BMS-191011, an opener of large-conductance Ca2+-activated potassium channels, dilates rat retinal arterioles in vivo

Endogenous noise of neocortical neurons correlates with atypical sensory response variability in the Fmr1-/y mouse model of autism

Excessive neural variability of sensory responses is a hallmark of atypical sensory processing in autistic individuals with cascading effects on other core autism symptoms but unknown neurobiological substrate. Here, by recording neocortical single neuron activity in a well-established mouse model of Fragile X syndrome and autism, we characterized atypical sensory processing and probed the role of endogenous noise sources in exaggerated response variability in males. The analysis of sensory stimulus evoked activity and spontaneous dynamics, as well as neuronal features, reveals a complex cellular and network phenotype. Neocortical sensory information processing is more variable and temporally imprecise. Increased trial-by-trial and inter-neuronal response variability is strongly related to key endogenous noise features, and may give rise to behavioural sensory responsiveness variability in autism. We provide a novel preclinical framework for understanding the sources of endogenous noise and its contribution to core autism symptoms, and for testing the functional consequences for mechanism-based manipulation of noise.

N-methyl-D-aspartic acid receptor-mediated vasodilation is attenuated in the retinas of diabetic rats

PURPOSE

Activation of N-methyl-d-aspartic acid (NMDA) receptors enhances nitric oxide (NO) production in retinal neuronal cells, and in turn, NO released from neuronal cells induces glial cell-mediated dilation of retinal arterioles in rats. The purpose of this study was to examine how neuronal cell-dependent, glial cell-mediated vasodilation is impacted in diabetic rat retinas.

MATERIALS AND METHODS

Diabetes was induced in 6-week-old male Wistar rats by combining streptozotocin injection and D-glucose feeding. Two weeks later, the dilator function of retinal arterioles was assessed.

RESULTS

Compared with non-diabetic rats, the dilator responses of retinal arterioles induced by intravitreal injection of NMDA and NOR3, an NO donor, were reduced in diabetic rats. Following the blockade of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels with iberiotoxin, no significant difference in the retinal vasodilator response to NOR3 was observed between non-diabetic and diabetic rats. Intravitreal injection of 14,15-epoxyeicosatrienoic acid, a vasodilatory factor released from glial cells, dilated retinal arterioles, and the response was diminished by diabetes.

CONCLUSION

These findings suggest that the impaired BK_Ca channel function in vascular cells is responsible for the diminished neuronal cell-dependent, glial cell-mediated dilation of retinal arterioles during the early stage of diabetes.

NS-11021 Modulates Cancer-Associated Processes Independently of BK Channels in Melanoma and Pancreatic Duct Adenocarcinoma Cell Lines

Potassium channels have emerged as regulators of carcinogenesis, thus introducing possible new therapeutic strategies in the fight against cancer. In particular, the large-conductance Ca2+-activated K+ channel, often referred to as BK channel, is involved in several cancer-associated processes. Here, we investigated the effects of different BK activators, NS-11021, NS-19504, and BMS-191011, in IGR39 (primary melanoma cell line) and Panc-1 (primary pancreatic duct carcinoma cell line), highly expressing the channel, and in IGR37 (metastatic melanoma cell line) that barely express BK. Our data showed that NS-11021 and NS-19504 potently activated BK channels in IGR39 and Panc-1 cells, while no effect on channel activation was detected in IGR37 cells. On the contrary, BK channel activator BMS-191011 was less effective. However, only NS-11021 showed significant effects in cancer-associated processes, such as cell survival, migration, and proliferation in these cancer cell lines. Moreover, NS-11021 led to an increase of intracellular Ca2+ concentration, independent of BK channel activation, thus complicating any interpretation of its role in the regulation of cancer-associated mechanisms. Overall, we conclude that the activation of the BK channel by itself is not sufficient to produce beneficial anti-cancer effects in the melanoma and PDAC cell lines examined. Importantly, our results raise an alarm flag regarding the use of presumably specific BK channel openers as anti-cancer agents.

Coronary Large Conductance Ca2+-Activated K+ Channel Dysfunction in Diabetes Mellitus

Diabetes mellitus (DM) is an independent risk of macrovascular and microvascular complications, while cardiovascular diseases remain a leading cause of death in both men and women with diabetes. Large conductance Ca2+-activated K+ (BK) channels are abundantly expressed in arteries and are the key ionic determinant of vascular tone and organ perfusion. It is well established that the downregulation of vascular BK channel function with reduced BK channel protein expression and altered intrinsic BK channel biophysical properties is associated with diabetic vasculopathy. Recent efforts also showed that diabetes-associated changes in signaling pathways and transcriptional factors contribute to the downregulation of BK channel expression. This manuscript will review our current understandings on the molecular, physiological, and biophysical mechanisms that underlie coronary BK channelopathy in diabetes mellitus.

Role of Epoxyeicosatrienoic Acids in Acetylcholine-Induced Dilation of Rat Retinal Arterioles in Vivo

CYP epoxygenase-derived epoxyeicosatrienoic acids (EETs) contribute to endothelium-dependent hyperpolarization (EDH)-related dilation in multiple vascular beds. The present study aimed to determine the role of EETs in the acetylcholine (ACh)-induced dilation of retinal arterioles in rats in vivo. The vasodilator responses were assessed by determining the change in diameter of the retinal arterioles on images of the ocular fundus. The intravitreal injection of 17-octadecynoic acid (1.4 nmol/eye), an inhibitor of CYP epoxygenase, and 14,15-epoxyeicosa-5(Z)-enoic acid (14,15-EE-5(Z)-E; 2 nmol/eye), an antagonist of EETs, reduced the ACh (0.3-10 µg/kg/min)-induced dilation of the retinal arterioles. The EET antagonist attenuated the vasodilator response to ACh under blockade of nitric oxide (NO) synthases and cyclooxygenases with N^G-nitro-L-arginine methyl ester (30 mg/kg) plus indomethacin (5 mg/kg). Intravitreal injection of 14,15-EET (0.5 nmol/eye) dilated retinal arterioles and the response was prevented by iberiotoxin, an inhibitor of large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels (20 pmol/eye). These results suggest that ACh stimulates the production of EETs, thereby dilating the retinal arterioles via activation of BK_Ca channels. CYP epoxygenase-derived EETs may be involved in the EDH-related component of the ACh-induced dilation of the retinal arterioles.

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.

BK Channels in the Central Nervous System

Large conductance Ca(2+)- and voltage-activated K(+) (BK) channels are widely distributed in the postnatal central nervous system (CNS). BK channels play a pleiotropic role in regulating the activity of brain and spinal cord neural circuits by providing a negative feedback mechanism for local increases in intracellular Ca(2+) concentrations. In neurons, they regulate the timing and duration of K(+) influx such that they can either increase or decrease firing depending on the cellular context, and they can suppress neurotransmitter release from presynaptic terminals. In addition, BK channels located in astrocytes and arterial myocytes modulate cerebral blood flow. Not surprisingly, both loss and gain of BK channel function have been associated with CNS disorders such as epilepsy, ataxia, mental retardation, and chronic pain. On the other hand, the neuroprotective role played by BK channels in a number of pathological situations could potentially be leveraged to correct neurological dysfunction.

5-hydroxytryptamine has an endothelium-derived hyperpolarizing factor-like effect on coronary flow in isolated rat hearts

Background

5-hydroxytryptamine (5-HT)-induced coronary artery responses have both vasoconstriction and vasorelaxation components. The vasoconstrictive effects of 5-HT have been well studied while the mechanism(s) of how 5-HT causes relaxation of coronary arteries has been less investigated. In isolated rat hearts, 5-HT-induced coronary flow increases are partially resistant to the nitric oxide synthase inhibitor Nω-Nitro-L-arginine methyl ester (L-NAME) and are blocked by 5-HT₇ receptor antagonists. In the present study, we investigated the role of 5-HT₇ receptor in 5-HT-induced coronary flow increases in isolated rat hearts in the absence of L-NAME, and we also evaluated the involvement of endothelium-derived hyperpolarizing factor (EDHF) in 5-HT-induced coronary flow increases in L-NAME-treated hearts with the inhibitors of arachidonic acid metabolism and the blockers of Ca²⁺-activated K⁺ channels.

Results

In isolated rat hearts, 5-HT and the 5-HT₇ receptor agonist 5-carboxamidotryptamine induced coronary flow increases, and both of these effects were blocked by the selective 5-HT₇ receptor antagonist SB269970; in SB269970-treated hearts, 5-HT induced coronary flow decreases, which effect was blocked by the 5-HT_2A receptor blocker R96544. In L-NAME-treated hearts, 5-HT-induced coronary flow increases were blocked by the phospholipase A₂ inhibitor quinacrine and the cytochrome P450 inhibitor SKF525A, but were not inhibited by the cyclooxygenase inhibitor indomethacin. As to the effects of the Ca²⁺-activated K⁺ channel blockers, 5-HT-induced coronary flow increases in L-NAME-treated hearts were inhibited by TRAM-34 (intermediate-conductance Ca²⁺-activated K⁺ channel blocker) and UCL1684 (small-conductance Ca²⁺-activated K⁺ channel blocker), but effects of the large-conductance Ca²⁺-activated K⁺ channel blockers on 5-HT-induced coronary flow increases were various: penitrem A and paxilline did not significantly affect 5-HT-induced coronary flow responses while tetraethylammonium suppressed the coronary flow increases elicited by 5-HT.

Conclusion

In the present study, we found that 5-HT-induced coronary flow increases are mediated by the activation of 5-HT₇ receptor in rat hearts in the absence of L-NAME. Metabolites of cytochrome P450s, small-conductance Ca²⁺-activated K⁺ channel, and intermediate-conductance Ca²⁺-activated K⁺ channel are involved in 5-HT-induced coronary flow increases in L-NAME-treated hearts, which resemble the mechanisms of EDHF-induced vasorelaxation. The role of large-conductance Ca²⁺-activated K⁺ channel in 5-HT-induced coronary flow increases in L-NAME-treated hearts needs further investigation.

Regulation of large conductance Ca2+-activated K+ (BK) channel β1 subunit expression by muscle RING finger protein 1 in diabetic vessels

The large conductance Ca(2+)-activated K(+) (BK) channel, expressed abundantly in vascular smooth muscle cells (SMCs), is a key determinant of vascular tone. BK channel activity is tightly regulated by its accessory β1 subunit (BK-β1). However, BK channel function is impaired in diabetic vessels by increased ubiquitin/proteasome-dependent BK-β1 protein degradation. Muscle RING finger protein 1 (MuRF1), a muscle-specific ubiquitin ligase, is implicated in many cardiac and skeletal muscle diseases. However, the role of MuRF1 in the regulation of vascular BK channel and coronary function has not been examined. In this study, we hypothesized that MuRF1 participated in BK-β1 proteolysis, leading to the down-regulation of BK channel activation and impaired coronary function in diabetes. Combining patch clamp and molecular biological approaches, we found that MuRF1 expression was enhanced, accompanied by reduced BK-β1 expression, in high glucose-cultured human coronary SMCs and in diabetic vessels. Knockdown of MuRF1 by siRNA in cultured human SMCs attenuated BK-β1 ubiquitination and increased BK-β1 expression, whereas adenoviral expression of MuRF1 in mouse coronary arteries reduced BK-β1 expression and diminished BK channel-mediated vasodilation. Physical interaction between the N terminus of BK-β1 and the coiled-coil domain of MuRF1 was demonstrated by pulldown assay. Moreover, MuRF1 expression was regulated by NF-κB. Most importantly, pharmacological inhibition of proteasome and NF-κB activities preserved BK-β1 expression and BK-channel-mediated coronary vasodilation in diabetic mice. Hence, our results provide the first evidence that the up-regulation of NF-κB-dependent MuRF1 expression is a novel mechanism that leads to BK channelopathy and vasculopathy in diabetes.

Ca(2+) sparks promote myogenic tone in retinal arterioles

BACKGROUND AND PURPOSE

Ca(2+) imaging reveals subcellular Ca(2+) sparks and global Ca(2+) waves/oscillations in vascular smooth muscle. It is well established that Ca(2+) sparks can relax arteries, but we have previously reported that sparks can summate to generate Ca(2+) waves/oscillations in unpressurized retinal arterioles, leading to constriction. We have extended these studies to test the functional significance of Ca(2+) sparks in the generation of myogenic tone in pressurized arterioles.

EXPERIMENTAL APPROACH

Isolated retinal arterioles (25-40 μm external diameter) were pressurized to 70 mmHg, leading to active constriction. Ca(2+) signals were imaged from arteriolar smooth muscle in the same vessels using Fluo4 and confocal laser microscopy.

KEY RESULTS

Tone development was associated with an increased frequency of Ca(2+) sparks and oscillations. Vasomotion was observed in 40% of arterioles and was associated with synchronization of Ca(2+) oscillations, quantifiable as an increased cross-correlation coefficient. Inhibition of Ca(2+) sparks with ryanodine, tetracaine, cyclopiazonic acid or nimodipine, or following removal of extracellular Ca(2+) , resulted in arteriolar relaxation. Cyclopiazonic acid-induced dilatation was associated with decreased Ca(2+) sparks and oscillations but with a sustained rise in the mean global cytoplasmic [Ca(2+) ] ([Ca(2+) ]c ), as measured using Fura2 and microfluorimetry.

CONCLUSIONS AND IMPLICATIONS

This study provides direct evidence that Ca(2+) sparks can play an excitatory role in pressurized arterioles, promoting myogenic tone. This contrasts with the generally accepted model in which sparks promote relaxation of vascular smooth muscle. Changes in vessel tone in the presence of cyclopiazonic acid correlated more closely with changes in spark and oscillation frequency than global [Ca(2+) ]c , underlining the importance of frequency-modulated signalling in vascular smooth muscle.

Prodrug approaches to reduce hyperexcitation in the CNS

Hyperexcitation in the central nervous system is the root cause of a number of disorders of the brain ranging from acute injury to chronic and progressive diseases. The major limitation to treatment of these ailments is the miniscule, yet formidable blood-brain barrier. To deliver therapeutic agents to the site of desired action, a number of biomedical engineering strategies have been developed including prodrug formulations that allow for either passive diffusion or active transport across this barrier. In the case of prodrugs, once in the brain compartment, the active therapeutic agent is released. In this review, we discuss in some detail a number of factors related to treatment of central nervous system hyperexcitation including molecular targets, disorders, prodrug strategies, and focused case studies of a number of therapeutics that are at a variety of stages of clinical development.