Voltage-dependent potentiation of L-type Ca2+ channels due to phosphorylation by cAMP-dependent protein kinase

Sympathetic innervation in skeletal muscle and its role at the neuromuscular junction

Neuromuscular junctions are the synapses between motor neurons and skeletal muscle fibers, which mediate voluntary muscle movement. Since neuromuscular junctions are also tightly associated with the capping function of terminal Schwann cells, these synapses have been classically regarded as tripartite chemical synapses. Although evidences from sympathetic innervation of neuromuscular junctions was described approximately a century ago, the essential presence and functional relevance of sympathetic contribution to the maintenance and modulation of neuromuscular junctions was demonstrated only recently. These findings shed light on the pathophysiology of different clinical conditions and can optimize surgical and clinical treatment modalities for skeletal muscle disorders.

IP3R1 underlies diastolic ANO1 activation and pressure-dependent chronotropy in lymphatic collecting vessels

Pressure-dependent chronotropy of murine lymphatic collecting vessels relies on the activation of the Ca2+-activated chloride channel encoded by Anoctamin 1 (Ano1) in lymphatic muscle cells. Genetic ablation or pharmacological inhibition of ANO1 results in a significant reduction in basal contraction frequency and essentially complete loss of pressure-dependent frequency modulation by decreasing the rate of the diastolic depolarization phase of the ionic pacemaker in lymphatic muscle cells (LMCs). Oscillating Ca2+ release from sarcoendoplasmic reticulum Ca2+ channels has been hypothesized to drive ANO1 activity during diastole, but the source of Ca2+ for ANO1 activation in smooth muscle remains unclear. Here, we investigated the role of the inositol triphosphate receptor 1 (Itpr1; Ip3r1) in this process using pressure myography, Ca2+ imaging, and membrane potential recordings in LMCs of ex vivo pressurized inguinal-axillary lymphatic vessels from control or Myh11CreERT2;Ip3r1fl/fl (Ip3r1ismKO) mice. Ip3r1ismKO vessels had significant reductions in contraction frequency and tone but an increased contraction amplitude. Membrane potential recordings from LMCs of Ip3r1ismKO vessels revealed a depressed diastolic depolarization rate and an elongation of the plateau phase of the action potential (AP). Ca2+ imaging of LMCs using the genetically encoded Ca2+ sensor GCaMP6f demonstrated an elongation of the Ca2+ flash associated with an AP-driven contraction. Critically, diastolic subcellular Ca2+ transients were absent in LMCs of Ip3r1ismKO mice, demonstrating the necessity of IP3R1 activity in controlling ANO1-mediated diastolic depolarization. These findings indicate a critical role for IP3R1 in lymphatic vessel pressure-dependent chronotropy and contractile regulation.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Effects of Dexmedetomidine on Basic Cardiac Electrophysiology in Adults; a Descriptive Review and a Prospective Case Study

Dexmedetomidine (DEX) is a commonly used sedative agent with no or minimal effects on breathing. DEX may also be beneficial in myocardial protection. Since the mechanisms of cardiac effects are not well known, we carried out a descriptive review and examined the effects of DEX on myocardial electrical conduction in a prospective and controlled manner. For the review, clinical studies exploring DEX in myocardial protection published between 2020-2022 were explored. A case study included 11 consecutive patients at a median (range) age of 48 (38-59), scheduled for elective radiofrequency ablation of paroxysmal atrial fibrillation. A bolus dose of DEX 1 µg/kg given in 15 min was followed by a continuous infusion of 0.2-0.7 µg/kg/h. Direct intracardiac electrophysiologic measurements, hemodynamics and oxygenation were measured before and after the DEX bolus. Experimental studies show that DEX protects the heart both via stabilizing cardiac electrophysiology and reducing apoptosis and autophagy after cell injury. The clinical evidence shows that DEX provides cardiac protection during different surgeries. In a clinical study, DEX increased the corrected sinus node recovery time, prolongated the atrioventricular (AV) nodal refractory period and cycle length producing AV nodal Wenckebach retrograde conduction block. DEX has a putative role in organ protection against hypoxic, oxidative and reperfusion injury. DEX slows down the firing of the sinus node and prolongs AV refractoriness.

Applications of Dexmedetomidine in Palliative and Hospice Care

Although the use of dexmedetomidine is currently approved by the US Food and Drug Administration in the adult population for monitored anesthesia care and sedation during mechanical ventilation, clinical experience suggests the potential application of dexmedetomidine in the palliative care arena. The medication can provide sedation with lower risk of delirium, control or minimize the adverse effects of other medications, and augment analgesia from opioids. We conducted a computerized bibliographic search of the literature regarding dexmedetomidine use for the treatment of pain and provision of sedation during palliative and hospice care in adult and pediatric patients. The objective was to provide a general descriptive account of the physiologic effects of dexmedetomidine and review its potential applications in the field of palliative and hospice care in adult and pediatric patients. The sedative and analgesic effects of dexmedetomidine have been well studied in animal and human models. Published experience from both single case reports and small case series has demonstrated the potential therapeutic applications of dexmedetomidine in palliative and hospice care. In addition to intravenous administration, case reports have demonstrated its successful use by both the intranasal and subcutaneous routes. Although these experiences have suggested its safety and efficacy, larger series and additional clinical experience with prospective comparison to other agents are needed to further define its efficacy and role in palliative and hospice care.

Aldosterone-Regulated Sodium Transport and Blood Pressure

Aldosterone is a major mineralocorticoid steroid hormone secreted by glomerulosa cells in the adrenal cortex. It regulates a variety of physiological responses including those to oxidative stress, inflammation, fluid disruption, and abnormal blood pressure through its actions on various tissues including the kidney, heart, and the central nervous system. Aldosterone synthesis is primarily regulated by angiotensin II, K⁺ concentration, and adrenocorticotrophic hormone. Elevated serum aldosterone levels increase blood pressure largely by increasing Na⁺ re-absorption in the kidney through regulating transcription and activity of the epithelial sodium channel (ENaC). This review focuses on the signaling pathways involved in aldosterone synthesis and its effects on Na⁺ reabsorption through ENaC.

Absence of gravin-mediated signaling inhibits development of high-fat diet-induced hyperlipidemia and atherosclerosis

Gravin, an A-kinase anchoring protein, is known to play a role in regulating key processes that lead to inflammation and atherosclerosis development, namely, cell migration, proliferation, and apoptosis. We investigated the role of gravin in the development of high-fat diet (HFD)-induced atherosclerosis and hyperlipidemia. Five-week-old male wild-type (WT) and gravin-t/t mice were fed a normal diet or an HFD for 16 wk. Gravin-t/t mice showed significantly lower liver-to-body-weight ratio, cholesterol, triglyceride, and very low-density lipoprotein levels in serum as compared with WT mice on HFD. Furthermore, there was less aortic plaque formation coupled with decreased lipid accumulation and liver damage, as the gravin-t/t mice had lower levels of serum alanine aminotransferase and aspartate aminotransferase. Additionally, gravin-t/t HFD-fed mice had decreased expression of liver 3-hydroxy-3-methyl-glutaryl-CoA reductase, an essential enzyme for cholesterol synthesis and lower fatty acid synthase expression. Gravin-t/t HFD-fed mice also exhibited inhibition of sterol regulatory element binding protein-2 (SREBP-2) expression, a liver transcription factor associated with the regulation of lipid transportation. In response to platelet-derived growth factor receptor treatment, gravin-t/t vascular smooth muscle cells exhibited lower intracellular calcium transients and decreased protein kinase A- and protein kinase C-dependent substrate phosphorylation, notably involving the Erk1/2 signaling pathway. Collectively, these results suggest the involvement of gravin-dependent regulation of lipid metabolism via the reduction of SREBP-2 expression. The absence of gravin-mediated signaling lowers blood pressure, reduces plaque formation in the aorta, and decreases lipid accumulation and damage in the liver of HFD mice. Through these processes, the absence of gravin-mediated signaling complex delays the HFD-induced hyperlipidemia and atherosclerosis.NEW & NOTEWORTHY The gravin scaffolding protein plays a key role in the multiple enzymatic pathways of lipid metabolism. We have shown for the first time the novel role of gravin in regulating the pathways related to the initiation and progression of atherosclerosis. Specifically, an absence of gravin-mediated signaling decreases the lipid levels (cholesterol, triglyceride, and VLDL) that are associated with sterol regulatory element binding protein-2 downregulation.

Postnatal Development and Distribution of Sympathetic Innervation in Mouse Skeletal Muscle

Vertebrate neuromuscular junctions (NMJs) have been conceived as tripartite synapses composed of motor neuron, Schwann cell, and muscle fiber. Recent work has shown the presence of sympathetic neurons in the immediate vicinity of NMJs and experimental and clinical findings suggest that this plays an eminent role in adult NMJ biology. The present study examined the postnatal development and distribution of sympathetic innervation in different muscles using immunofluorescence, confocal microscopy, and Western blot. This demonstrates the proximity of sympathetic neurons in diaphragm, extensor digitorum longus, tibialis anterior, soleus, and levator auris longus muscles. In extensor digitorum longus muscle, sympathetic innervation of NMJs was quantified from perinatal to adult stage and found to increase up to two months of age. In diaphragm muscle, an extensive network of sympathetic neurons was prominent along the characteristic central synapse band. In summary, these data demonstrate that an elaborate sympathetic innervation is present in several mouse skeletal muscles and that this is often next to NMJs. Although the presence of sympathetic neurons at the perisynaptic region of NMJs increased during postnatal development, many synapses were already close to sympathetic neurons at birth. Potential implications of these findings for treatment of neuromuscular diseases are discussed.

Voltage-Sensitive Calcium Channels in the Brain: Relevance to Alcohol Intoxication and Withdrawal

Voltage-sensitive Ca²⁺ (Ca_V) channels are the primary route of depolarization-induced Ca²⁺ entry in neurons and other excitable cells, leading to an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). The resulting increase in [Ca²⁺]_i activates a wide range of Ca²⁺-dependent processes in neurons, including neurotransmitter release, gene transcription, activation of Ca²⁺-dependent enzymes, and activation of certain K⁺ channels and chloride channels. In addition to their key roles under physiological conditions, Ca_V channels are also an important target of alcohol, and alcohol-induced changes in Ca²⁺ signaling can disturb neuronal homeostasis, Ca²⁺-mediated gene transcription, and the function of neuronal circuits, leading to various neurological and/or neuropsychiatric symptoms and disorders, including alcohol withdrawal induced-seizures and alcoholism.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

Regulation of Cardiac Calcium Channels in the Fight-or-Flight Response

Intracellular calcium transients generated by activation of voltage-gated calcium (CaV) channels generate local signals, which initiate physiological processes such as secretion, synaptic transmission, and excitation-contraction coupling. Regulation of calcium entry through CaV channels is crucial for control of these physiological processes. In this article, I review experimental results that have emerged over several years showing that cardiac CaV1.2 channels form a local signaling complex, in which their proteolytically processed distal C-terminal domain, an A-Kinase Anchoring Protein, and cyclic AMP-dependent protein kinase (PKA) interact directly with the transmembrane core of the ion channel through the proximal C-terminal domain. This signaling complex is the substrate for β-adrenergic up-regulation of the CaV1.2 channel in the heart during the fight-or-flight response. Protein phosphorylation of two sites at the interface between the distal and proximal C-terminal domains contributes importantly to control of basal CaV1.2 channel activity, and phosphorylation of Ser1700 by PKA at that interface up-regulates CaV1.2 activity in response to β-adrenergic signaling. Thus, the intracellular C-terminal domain of CaV1.2 channels serves as a signaling platform, mediating beat-to-beat physiological regulation of channel activity and up-regulation by β-adrenergic signaling in the fight-or-flight response.

Role of the cAMP Pathway in Glucose and Lipid Metabolism

3'-5'-Cyclic adenosine monophosphate (cyclic AMP or cAMP) was first described in 1957 as an intracellular second messenger mediating the effects of glucagon and epinephrine on hepatic glycogenolysis (Berthet et al., J Biol Chem 224(1):463-475, 1957). Since this initial characterization, cAMP has been firmly established as a versatile molecular signal involved in both central and peripheral regulation of energy homeostasis and nutrient partitioning. Many of these effects appear to be mediated at the transcriptional level, in part through the activation of the transcription factor CREB and its coactivators. Here we review current understanding of the mechanisms by which the cAMP signaling pathway triggers metabolic programs in insulin-responsive tissues.

Two-pore domain potassium channels in the adrenal cortex

The physiological control of steroid hormone secretion from the adrenal cortex depends on the function of potassium channels. The "two-pore domain K(+) channels" (K2P) TWIK-related acid sensitive K(+) channel 1 (TASK1), TASK3, and TWIK-related K(+) channel 1 (TREK1) are strongly expressed in adrenocortical cells. They confer a background K(+) conductance to these cells which is important for the K(+) sensitivity as well as for angiotensin II and adrenocorticotropic hormone-dependent stimulation of aldosterone and cortisol synthesis. Mice with single deletions of the Task1 or Task3 gene as well as Task1/Task3 double knockout mice display partially autonomous aldosterone synthesis. It appears that TASK1 and TASK3 serve different functions: TASK1 affects cell differentiation and prevents expression of aldosterone synthase in the zona fasciculata, while TASK3 controls aldosterone secretion in glomerulosa cells. TREK1 is involved in the regulation of cortisol secretion in fasciculata cells. These data suggest that a disturbed function of K2P channels could contribute to adrenocortical pathologies in humans.

Apparent lack of physical or functional interaction between CaV1.1 and its distal C terminus

The distal C-terminal domain of Ca_V1.1 is not required for depolarization-induced potentiation of L-type Ca²⁺ current in skeletal muscle.

Ca_V1.1 acts as both the voltage sensor that triggers excitation–contraction coupling in skeletal muscle and as an L-type Ca²⁺ channel. It has been proposed that, after its posttranslational cleavage, the distal C terminus of Ca_V1.1 remains noncovalently associated with proximal Ca_V1.1, and that tethering of protein kinase A to the distal C terminus is required for depolarization-induced potentiation of L-type Ca²⁺ current in skeletal muscle. Here, we report that association of the distal C terminus with proximal Ca_V1.1 cannot be detected by either immunoprecipitation of mouse skeletal muscle or by colocalized fluorescence after expression in adult skeletal muscle fibers of a Ca_V1.1 construct labeled with yellow fluorescent protein (YFP) and cyan fluorescent protein on the N and C termini, respectively. We found that L-type Ca²⁺ channel activity was similar after expression of constructs that either did (YFP-Ca_V1.1₁₈₆₀) or did not (YFP-Ca_V1.1₁₆₆₆) contain coding sequence for the distal C-terminal domain in dysgenic myotubes null for endogenous Ca_V1.1. Furthermore, in response to strong (up to 90 mV) or long-lasting prepulses (up to 200 ms), tail current amplitudes and decay times were equally increased in dysgenic myotubes expressing either YFP-Ca_V1.1₁₈₆₀ or YFP-Ca_V1.1₁₆₆₆, suggesting that the distal C-terminal domain was not required for depolarization-induced potentiation. Thus, our experiments do not support the existence of either biochemical or functional interactions between proximal Ca_V1.1 and the distal C terminus.

A HCO(3)(-)-dependent mechanism involving soluble adenylyl cyclase for the activation of Ca²⁺ currents in locus coeruleus neurons

Hypercapnic acidosis activates Ca²⁺ channels and increases intracellular Ca²⁺ levels in neurons of the locus coeruleus, a known chemosensitive region involved in respiratory control. We have also shown that large conductance Ca²⁺-activated K⁺ channels, in conjunction with this pathway, limits the hypercapnic-induced increase in firing rate in locus coeruleus neurons. Here, we present evidence that the Ca²⁺ current is activated by a HCO(3)(-)-sensitive pathway. The increase in HCO(3)(-) associated with hypercapnia activates HCO(3)(-)-sensitive adenylyl cyclase (soluble adenylyl cyclase). This results in an increase in cyclic adenosine monophosphate levels and activation of Ca²⁺ channels via cyclic adenosine monophosphate-activated protein kinase A. We also show the presence of soluble adenylyl cyclase in the cytoplasm of locus coeruleus neurons, and that the cyclic adenosine monophosphate analogue db-cyclic adenosine monophosphate increases Ca²⁺i. Disrupting this pathway by decreasing HCO(3)(-) levels during acidification or inhibiting either soluble adenylyl cyclase or protein kinase A, but not transmembrane adenylyl cyclase, can increase the magnitude of the firing rate response to hypercapnia in locus coeruleus neurons from older neonates to the same extent as inhibition of K⁺ channels. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Local cAMP signaling in disease at a glance

The second messenger cyclic AMP (cAMP) operates in discrete subcellular regions within which proteins that synthesize, break down or respond to the second messenger are precisely organized. A burgeoning knowledge of compartmentalized cAMP signaling is revealing how the local control of signaling enzyme activity impacts upon disease. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight how misregulation of local cyclic AMP signaling can have pathophysiological consequences. We first introduce the core molecular machinery for cAMP signaling, which includes the cAMP-dependent protein kinase (PKA), and then consider the role of A-kinase anchoring proteins (AKAPs) in coordinating different cAMP-responsive proteins. The latter sections illustrate the emerging role of local cAMP signaling in four disease areas: cataracts, cancer, diabetes and cardiovascular diseases.

Effect of dexmedetomidine on the corrected QT and Tp-e intervals during spinal anesthesia

PURPOSE

The aim of this study is to evaluate the effect of dexmedetomidine on corrected QT (QTc) and Tp-e intervals in patients undergoing spinal anesthesia.

MATERIALS AND METHODS

We studied 50 patients who were scheduled to undergo spinal anesthesia before orthopedic surgeries. Patients were allocated to receive either an infusion of dexmedetomidine or normal saline after spinal anesthesia.

RESULTS

QTc intervals were significantly prolonged after spinal anesthesia, and the prolonged QTc interval returned to baseline values 10 minutes after either normal saline or dexmedetomidine administration in both groups. The QTc interval values after dexmedetomidine administration were significantly shorter compared to the QTc interval values just before dexmedetomidine administration.

CONCLUSION

Dexmedetomidine could promote the return of a prolonged QTc interval induced by spinal anesthesia and might be helpful in patients who have a prolonged QTc interval.

Enhanced cardiac function in Gravin mutant mice involves alterations in the β-adrenergic receptor signaling cascade

Gravin, an A-kinase anchoring protein, targets protein kinase A (PKA), protein kinase C (PKC), calcineurin and other signaling molecules to the beta2-adrenergic receptor (β2-AR). Gravin mediates desensitization/resensitization of the receptor by facilitating its phosphorylation by PKA and PKC. The role of gravin in β-AR mediated regulation of cardiac function is unclear. The purpose of this study was to determine the effect of acute β-AR stimulation on cardiac contractility in mice lacking functional gravin. Using echocardiographic analysis, we observed that contractility parameters such as left ventricular fractional shortening and ejection fraction were increased in gravin mutant (gravin-t/t) animals lacking functional protein compared to wild-type (WT) animals both at baseline and following acute isoproterenol (ISO) administration. In isolated gravin-t/t cardiomyocytes, we observed increased cell shortening fraction and decreased intracellular Ca(2+) in response to 1 µmol/L ISO stimulation. These physiological responses occurred in the presence of decreased β2-AR phosphorylation in gravin-t/t hearts, where PKA-dependent β2-AR phosphorylation has been shown to lead to receptor desensitization. cAMP production, PKA activity and phosphorylation of phospholamban and troponin I was comparable in WT and gravin-t/t hearts both with and without ISO stimulation. However, cardiac myosin binding protein C (cMyBPC) phosphorylation site at position 273 was significantly increased in gravin-t/t versus WT hearts, in the absence of ISO. Additionally, the cardioprotective heat shock protein 20 (Hsp20) was significantly more phosphorylated in gravin-t/t versus WT hearts, in response to ISO. Our results suggest that disruption of gravin's scaffold mediated signaling is able to increase baseline cardiac function as well as to augment contractility in response to acute β-AR stimulation by decreasing β2-AR phosphorylation and thus attenuating receptor desensitization and perhaps by altering PKA localization to increase the phosphorylation of cMyBPC and the nonclassical PKA substrate Hsp20.

Enhanced cardiac function in Gravin mutant mice involves alterations in the β-adrenergic receptor signaling cascade

Gravin, an A-kinase anchoring protein, targets protein kinase A (PKA), protein kinase C (PKC), calcineurin and other signaling molecules to the beta2-adrenergic receptor (β₂-AR). Gravin mediates desensitization/resensitization of the receptor by facilitating its phosphorylation by PKA and PKC. The role of gravin in β-AR mediated regulation of cardiac function is unclear. The purpose of this study was to determine the effect of acute β-AR stimulation on cardiac contractility in mice lacking functional gravin. Using echocardiographic analysis, we observed that contractility parameters such as left ventricular fractional shortening and ejection fraction were increased in gravin mutant (gravin-t/t) animals lacking functional protein compared to wild-type (WT) animals both at baseline and following acute isoproterenol (ISO) administration. In isolated gravin-t/t cardiomyocytes, we observed increased cell shortening fraction and decreased intracellular Ca²⁺ in response to 1 µmol/L ISO stimulation. These physiological responses occurred in the presence of decreased β₂-AR phosphorylation in gravin-t/t hearts, where PKA-dependent β₂-AR phosphorylation has been shown to lead to receptor desensitization. cAMP production, PKA activity and phosphorylation of phospholamban and troponin I was comparable in WT and gravin-t/t hearts both with and without ISO stimulation. However, cardiac myosin binding protein C (cMyBPC) phosphorylation site at position 273 was significantly increased in gravin-t/t versus WT hearts, in the absence of ISO. Additionally, the cardioprotective heat shock protein 20 (Hsp20) was significantly more phosphorylated in gravin-t/t versus WT hearts, in response to ISO. Our results suggest that disruption of gravin’s scaffold mediated signaling is able to increase baseline cardiac function as well as to augment contractility in response to acute β-AR stimulation by decreasing β₂-AR phosphorylation and thus attenuating receptor desensitization and perhaps by altering PKA localization to increase the phosphorylation of cMyBPC and the nonclassical PKA substrate Hsp20.

A review of the pharmacology and clinical uses of pimobendan

OBJECTIVE

To review the pharmacology, research developments, and clinical uses of pimobendan

DATA SOURCES

Original research articles and clinical studies from 1984 to August 2011.

VETERINARY DATA SYNTHESIS

Pimobendan is approved for use in dogs for the treatment of congestive heart failure (CHF) secondary to chronic valvular heart disease (CVHD) and dilated cardiomyopathy (DCM). Expert-based veterinary guidelines recommend the use of pimobendan in the management of acute, hospital-based therapy for patients with CHF attributable to CVHD.

CONCLUSIONS

The use of pimobendan, an inodilator with phosphodiesterase 3 (PDE3) inhibitory and calcium-sensitizing properties, is regarded as a component of the standard of care in the management of dogs with CHF secondary to both DCM and CVHD. Further studies are warranted to confirm the safety and efficacy of pimobendan for the off-label use of this drug in asymptomatic CVHD, pulmonary arterial hypertension, asymptomatic myocardial diseases, CHF from all other causes and in cats with CHF.

Functional Anchoring of the cAMP-Dependent Protein Kinase

The precision of cAMP-responsive events is controlled in part through compartmentalization of the signal transduction machinery. Recent evidence suggests that the cAMP-dependent protein kinase (PKA) is localized to specific subcellular compartments through association with A Kinase Anchoring Proteins (AKAPs). The AKAPs now represent a functionally related family of regulatory proteins that contain a conserved PKA binding domain and unique targeting sequences that direct the PKA-AKAP complex to subcellular structures. In this review, the recent evidence suggesting that AKAPs facilitate PKA anchoring close to key membrane substrates, such as glutamate receptors, calcium-activated potassium channels, and skeletal or cardiac muscle calcium channels, is surveyed.

cAMP signaling in skeletal muscle adaptation: hypertrophy, metabolism, and regeneration

Among organ systems, skeletal muscle is perhaps the most structurally specialized. The remarkable subcellular architecture of this tissue allows it to empower movement with instructions from motor neurons. Despite this high degree of specialization, skeletal muscle also has intrinsic signaling mechanisms that allow adaptation to long-term changes in demand and regeneration after acute damage. The second messenger adenosine 3',5'-monophosphate (cAMP) not only elicits acute changes within myofibers during exercise but also contributes to myofiber size and metabolic phenotype in the long term. Strikingly, sustained activation of cAMP signaling leads to pronounced hypertrophic responses in skeletal myofibers through largely elusive molecular mechanisms. These pathways can promote hypertrophy and combat atrophy in animal models of disorders including muscular dystrophy, age-related atrophy, denervation injury, disuse atrophy, cancer cachexia, and sepsis. cAMP also participates in muscle development and regeneration mediated by muscle precursor cells; thus, downstream signaling pathways may potentially be harnessed to promote muscle regeneration in patients with acute damage or muscular dystrophy. In this review, we summarize studies implicating cAMP signaling in skeletal muscle adaptation. We also highlight ligands that induce cAMP signaling and downstream effectors that are promising pharmacological targets.

Acute and chronic regulation of aldosterone production

Aldosterone is the major mineralocorticoid synthesized by the adrenal and plays an important role in the regulation of systemic blood pressure through the absorption of sodium and water. Aldosterone production is regulated tightly by selective expression of aldosterone synthase (CYP11B2) in the adrenal outermost zone, the zona glomerulosa. Angiotensin II (Ang II), potassium (K(+)) and adrenocorticotropin (ACTH) are the main physiological agonists which regulate aldosterone secretion. Aldosterone production is regulated within minutes of stimulation (acutely) through increased expression and phosphorylation of the steroidogenic acute regulatory (StAR) protein and over hours to days (chronically) by increased expression of the enzymes involved in the synthesis of aldosterone, particularly CYP11B2. Imbalance in any of these processes may lead to several disorders of aldosterone excess. In this review we attempt to summarize the key molecular events involved in the acute and chronic phases of aldosterone secretion.

Protein phosphatase 2A interacts with the Na,K-ATPase and modulates its trafficking by inhibition of its association with arrestin

BACKGROUND

The P-type ATPase family constitutes a collection of ion pumps that form phosphorylated intermediates during ion transport. One of the best known members of this family is the Na⁺,K⁺-ATPase. The catalytic subunit of the Na⁺,K⁺-ATPase includes several functional domains that determine its enzymatic and trafficking properties.

METHODOLOGY/PRINCIPAL FINDINGS

Using the yeast two-hybrid system we found that protein phosphatase 2A (PP2A) catalytic C-subunit is a specific Na⁺,K⁺-ATPase interacting protein. PP-2A C-subunit interacted with the Na⁺,K⁺-ATPase, but not with the homologous sequences of the H⁺,K⁺-ATPase. We confirmed that the Na⁺,K⁺-ATPase interacts with a complex of A- and C-subunits in native rat kidney. Arrestins and G-protein coupled receptor kinases (GRKs) are important regulators of G-protein coupled receptor (GPCR) signaling, and they also regulate Na⁺,K⁺-ATPase trafficking through direct association. PP2A inhibits association between the Na⁺,K⁺-ATPase and arrestin, and diminishes the effect of arrestin on Na⁺,K⁺-ATPase trafficking. GRK phosphorylates the Na⁺,K⁺-ATPase and PP2A can at least partially reverse this phosphorylation.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data demonstrate that the sodium pump belongs to a growing list of ion transport proteins that are regulated through direct interactions with the catalytic subunit of a protein phosphatase.

Effects of calcium channel antagonists on the motivational effects of nicotine and morphine in conditioned place aversion paradigm

The motivational component of drug withdrawal may contribute to drug seeking and relapse through the negative reinforcement-related process; thus, it is important to understand the mechanisms that mediate affective withdrawal behaviors. The present study was undertaken to examine the calcium-dependent mechanism of negative motivational symptoms of nicotine and morphine withdrawal using the conditioned place aversion (CPA) paradigm. Rats were chronically treated with nicotine (1.168 mg/kg, free base, s.c., 11 days, three times daily) or morphine (10 mg/kg,s.c., 11 days, twice daily). Then, during conditioning, rats pre-treated with nicotine or morphine received a nicotinic receptor antagonist mecamylamine (3.5 mg/kg) or an opioid receptor antagonist naloxone (1 mg/kg) to precipitate withdrawal in their initially preferred compartment, or saline in their non-preferred compartment. Our results demonstrated that after three conditioning sessions, mecamylamine induced a clear place aversion in rats that had previously received nicotine injections, and naloxone induced a significant place aversion in rats that had previously received morphine injections. Further, the major findings showed that calcium channel antagonists, i.e., nimodipine, verapamil and flunarizine (5 and 10 mg/kg, i.p.), injected before the administration of mecamylamine or naloxone, attenuated nicotine or morphine place aversion. As an outcome, these findings support the hypothesis that similar calcium-dependent mechanisms are involved in aversive motivational component associated with nicotine a morphine withdrawal. We can suggest that calcium channel blockers have potential for alleviating nicotine and morphine addiction by selectively decreasing the incentive motivational properties of both drugs, and may be beneficial as smoking cessation or opioid dependence pharmacotherapies.

Voltage-gated calcium channels

Voltage-gated calcium (Ca(2+)) channels are key transducers of membrane potential changes into intracellular Ca(2+) transients that initiate many physiological events. There are ten members of the voltage-gated Ca(2+) channel family in mammals, and they serve distinct roles in cellular signal transduction. The Ca(V)1 subfamily initiates contraction, secretion, regulation of gene expression, integration of synaptic input in neurons, and synaptic transmission at ribbon synapses in specialized sensory cells. The Ca(V)2 subfamily is primarily responsible for initiation of synaptic transmission at fast synapses. The Ca(V)3 subfamily is important for repetitive firing of action potentials in rhythmically firing cells such as cardiac myocytes and thalamic neurons. This article presents the molecular relationships and physiological functions of these Ca(2+) channel proteins and provides information on their molecular, genetic, physiological, and pharmacological properties.

Down-modulation of Ca2+ channels by endogenously released ATP and opioids: from the isolated chromaffin cell to the slice of adrenal medullae

Modifications in Ca(2+) influx may lead to profound changes in the cell activity associated with Ca(2+)-dependent processes, from muscle contraction and neurotransmitter release to calcium-mediated cell death. Therefore, calcium entry into the cell requires fine regulation. In this context, understanding of the modulation of voltage-dependent Ca(2+) channels seems to be critical. The modulatory process results in the enhancement or decrement of calcium influx that may regulate the local and global cytosolic Ca(2+) concentrations. Here, we summarize the well-established data on this matter described in isolated chromaffin cells by our laboratory and others, and the new results we have obtained in a more physiological preparation: freshly isolated slices of mouse adrenal medullae.

Fibronectin increases the force production of mouse papillary muscles via α5β1 integrin

The extracellular matrix (ECM) protein-integrin-cytoskeleton axis plays a central role as a mechanotransducing protein assemblage in many cell types. However, how the process of mechanotransduction and the mechanically generated signals arising from this axis affect myofilament function in cardiac muscle are not completely understood. We hypothesize that ECM proteins can regulate cardiac function through integrin binding, and thereby alter the intracellular calcium concentration ([Ca(2+)](i)) and/or modulate myofilament activation processes. Force measurements made in mouse papillary muscle demonstrated that in the presence of the soluble form of the ECM protein, fibronectin (FN), active force was increased significantly by 40% at 1 Hz, 54% at 2 Hz, 35% at 5 Hz and 16% at 9 Hz stimulation frequencies. Furthermore, increased active force in the presence of FN was associated with 12-33% increase in [Ca(2+)](i) and 20-50% increase in active force per unit Ca(2+). A function blocking antibody for α5 integrin prevented the effects of the FN on the changes in force and [Ca(2+)](i), whereas a function blocking α3 integrin antibody did not reverse the effects of FN. The effects of FN were reversed by an L-type Ca(2+) channel blocker, verapamil or PKA inhibitor. Freshly isolated cardiomyocytes exhibited a 39% increase in contraction force and a 36% increase in L-type Ca(2+) current in the presence of FN. Fibers treated with FN showed a significant increase in the phosphorylation of phospholamban; however, the phosphorylation of troponin I was unchanged. These results demonstrate that FN acts via α5β1 integrin to increase force production in myocardium and that this effect is partly mediated by increases in [Ca(2+)](i) and Ca(2+) sensitivity, PKA activation and phosphorylation of phospholamban.

Beta-adrenergic-regulated phosphorylation of the skeletal muscle Ca(V)1.1 channel in the fight-or-flight response

Ca(V)1 channels initiate excitation-contraction coupling in skeletal and cardiac muscle. During the fight-or-flight response, epinephrine released by the adrenal medulla and norepinephrine released from sympathetic nerves increase muscle contractility by activation of the β-adrenergic receptor/cAMP-dependent protein kinase pathway and up-regulation of Ca(V)1 channels in skeletal and cardiac muscle. Although the physiological mechanism of this pathway is well defined, the molecular mechanism and the sites of protein phosphorylation required for Ca(V)1 channel regulation are unknown. To identify the regulatory sites of phosphorylation under physiologically relevant conditions, Ca(V)1.1 channels were purified from skeletal muscle and sites of phosphorylation on the α1 subunit were identified by mass spectrometry. Two phosphorylation sites were identified in the proximal C-terminal domain, serine 1575 (S1575) and threonine 1579 (T1579), which are conserved in cardiac Ca(V)1.2 channels (S1700 and T1704, respectively). In vitro phosphorylation revealed that Ca(V)1.1-S1575 is a substrate for both cAMP-dependent protein kinase and calcium/calmodulin-dependent protein kinase II, whereas Ca(V)1.1-T1579 is a substrate for casein kinase 2. Treatment of rabbits with isoproterenol to activate β-adrenergic receptors increased phosphorylation of S1575 in skeletal muscle Ca(V)1.1 channels in vivo, and treatment with propranolol to inhibit β-adrenergic receptors reduced phosphorylation. As S1575 and T1579 in Ca(V)1.1 channels and their homologs in Ca(V)1.2 channels are located at a key regulatory interface between the distal and proximal C-terminal domains, it is likely that phosphorylation of these sites in skeletal and cardiac muscle is directly involved in calcium channel regulation in response to the sympathetic nervous system in the fight-or-flight response.

Signaling complexes of voltage-gated sodium and calcium channels

Membrane depolarization and intracellular Ca(2+) transients generated by activation of voltage-gated Na+ and Ca(2+) channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation-contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. This article reviews experimental results showing that Na+ and Ca(2+) channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein-protein interactions are important for regulation of cellular plasticity through modulation of Na+ channel function in brain neurons, for short-term synaptic plasticity through modulation of presynaptic Ca(V)2 channels, and for the fight-or-flight response through regulation of postsynaptic Ca(V)1 channels in skeletal and cardiac muscle. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation-contraction coupling.

Preliminary experience with dexmedetomidine for monitored anesthesia care during ENT surgical procedures

Dexmedetomidine is an alpha2-adrenergic agonist that produces anxiolysis, amnesia, sedation, potentiation of opioid analgesia, and sympatholysis. It is currently approved by the U.S. Food & Drug Administration for the sedation of adults in the intensive care setting for up to 24 hours during mechanical ventilation. Given its beneficial sedative and anxiolytic properties and limited adverse effect profile, it has been used in several other clinical scenarios. The authors present their experience using dexmedetomidine for monitored anesthesia care (MAC) during "awake" ENT procedures such as thyroplasty, a procedure requiring a patient to verbalize when requested but to otherwise remain immobile to allow for completion of the procedure, and in a patient with post-polio syndrome with poor pulmonary reserve requiring esophagoscopy with dilation and botulinum toxin injection for cricopharyngeal dysfunction. Our preliminary experience suggests that dexmedetomidine provides effective sedation as the primary agent for MAC during such procedures in adult patients. The end-organ effects of dexmedetomidine and previous reports of its use during MAC are reviewed.

PKA microdomain organisation and cAMP handling in healthy and dystrophic muscle in vivo

Signalling through protein kinase A (PKA) triggers a multitude of intracellular effects in response to a variety of extracellular stimuli. To guarantee signal specificity, different PKA isoforms are compartmentalised by Akinase anchoring proteins (AKAPs) into functional microdomains. By using genetically encoded fluorescent reporters of cAMP concentration that are targeted to the intracellular sites where PKA type I and PKA type II isoforms normally reside, we directly show for the first time spatially and functionally separate PKA microdomains in mouse skeletal muscle in vivo. The reporters localised into clearly distinct patterns within sarcomers, from where they could be displaced by means of AKAP disruptor peptides indicating the presence of disparate PKA type I and PKA type II anchor sites within skeletal muscle fibres. The functional relevance of such differential localisation was underscored by the finding of mutually exclusive and AKAP-dependent increases in [cAMP] in the PKA type I and PKA type II microdomains upon application of different cAMP agonists. Specifically, the sensors targeted to the PKA type II compartment responded only to norepinephrine, whereas those targeted to the PKA type I compartment responded only to alpha-calcitonin gene-related peptide. Notably, in dystrophic mdx mice the localisation pattern of the reporters was altered and the functional separation of the cAMP microdomains was abolished. In summary, our data indicate that an efficient organisation in microdomains of the cAMP/PKA pathway exists in the healthy skeletal muscle and that such organisation is subverted in dystrophic skeletal muscle.

Regulation of Sodium and Calcium Channels by Signaling Complexes

Membrane depolarization and intracellular calcium transients generated by activation of voltage-gated sodium and calcium channels are local signals, which initiate physiological processes such as action potential conduction, synaptic transmission, and excitation-contraction coupling. Targeting of effector proteins and regulatory proteins to ion channels is an important mechanism to ensure speed, specificity, and precise regulation of signaling events in response to local stimuli. In this article, we review recent experimental results showing that sodium and calcium channels form local signaling complexes, in which effector proteins, anchoring proteins, and regulatory proteins interact directly with ion channels. The intracellular domains of these channels serve as signaling platforms, mediating their participation in intracellular signaling processes. These protein-protein interactions are important for efficient synaptic transmission and for regulation of ion channels by neurotransmitters and intracellular second messengers. These localized signaling complexes are essential for normal function and regulation of electrical excitability, synaptic transmission, and excitation-contraction coupling.

The role of ether-a-go-go-related gene K(+) channels in glucocorticoid inhibition of adrenocorticotropin release by rat pituitary cells

The present study investigated the role of K(+) channels in the inhibitory effect of glucocorticoid on adrenocorticotropin (ACTH) release induced by corticotropin-releasing hormone (CRH) using cultured rat anterior pituitary cells. Apamin and charybdotoxin (CTX) did not have a significant effect on ACTH release induced by CRH (1 nM). Tetraethylammonium (TEA), a broad spectrum K(+) channel blocker, increased the ACTH response to CRH only at the highest concentration (10 mM). The exposure to 100 nM corticosterone for 60 min inhibited the CRH-induced ACTH release. Neither TEA, apamin, nor CTX affected the inhibitory effect of corticosterone. In contrast, astemizole (Ast) and E-4031, ether-a-go-go-related gene (erg) K(+) channel blockers, abolished the inhibitory effect of corticosterone on CRH-induced ACTH release (1.25+/-0.10 vs. 1.45+/-0.11 ng/well at 10 microM Ast, p>0.05, 1.71+/-0.16 vs. 1.91+/-0.32 ng/well at 10 microM E-4031, p>0.05, vehicle vs. corticosterone). RT-PCR demonstrated all three subtypes of rat-erg mRNAs in the pituitary and corticosterone increased only erg1 mRNA up to 2.47+/-0.54 fold. In conclusion, erg K(+) channels were up-regulated by glucocorticoid, and have indispensable roles in delayed glucocorticoid inhibition of CRH-induced ACTH release by rat pituitary cells.

Calcium channels in chromaffin cells: focus on L and T types

Voltage-gated Ca2+ channels (Cav) are highly expressed in the adrenal chromaffin cells of mammalian species. Besides shaping action potential waveforms, they are directly involved in the excitation-secretion coupling underlying catecholamine release and, possibly, control other Ca2+-dependent events that originate near the membrane. These functions are shared by a number of Cav channel types (L, N, P/Q, R and T) which have different structure-function characteristics and whose degree of expression changes remarkably among mammalian species. Understanding precisely the functioning of each voltage-gated Ca2+ channels is a crucial task that helps clarifying the Ca2+-dependent mechanisms controlling exocytosis during physiological and pathological conditions. In this paper, we focus on classical and new roles that L- and T-type channels play in the control of chromaffin cell excitability and neurotransmitter release. Interestingly, L-type channels are shown to be implicated in the spontaneous autorhythmicity of chromaffin cells, while T-type channels, which are absent in adult chromaffin cells, are coupled with secretion and can be recruited following long-term beta-adrenergic stimulation or chronic hypoxia. This suggests that like other cells, adrenal chromaffin cells undergo effective remodelling of membrane ion channels and cell functioning during prolonged stress conditions.

Ca2+/CaM-dependent inactivation of the skeletal muscle L-type Ca2+ channel (Cav1.1)

Ca2+-dependent modulation via calmodulin (CaM) has been documented for most high-voltage-activated Ca2+ channels, but whether the skeletal muscle L-type channel (Cav1.1) exhibits this property has been unknown. In this paper, whole-cell current and fluorescent resonance energy transfer (FRET) recordings were obtained from cultured mouse myotubes to test for potential involvement of CaM in function of Cav1.1. When prolonged depolarization (800 ms) was used to evoke Cav1.1 currents in normal myotubes, the fraction of current remaining at the end of the pulse displayed classic signs of Ca2+-dependent inactivation (CDI), including U-shaped voltage dependence, maximal inactivation (approximately 30%) at potentials eliciting maximal inward current, and virtual elimination of inactivation when Ba2+ replaced external Ca2+ or when 10 mM BAPTA was included in the pipette solution. Furthermore, CDI was virtually eliminated (from 30 to 8%) in normal myotubes overexpressing mutant CaM (CaM1234) that does not bind Ca2+, whereas CDI was unaltered in myotubes overexpressing wild-type CaM (CaMwt). In addition, a significant FRET signal (E=4.06%) was detected between fluorescently tagged Cav1.1 and CaMwt coexpressed in dysgenic myotubes, demonstrating for the first time that these two proteins associate in vivo. These findings show that CaM associates with and modulates Cav1.1.

Voltage-dependent and Src-mediated regulation of NMDA receptor single channel outward currents in cortical neurons

A voltage-dependent but Ca2+-independent regulation of N-methyl-D-aspartate (NMDA) receptor outward activity was studied at the single channel level using outside-out patches of cultured mouse cortical neurons. Unlike the inward activity associated with Ca2+ and Na+ influx, the NMDA receptor outward K+ conductance was unaffected by changes in Ca2+ concentration. Following a depolarizing pre-pulse, the single channel open probability (NP o), amplitude, and open duration of the NMDA inward current decreased, whereas the same pre-depolarization increased those parameters of the NMDA outward current (pre-pulse facilitation). The outward NP o was increased by the pre-pulse facilitation, disregarding Ca2+ changes. The voltage-current relationships of the inward and outward currents were shifted by the pre-depolarization toward opposite directions. The Src family kinase inhibitor, PP1, and the Src kinase antibody, but not the anti-Fyn antibody, blocked the pre-pulse facilitation of the NMDA outward activity. On the other hand, a hyperpolarizing pre-pulse showed no effect on NMDA inward currents but inhibited outward currents (pre-pulse depression). Application of Src kinase, but not Fyn kinase, prevented the pre-pulse depression. We additionally showed that a depolarization pre-pulse potentiated miniature excitatory synaptic currents (mEPSCs). The effect was blocked by application of the NMDA receptor antagonist AP-5 during depolarization. These data suggest a voltage-sensitive regulation of NMDA receptor channels mediated by Src kinase. The selective changes in the NMDA receptor-mediated K+ efflux may represent a physiological and pathophysiological plasticity at the receptor level in response to dynamic changes in the membrane potential of central neurons.

Diversity of cAMP-Dependent Protein Kinase Isoforms and Their Anchoring Proteins in Mouse Ventricular Tissue

Using a chemical proteomics approach, we efficiently enriched for the generally low abundant cAMP signaling proteins, and their interactors, directly from mouse ventricular tissue. The presence of undesired contaminating (noncyclic) nucleotide-binding proteins was diminished using a tailored sequential elution protocol. Through further optimization of this affinity purification and elution protocol, we were able to detect all known protein kinase A regulatory isoforms (PKA-R). Furthermore, 11 different A-kinase anchoring proteins (AKAPs) were detected. A proposed fusion protein of paralemmin 2 and AKAP2 could be decisively established as a novel AKAP at the protein level in ventricular tissue. When comparing this dataset of cAMP-affinity purified proteins with earlier data obtained with immobilized cGMP from rat ventricular tissue, we observe a large overlap in the retained proteins but also some clear differences. Furthermore, implementation of an in-depth analysis of in vivo phosphorylation sites on PKA-R revealed the presence of several differentially phosphorylated PKA-R isoforms. This illustrates yet another layer of functional regulation in cyclic nucleotide signaling. In general, our improved chemical proteomics screen offers a broad, but detailed, view on nature's complex diversity in cyclic nucleotide signaling mechanisms. Possibly different AKAP-isoforms may direct differentially phosphorylated PKA-R isoforms to different cellular compartments, providing a multifaceted platform for just this kinase.

Dexmedetomidine: applications in pediatric critical care and pediatric anesthesiology

OBJECTIVE

To provide a general descriptive account of the end-organ effects of dexmedetomidine and to provide an evidence-based review of the literature regarding its use in infants and children.

DATA SOURCE

A computerized bibliographic search of the literature regarding dexmedetomidine.

MAIN RESULTS

The end-organ effects of dexmedetomidine have been well studied in animal and adult human models. Adverse cardiovascular effects include occasional episodes of bradycardia with rare reports of sinus pause or cardiac arrest. Hypotension has also been reported as well as hypertension, the latter thought to be due to peripheral alpha2B agonism with peripheral vasoconstriction. Although dexmedetomidine has no direct effects on myocardial function, decreased cardiac output may result from changes in heart rate or increases in afterload. There are somewhat conflicting reports in the literature regarding its effects on ventilatory function, with some studies (both human and animal) suggesting a mild degree of respiratory depression, decreased minute ventilation, and decreased response to CO2 challenge whereas others demonstrate no effect. The central nervous system effects include sedation and analgesia with prevention of recall and memory at higher doses. Dexmedetomidine may also provide some neuroprotective activity during periods of ischemia. Applications in infants and children have included sedation during mechanical ventilation, prevention of emergence agitation following general anesthesia, provision of procedural sedation, and the prevention of withdrawal following the prolonged administration of opioids and benzodiazepines.

CONCLUSIONS

The literature contains reports of the use of dexmedetomidine in approximately 800 pediatric patients. Given its favorable sedative and anxiolytic properties combined with its limited effects on hemodynamic and respiratory function, there is growing interest in and reports of its use in the pediatric population in various clinical scenarios.

Inhibitors of cyclic nucleotide phosphodiesterase 3 and 5 as therapeutic agents in heart failure

Cyclic nucleotide phosphodiesterases (PDE) 3 and 5 regulate cAMP and cGMP signalling in cardiac and smooth muscle myocytes. Important advances in the understanding of the roles of these enzymes have recently been made. PDE3 inhibitors have inotropic and vasodilatory properties, and although they acutely improve haemodynamics in patients with heart failure, they do not improve long-term morbidity and mortality. Although combination therapy with beta-adrenergic receptor antagonists or selective inhibition of specific PDE3 isoforms might result in a more favourable long-term outcome, more clinical data are needed to test this proposition. The role of PDE5 inhibitors in the treatment of cardiac disease is evolving. PDE5 inhibitors cause pulmonary and systemic vasodilation. How these drugs will compare with other vasodilators in terms of long-term outcomes in patients with heart failure is unknown. Recent studies also suggest that PDE5 inhibitors may have antihypertropic effects, exerted through increased myocardial cGMP signalling, that could be of additional benefit in patients with heart failure.

Merg1a K+ channel induces skeletal muscle atrophy by activating the ubiquitin proteasome pathway

Skeletal muscle atrophy results from an imbalance in protein degradation and protein synthesis and occurs in response to injury, various disease states, disuse, and normal aging. Current treatments for this debilitating condition are inadequate. More information about mechanisms involved in the onset and progression of muscle atrophy is necessary for development of more effective therapies. Here we show that expression of the mouse ether-a-go-go related gene (Merg1a) K+ channel is up-regulated in skeletal muscle of mice experiencing atrophy as a result of both malignant tumor expression and disuse. Further, ectopic expression of Merg1a in vivo induces atrophy in healthy wt-bearing mice, while expression of a dysfunctional Merg1a mutant suppresses atrophy in hindlimb-suspended mice. Treatment of hindlimb-suspended mice with astemizole, a known Merg1a channel blocker, inhibits atrophy in these animals. Importantly, in vivo expression of Merg1a in mouse skeletal muscle activates the ubiquitin proteasome pathway that is responsible for the majority of protein degradation that causes muscle atrophy, yet expression of a dysfunctional Merg1a mutant decreases levels of ubiquitin-proteasome proteolysis. Thus, expression of Merg1a likely initiates atrophy by activating ubiquitin-proteasome proteolysis. This gene and its product are potential targets for prevention and treatment of muscle atrophy.

Integrin Receptor Activation Triggers Converging Regulation of Cav1.2 Calcium Channels by c-Src and Protein Kinase A Pathways

L-type, voltage-gated Ca2+ channels (CaL) play critical roles in brain and muscle cell excitability. Here we show that currents through heterologously expressed neuronal and smooth muscle CaL channel isoforms are acutely potentiated following alpha5beta1 integrin activation. Only the alpha1C pore-forming channel subunit is critical for this process. Truncation and site-directed mutagenesis strategies reveal that regulation of Cav1.2 by alpha5beta1 integrin requires phosphorylation of alpha1C C-terminal residues Ser1901 and Tyr2122. These sites are known to be phosphorylated by protein kinase A (PKA) and c-Src, respectively, and are conserved between rat neuronal (Cav1.2c) and smooth muscle (Cav1.2b) isoforms. Kinase assays are consistent with phosphorylation of these two residues by PKA and c-Src. Following alpha5beta1 integrin activation, native CaL channels in rat arteriolar smooth muscle exhibit potentiation that is completely blocked by combined PKA and Src inhibition. Our results demonstrate that integrin-ECM interactions are a common mechanism for the acute regulation of CaL channels in brain and muscle. These findings are consistent with the growing recognition of the importance of integrin-channel interactions in cellular responses to injury and the acute control of synaptic and blood vessel function.

Direct in vivo monitoring of sarcoplasmic reticulum Ca2+ and cytosolic cAMP dynamics in mouse skeletal muscle

Skeletal muscle contraction depends on the release of Ca(2+) from the sarcoplasmic reticulum (SR), but the dynamics of the SR free Ca(2+) concentration ([Ca(2+)](SR)), its modulation by physiological stimuli such as catecholamines, and the concomitant changes in cAMP handling have never been directly determined. We used two-photon microscopy imaging of GFP-based probes expressed in mouse skeletal muscles to monitor, for the first time in a live animal, the dynamics of [Ca(2+)](SR) and cAMP. Our data, which were obtained in highly physiological conditions, suggest that free [Ca(2+)](SR) decreases by approximately 50 microM during single twitches elicited through nerve stimulation. We also demonstrate that cAMP levels rise upon beta-adrenergic stimulation, leading to an increased efficacy of the Ca(2+) release/reuptake cycle during motor nerve stimulation.

Inward rectification of the AKT2 channel abolished by voltage-dependent phosphorylation

The Arabidopsis K(+) channel AKT2 possesses the remarkable property that its voltage threshold for activation can be either within the physiological range (gating mode 1), or shifted towards considerably more positive voltages (gating mode 2). Gating mode 1 AKT2 channels behave as delayed K(+)-selective inward rectifiers; while gating mode 2 AKT2 channels are K(+)-selective 'open leaks' in the physiological range of membrane potential. In the present study we have investigated modulation of AKT2 current by effectors of phosphatases/kinases in COS cells and Xenopus oocytes. These experiments show that (i) dephosphorylation can result in AKT2 channel silencing; and (ii) phosphorylation by protein kinase A (PKA) favors both recruitment of silenced AKT2 channels and transition from gating mode 1 to gating mode 2. Interestingly, phosphorylation of AKT2 by PKA in COS cells and Xenopus oocytes is favored by hyperpolarization. Two PKA phosphorylation sites (S210 and S329) were pinpointed in the region of the pore inner mouth. The role of these phosphorylation sites in the switch between the two gating modes was assessed by electrophysiological characterization of mutant channels. The molecular aspects of AKT2 regulation by phosphorylation, and the possible physiological meaning of such regulation in the plant context, are discussed.

Activation of mGluR5 modulates Ca2+ currents in retinal amacrine cells from the chick

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.