular and neural cells. Furthermore, data indicate that the cytosolic isoform arginase I (AI) is involved in hyperglycemia-induced dysfunction and injury of vascular endothelial cells whereas the mitochondrial isoform arginase II (AII) is involved in neurovascular dysfunction and death following hyperoxia exposure. Thus, we postulate that activation of the arginase pathway causes neurovascular injury by uncoupling NOS and inducing polyamine oxidation and glutamate formation, thereby reducing NO and increasing oxidative stress, all of which contribute to the retinopathic process.

Long distance conduction of vasodilation: a passive or regenerative process?

The mechanism enabling coordination of the resistance of feed arteries with microcirculatory arterioles to rapidly regulate tissue blood flow in line with changes in metabolic demand has preoccupied scientists for a quarter of a century. As experiments uncovered the underlying electrical events, it was frequently questioned how vasodilation could conduct over long distances without appreciable attenuation. This perspective reviews the data pertinent to this phenomenon and provides evidence that this remarkable response could be made possible by a simple mechanism based on the steep relationship between membrane potential and calcium entry demonstrated by the voltage-dependent calcium channels which mediate the control of vascular tone in vivo.

Cellular and physiological mechanisms underlying blood flow regulation in the retina and choroid in health and disease

We review the cellular and physiological mechanisms responsible for the regulation of blood flow in the retina and choroid in health and disease. Due to the intrinsic light sensitivity of the retina and the direct visual accessibility of fundus blood vessels, the eye offers unique opportunities for the non-invasive investigation of mechanisms of blood flow regulation. The ability of the retinal vasculature to regulate its blood flow is contrasted with the far more restricted ability of the choroidal circulation to regulate its blood flow by virtue of the absence of glial cells, the markedly reduced pericyte ensheathment of the choroidal vasculature, and the lack of intermediate filaments in choroidal pericytes. We review the cellular and molecular components of the neurovascular unit in the retina and choroid, techniques for monitoring retinal and choroidal blood flow, responses of the retinal and choroidal circulation to light stimulation, the role of capillaries, astrocytes and pericytes in regulating blood flow, putative signaling mechanisms mediating neurovascular coupling in the retina, and changes that occur in the retinal and choroidal circulation during diabetic retinopathy, age-related macular degeneration, glaucoma, and Alzheimer's disease. We close by discussing issues that remain to be explored.

Vulnerability of the retinal microvasculature to oxidative stress: ion channel-dependent mechanisms

Although oxidative stress is a hallmark of important vascular disorders such as diabetic retinopathy, it remains unclear why the retinal microvasculature is particularly vulnerable to this pathophysiological condition. We postulated that redox-sensitive ion channels may play a role. Using H(2)O(2) to cause oxidative stress in microvascular complexes freshly isolated from the adult rat retina, we assessed ionic currents, cell viability, intracellular oxidants, and cell calcium by using perforated-patch recordings, trypan blue dye exclusion, and fura-2 fluorescence, respectively. Supporting a role for the oxidant-sensitive ATP-sensitive K (K(ATP)) channels, we found that these channels are activated during exposure of retinal microvessels to H(2)O(2). Furthermore, their inhibition by glibenclamide significantly lessened H(2)O(2)-induced microvascular cell death. Additional experiments established that by increasing the influx of calcium into microvascular cells, the K(ATP) channel-mediated hyperpolarization boosted the vulnerability of these cells to oxidative stress. In addition to the K(ATP) channel-dependent mechanism for increasing the lethality of oxidative stress, we also found that the vulnerability of cells in the capillaries, but not in the arterioles, was further boosted by a K(ATP) channel-independent mechanism, which our experiments indicated involves the oxidant-induced activation of calcium-permeable nonspecific cation channels. Taken together, our findings support a working model in which both K(ATP) channel-independent and K(ATP) channel-dependent mechanisms render the capillaries of the retina particularly vulnerable to oxidative stress. Identification of these previously unappreciated mechanisms for boosting the lethality of oxidants may provide new targets for pharmacologically limiting damage to the retinal microvasculature during periods of oxidative stress.

The electrotonic architecture of the retinal microvasculature: diabetes-induced alteration

Although microvascular cell death is a well established hallmark of diabetic retinopathy, which is a major cause of vision loss, much remains to be learned about the functional changes that precede the onset of morphological damage to retinal blood vessels. Early alterations of function are of interest since they may contribute to the development of irreversible pathological events. Because one of the earliest retinal effects of diabetes is the dysregulation of blood flow, we asked whether diabetes alters the functional organization of the capillary/arteriolar complex, which is the operational unit that plays an important role in regulating local perfusion. In this study, the effect of diabetes on the electrotonic architecture of the retinal microvasculature was characterized. To do this, we quantified the efficacy by which voltages are transmitted between pairs perforated-patch pipettes sealed onto abluminal cells located at well defined locations in capillary/arteriolar complexes freshly isolated from the retinas of rats made diabetic by streptozotocin. Results of these dual recording experiments were compared with data from similar experiments performed on non-diabetic retinal microvessels. These experiments revealed that diabetes caused a ∼5-fold increase in the rate at which a voltage decays as it axially spreads through the retinal microvasculature. In contrast, the efficacy of radial abluminal cell/endothelial cell transmission was not significantly affected by diabetes. Based on the results of this study, which is the first to characterize how diabetes affects voltage transmission in capillary/arteriolar complexes of any tissue, we concluded that by selectively inhibiting axial transmission, diabetes alters the electrotonic architecture of the retinal microvasculature. This diabetes-induced alteration in the functional organization of the capillary/arteriolar unit is likely to impair its ability to efficiently and effectively regulate blood flow and thereby, may contribute to the progression of sight-threatening complications of diabetic retinopathy.

Retinovascular physiology and pathophysiology: new experimental approach/new insights

An important challenge in visual neuroscience is to understand the physiology and pathophysiology of the intra-retinal vasculature, whose function is required for ophthalmoception by humans and most other mammals. In the quest to learn more about this highly specialized portion of the circulatory system, a newly developed method for isolating vast microvascular complexes from the rodent retina has opened the way for using techniques such as patch-clamping, fluorescence imaging and time-lapse photography to elucidate the functional organization of a capillary network and its pre-capillary arteriole. For example, the ability to obtain dual perforated-patch recordings from well-defined sites within an isolated microvascular complex permitted the first characterization of the electrotonic architecture of a capillary/arteriole unit. This analysis revealed that this operational unit is not simply a homogenous synctium, but has a complex functional organization that is dynamically modulated by extracellular signals such as angiotensin II. Another recent discovery is that a capillary and its pre-capillary arteriole have distinct physiological differences; capillaries have an abundance of ATP-sensitive potassium (K(ATP)) channels and a dearth of voltage-dependent calcium channels (VDCCs) while the converse is true for arterioles. In addition, voltage transmission between abluminal cells and the endothelium is more efficient in the capillaries. Thus, the capillary network is well-equipped to generate and transmit voltages, and the pre-capillary arteriole is well-adapted to transduce a capillary-generated voltage into a change in abluminal cell calcium and thereby, a vasomotor response. Use of microvessels isolated from the diabetic retina has led to new insights concerning retinal vascular pathophysiology. For example, soon after the onset of diabetes, the efficacy of voltage transmission through the endothelium is diminished; arteriolar VDCCs are inhibited, and there is increased vulnerability to purinergic vasotoxicity, which is a newly identified pathobiological mechanism. Other recent studies reveal that K(ATP) channels not only have an essential physiological role in generating vasomotor responses, but their activation substantially boosts the lethality of hypoxia. Thus, the pathophysiology of the retinal microvasculature is closely linked with its physiology.

Vulnerability of the retinal microvasculature to hypoxia: role of polyamine-regulated K(ATP) channels

PURPOSE

It is uncertain why retinal capillaries are particularly vulnerable to hypoxia. In this study, it was hypothesized that their specialized physiology, which includes being the predominant microvascular location of functional adenosine triphosphate-sensitive potassium (K(ATP)) channels, boosts their susceptibility to hypoxia-induced cell death.

METHODS

Cell viability, ionic currents, intracellular calcium, and pericyte contractility in microvascular complexes freshly isolated from the rat retina were assessed using trypan blue dye exclusion, perforated-patch recordings, fura-2 fluorescence, and time-lapse videos. Chemical hypoxia was induced by antimycin, an oxidative phosphorylation inhibitor.

RESULTS

In freshly isolated retinal microvascular complexes, chemical hypoxia caused more cell death in capillaries than in arterioles. Indicative of the role of polyamine-dependent K(ATP) channels, antimycin-induced capillary cell death was markedly decreased in microvessels treated with the polyamine synthesis inhibitor, difluoromethylornithine, or the K(ATP) channel inhibitor, glibenclamide. These inhibitors also diminished the antimycin-induced hyperpolarization, as well as the antimycin-induced intracellular calcium increase, which was significantly dependent on extracellular calcium and was diminished by the inhibitor of calcium-induced calcium release (CICR), dantrolene. Consistent with the importance of the CICR-dependent increase in capillary cell calcium, dantrolene significantly decreased hypoxia-induced capillary cell death. We also found that activation of the polyamine/K(ATP) channel/Ca(2+) influx/CICR pathway not only boosted the vulnerability of retinal capillaries to hypoxia, but also caused the contraction of capillary pericytes, whose vasoconstrictive effect may exacerbate hypoxia.

CONCLUSIONS

The vulnerability of retinal capillaries to hypoxia is boosted by a mechanism involving the polyamine/K(ATP) channel/Ca(2+) influx/CICR pathway. Discovery of this pathway should provide new targets for pharmacological interventions to minimize hypoxia-induced damage in retinal capillaries.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Measurement of the membrane potential in small cells using patch clamp methods

The resting membrane potential, E(m), of mammalian cells is a fundamental physiological parameter. Even small changes in E(m) can modulate excitability, contractility and rates of cell migration. At present accurate, reproducible measurements of E(m) and determination of its ionic basis remain significant challenges when patch clamp methods are applied to small cells. In this study, a mathematical model has been developed which incorporates many of the main biophysical principles which govern recordings of the resting potential of 'small cells'. Such a prototypical cell (approx. capacitance, 6 pF; input resistance 5 GΩ) is representative of neonatal cardiac myocytes, and other cells in the cardiovascular system (endothelium, fibroblasts) and small cells in other tissues, e.g. bone (osteoclasts) articular joints (chondrocytes) and the pancreas (β cells). Two common experimental conditions have been examined: (1) when the background K(+) conductance is linear; and (2) when this K(+) conductance is highly nonlinear and shows pronounced inward rectification. In the case of a linear K(+) conductance, the presence of a "leakage" current through the seal resistance between the cell membrane and the patch pipette always depolarizes E(m). Our calculations confirm that accurate characterization of E(m) is possible when the seal resistance is at least 5 times larger than the input resistance of the targeted cell. Measurement of E(m) under conditions in which the main background current includes a markedly nonlinear K(+) conductance (due to inward rectification) yields complex and somewhat counter-intuitive findings. In fact, there are at least two possible stable values of resting membrane potential for a cell when the nonlinear, inwardly rectifying K(+) conductance interacts with the seal current. This type of bistable behavior has been reported in a variety of small mammalian cells, including those from the heart, endothelium, smooth muscle and bone. Our theoretical treatment of these two common experimental situations provides useful mechanistic insights, and suggests practical methods by which these significant limitations, and their impact, can be minimized.

The electrotonic architecture of the retinal microvasculature: modulation by angiotensin II

The capillary/arteriole complex is the key operational unit regulating local perfusion to meet metabolic demand. However, much remains to be learned about how this multi cellular unit is functionally organized. To help address this challenge, we characterized the electrotonic architecture of the retinal microvasculature, which is particularly well adapted for the decentralized control of blood flow. In this study, we quantified the transmission of voltage between pairs of perforated-patch pipettes sealed onto abluminal cells located on microvascular complexes freshly isolated from the adult rat retina. These complexes consisted of capillaries,as well as tertiary and secondary arterioles. Dual recording experiments revealed that voltage spreading axially through a capillary, tertiary arteriole or secondary arteriole is transmitted very efficiently with a decay rate of only ∼5% per 100 μm. However, the retinal microvasculature is not simply a well-coupled syncytium since we detected significant voltage dissipation with radial abluminal cell-to-endothelium transmission and also at branch points between a capillary and its tertiary arteriole and between tertiary and secondary arterioles. Consistent with capillaries being particularly well-suited for the task of transmitting voltages induced by vasoactive signals, radial transmission is most efficient in this portion of the retinal microvasculature. Dual recordings also revealed that angiotensin II potently inhibits axial transmission. As a functional consequence, the geographical extent of the microvasculature's response to voltage-changing inputs is markedly restricted in the presence of angiotensin. In addition, this effect of angiotensin established that the electrotonic architecture of the retinal microvasculature is not static, but rather, is dynamically modulated by vasoactive signals.

Diabetes-induced inhibition of voltage-dependent calcium channels in the retinal microvasculature: role of spermine

PURPOSE

Although decentralized control of blood flow is particularly important in the retina, knowledge of the functional organization of the retinal microvasculature is limited. Here, the authors characterized the distribution and regulation of L-type voltage-dependent calcium channels (VDCCs) within the most decentralized operational complex of the retinal vasculature--the feeder vessel/capillary unit--which consists of a capillary network plus the vessel linking it with a myocyte-encircled arteriole.

METHODS

Perforated-patch recordings, calcium-imaging, and time-lapse photography were used to assess VDCC-dependent changes in ionic currents, intracellular calcium, abluminal cell contractility, and lumen diameter, in microvascular complexes freshly isolated from the rat retina.

RESULTS

Topographical heterogeneity was found in the distribution of functional VDCCs; VDCC activity was markedly greater in feeder vessels than in capillaries. Experiments showed that this topographical distribution occurs, in large part, because of the inhibition of capillary VDCCs by a mechanism dependent on the endogenous polyamine spermine. An operational consequence of functional VDCCs predominantly located in the feeder vessels is that voltage-driven vasomotor responses are generated chiefly in this portion of the feeder vessel/capillary unit. However, early in the course of diabetes, this ability to generate voltage-driven vasomotor responses becomes profoundly impaired because of the inhibition of feeder vessel VDCCs by a spermine-dependent mechanism.

CONCLUSIONS

The regulation of VDCCs by endogenous spermine not only plays a critical role in establishing the physiological organization of the feeder vessel/capillary unit, but also may contribute to dysfunction of this decentralized operational unit in the diabetic retina.

Involvement of NADPH oxidase and protein kinase C in endothelin-1-induced superoxide production in retinal microvessels

Redox signaling has been implicated in pathophysiological changes in the vascular system. We examined whether endothelin-1 (ET-1) increases the formation of superoxide anions in retinal microvessels. Freshly isolated retinal microvessels from rats were exposed to ET-1 (100 nM), and the intracellular superoxide formation in the retinal pericytes was assessed semi-quantitatively by time-lapse fluorometric analyses using hydroethidine. The receptor mechanisms were determined by BQ-123 and BQ-788, receptor antagonists for ET(A) and ET(B) receptors, respectively, and also by IRL-1620, a selective agonist for ET(B) receptors. In addition, the changes induced by adding apocynin (10 microM), myr-PKC (1.0 microM), allopurinol (100 microM), rotenone (10 microM), or L-NAME (100 microM) with ET-1 were evaluated. Microvessels were incubated with phorbol 12-myristate 13-acetate (PMA, 10nM), a protein kinase C (PKC) activator. Fluorometric analyses showed ethidium fluorescence-positive regions that coincided well with the location of retinal pericytes. The intracellular superoxide levels were significantly increased after addition of ET-1 (100 nM), and this elevation was suppressed by apocynin or myr-PKC. Other enzyme inhibitors including L-NAME had no effect. The ET-1-induced increase of superoxide was significantly suppressed by BQ-123 (1.0 microM), while effects of adding BQ-788 (1.0 microM) were insignificant. IRL-1620 (100 nM) did not increase superoxide formation significantly. PMA (10nM) mimicked the effect of ET-1. These results suggest that ET-1 increases the formation of superoxides in the retinal microvascular pericytes most likely by activating NADPH oxidase through ET(A) receptors. The activation of PKC may be involved in the mechanism. Thus, ET-1 may augment its vasoconstrictive effects through the formation of superoxide, which may impair the bioavailability of nitric oxide in the retinal microvasculature.

Functional K(ATP) channels in the rat retinal microvasculature: topographical distribution, redox regulation, spermine modulation and diabetic alteration

The essential task of the circulatory system is to match blood flow to local metabolic demand. However, much remains to be learned about this process. To better understand how local perfusion is regulated, we focused on the functional organization of the retinal microvasculature, which is particularly well adapted for the local control of perfusion. Here, we assessed the distribution and regulation of functional K(ATP) channels whose activation mediates the hyperpolarization induced by adenosine. Using microvascular complexes freshly isolated from the rat retina, we found a topographical heterogeneity in the distribution of functional K(ATP) channels; capillaries generate most of the K(ATP) current. The initiation of K(ATP)-induced responses in the capillaries supports the concept that the regulation of retinal perfusion is highly decentralized. Additional study revealed that microvascular K(ATP) channels are redox sensitive, with oxidants increasing their activity. Furthermore, the oxidant-mediated activation of these channels is driven by the polyamine spermine, whose catabolism produces oxidants. In addition, our observation that spermine-dependent oxidation occurs predominately in the capillaries accounts for why they generate most of the K(ATP) current detected in retinal microvascular complexes. Here, we also analysed retinal microvessels of streptozotocin-injected rats. We found that soon after the onset of diabetes, an increase in spermine-dependent oxidation at proximal microvascular sites boosts their K(ATP) current and thereby virtually eliminates the topographical heterogeneity of functional K(ATP) channels. We conclude that spermine-dependent oxidation is a previously unrecognized mechanism by which this polyamine modulates ion channels; in addition to a physiological role, spermine-dependent oxidation may also contribute to microvascular dysfunction in the diabetic retina.