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Presents information on a study which demonstrated that induction of the cholesterol metabolic pathway by growth of embryonic chicken atrial cells in medium supplemented with lipoprotein-depleted serum. Materials and methods; Results; Discussion of the results.

The pathophysiology of pulmonary hypertension (PH) and heart failure (HF) includes fibrogenic remodeling associated with the loss of pulmonary arterial (PA) and cardiac compliance. We and others have previously identified transglutaminase 2 (TG2) as a participant in adverse fibrogenic remodeling. However, little is known about the biologic mechanisms that regulate TG2 function. We examined physiological mouse models of experimental PH, HF, and type 1 diabetes that are associated with altered glucose metabolism/glycolysis and report here that TG2 expression and activity are elevated in pulmonary and cardiac tissues under all these conditions. We additionally used PA adventitial fibroblasts to test the hypothesis that TG2 is an intermediary between enhanced tissue glycolysis and fibrogenesis. Our in vitro results show that glycolytic enzymes and TG2 are upregulated in fibroblasts exposed to high glucose, which stimulates cellular glycolysis as measured by Seahorse analysis. We examined the relationship of TG2 to a terminal glycolytic enzyme, pyruvate kinase M2 (PKM2), and found that PKM2 regulates glucose‐induced TG2 expression and activity as well as fibrogenesis. Our studies further show that TG2 inhibition blocks glucose‐induced fibrogenesis and cell proliferation. Our findings support a novel role for glycolysis‐mediated TG2 induction and tissue fibrosis associated with experimental PH, HF, and hyperglycemia. [ABSTRACT FROM AUTHOR]

Type 1 diabetic Akita mice develop severe cardiac parasympathetic dysfunction that we have previously demonstrated is due at least in part to an abnormality in the response of the end organ to parasympathetic stimulation. Specifically, we had shown that hypoinsulinemia in the diabetic heart results in attenuation of the G-protein coupled inward rectifying K channel (GIRK) which mediates the negative chronotropic response to parasympathetic stimulation due at least in part to decreased expression of the GIRK1 and GIRK4 subunits of the channel. We further demonstrated that the expression of GIRK1 and GIRK4 is under the control of the Sterol Regulatory element Binding Protein (SREBP-1), which is also decreased in response to hypoinsulinemia. Finally, given that hyperactivity of Glycogen Synthase Kinase (GSK)3β, had been demonstrated in the diabetic heart, we demonstrated that treatment of Akita mice with Li+, an inhibitor of GSK3β, increased parasympathetic responsiveness and SREBP-1 levels consistent with the conclusion that GSK3β might regulate IKACh via an effect on SREBP-1. However, inhibitor studies were complicated by lack of specificity for GSK3β. Here we generated an Akita mouse with cardiac specific inducible knockout of GSK3β. Using this mouse, we demonstrate that attenuation of GSK3β expression is associated with an increase in parasympathetic responsiveness measured as an increase in the heart rate response to atropine from 17.3 ± 3.5% (n = 8) prior to 41.2 ± 5.4% (n = 8, P = 0.017), an increase in the duration of carbamylcholine mediated bradycardia from 8.43 ± 1.60 min (n = 7) to 12.71 ± 2.26 min (n = 7, P = 0.028) and an increase in HRV as measured by an increase in the high frequency fraction from 40.78 ± 3.86% to 65.04 ± 5.64 (n = 10, P = 0.005). Furthermore, patch clamp measurements demonstrated a 3-fold increase in acetylcholine stimulated peak IKACh in atrial myocytes from GSK3β deficiency mice compared with control. Finally, western blot analysis of atrial extracts from knockout mice demonstrated increased levels of SREBP-1, GIRK1 and GIRK4 compared with control. Taken together with our prior observations, these data establish a role of increased GSK3β activity in the pathogenesis of parasympathetic dysfunction in type 1 diabetes via the regulation of IKACh and GIRK1/4 expression. [ABSTRACT FROM AUTHOR]

Abdominal Aortic Aneurysm (AAA) is a major cause of mortality and morbidity in men over 65 years of age. Male apolipoprotein E knockout (ApoE −/− ) mice infused with angiotensin II (AngII) develop AAA. Although AngII stimulates both JAK/STAT and Toll-like receptor 4 (TLR4) signaling pathways, their involvement in AngII mediated AAA formation is unclear. Here we used the small molecule STAT3 inhibitor, S3I-201, the TLR4 inhibitor Eritoran and ApoE −/− TLR4 −/− mice to evaluate the interaction between STAT3 and TLR4 signaling in AngII-induced AAA formation. ApoE −/− mice infused for 28 days with AngII developed AAAs and increased STAT3 activation and TLR4 expression. Moreover, AngII increased macrophage infiltration and the ratio of M1 (pro-inflammatory)/M2 (healing) macrophages in aneurysmal tissue as early as 7–10 days after AngII infusion. STAT3 inhibition with S3I-201 decreased the incidence and severity of AngII-induced AAA formation and decreased MMP activity and the ratio of M1/M2 macrophages. Furthermore, AngII-mediated AAA formation, MMP secretion, STAT3 phosphorylation and the ratio of M1/M2 macrophages were markedly decreased in ApoE −/− TLR4 −/− mice, and in Eritoran-treated ApoE −/− mice. TLR4 and pSTAT3 levels were also increased in human aneurysmal tissue. These data support a role of pSTAT3 in TLR4 dependent AAA formation and possible therapeutic roles for TLR4 and/or STAT3 inhibition in AAA. [ABSTRACT FROM AUTHOR]

Decreased heart rate variability (HRV) is a major risk factor for sudden death and cardiovascular disease. We previously demonstrated that parasympathetic dysfunction in the heart of the Akita type 1 diabetic mouse was due to a decrease in the level of the sterol response element-binding protein (SREBP-1). Here we demonstrate that hyperactivity of glycogen synthase kinase-3β (GSK3β) in the atrium of the Akita mouse results in decreased SREBP-1, attenuation of parasympathetic modulation of heart rate, measured as a decrease in the high-frequency (HF) fraction of HRV in the presence of propranolol, and a decrease in expression of the G-protein coupled inward rectifying K+ (GIRK4) subunit of the acetylcholine (ACh)-activated inward-rectifying K+ channel (IKACh), the ion channel that mediates the heart rate response to parasympathetic stimulation. Treatment of atrial myocytes with the GSK3β inhibitor Kenpaullone increased levels of SREBP-1 and expression of GIRK4 and IKACh, whereas a dominant-active GSK3β mutant decreased SREBP-1 and GIRK4 expression. In Akita mice treated with GSK3β inhibitors Li+ and/or CHIR-99021, Li+ increased IKACh, and Li+ and CHIR-99021 both partially reversed the decrease in HF fraction while increasing GIRK4 and SREBP-1 expression. These data support the conclusion that increased GSK3β activity in the type 1 diabetic heart plays a critical role in parasympathetic dysfunction through an effect on SREBP-1, supporting GSK3β as a new therapeutic target for diabetic autonomic neuropathy. [ABSTRACT FROM AUTHOR]

Although a reduction in the high-frequency (HF) component of heart rate variability (HRV) is a major complication of diabetes and a risk factor for sudden death, its relationship to ventricular tachycardia (VT) is unknown. We developed a mouse model for the study of VT and its relationship to changes in HRV in the Akita type 1 diabetic mouse. Programmed ventricular stimulation of anesthetized mice demonstrated that Akita mice were more inducible for VT compared with wild-type mice: 78.6% versus 28.6% (P < 0.007). Optical mapping of perfused hearts demonstrated multifocal breakthroughs that occasionally gave rise to short-lived rotors consistent with focal initiation and maintenance of VT. Treatment of Akita mice with pravastatin, which had been previously shown clinically to decrease ventricular ectopy and to increase HRV, decreased the inducibility of VT: 36.8% compared with 75.0% with placebo treatment (P < 0.022). The HF fraction of HRV was reduced in Akita mice (48.6 ± 5.2% vs. 70.9 ± 4.8% in wild-type mice, P < 0.005) and was increased compared with placebo treatment in pravastatin-treated mice. Pretreatment of Akita mice with the muscarinic agonist carbamylcholine or the β-adrenergic receptor blocker propranolol decreased the inducibility of VT (P = 0.001). In conclusion, the increased inducibility of focally initiated VT and reduced HF fraction in Akita mice were partially reversed by both pravastatin treatment and pharmacologic reversal of parasympathetic dysfunction. In this new animal model for the study of the pathogenesis of VT in type 1 diabetes, pravastatin may play a role in the prevention of VT by attenuating parasympathetic dysfunction. [ABSTRACT FROM AUTHOR]

Background A test dose is used to detect intravascular injection during neuraxial block in pediatrics. Accidental intravascular epidural local anesthetic injection might be unrecognized in anesthetized children leading to potential life-threatening complications. In children, sevoflurane anesthesia blunts the hemodynamic response when intravascular cathecolamines are administered. No studies have explored the hemodynamics and the criteria for a positive test dose result following phenylephrine in sevoflurane anesthetized children. Methods Healthy children undergoing minor procedures were randomly assigned to receive intravenous placebo, or 5 μg∙kg−1 phenylephrine ( n = 11/group) during sevoflurane anesthesia. Hemodynamic response was assessed using electrocardiography, pulse oxymetry and non-invasive blood pressure monitoring for 5 min following drug administration in anesthetized patients. Results All patients receiving phenylephrine showed a decreased heart rate (HR) but not all of them met the positive criterion for test dose response. Overall, at 1 min, patients receiving phenylephrine showed a 25% decrease in HR from the baseline while an increase in blood pressure was noticed in 54% of patients receiving phenylephrine. Discussion Phenylephrine might be a future indicator of positive intravascular test dose. Further investigation is needed to find out the phenylephrine dose that elicits a reliable hemodynamic response and whether phenylephrine needs to be dose age-adjusted in order to appreciate relevant hemodynamic changes in children receiving neuraxial blocks undergoing general anesthesia. [ABSTRACT FROM AUTHOR]

Retrospective studies suggest that statins might exert an antiarrhythmic effect on the heart. The mechanism of this effect is unclear. Parasympathetic stimulation of the heart has been shown to protect against ventricular arrhythmias. The goal of this study was to determine the effect of statins on ventricular arrhythmias and its correlation with changes in parasympathetic responsiveness and Galpha(i2) expression. Patients were randomized to pravastatin and simvastatin in a double-blind crossover design. Ventricular arrhythmias were determined by analysis of 24-hour Holter recordings. Spectral RR interval analysis of Holter studies determined peak high-frequency power fraction, which reflects parasympathetic modulation of heart rate. Expression of Galpha(i2), a molecular component of the parasympathetic response pathway, was determined by Western blots of patients' lymphocytes. Pravastatin treatment decreased the incidence of ventricular premature complexes by 22.5 + or - 3.4% (n = 20, p <0.05), couplets, and runs of 3 to 6 beats of nonsustained ventricular tachycardia from 9.8 + or - 2.67 to 3.9 + or - 1.25 events/patient/24 hours (n = 12, p <0.05). Pravastatin increased peak high-frequency fraction by 29.8 + or - 4.3% (n = 33, p <0.001), while Galpha(i2) expression increased by 51.3 + or - 22.5% (n = 21, p <0.05). Effects of simvastatin on ventricular premature complexes and nonsustained ventricular tachycardia were not significant. Relative changes in couplets and nonsustained ventricular tachycardia in pravastatin-treated patients correlated negatively with changes in Galpha(i2) and high-frequency fraction (rho = -0.588 and rho = -0.763, respectively, n = 12, p <0.05). In conclusion, these data suggest that pravastatin might decrease cardiac irritability via an increase in parasympathetic responsiveness and that changes in Galpha(i2) expression might serve as a molecular marker for this effect, which might play a role in the molecular mechanism of the antiarrhythmic effect of statins. [ABSTRACT FROM AUTHOR]

Retrospective studies suggest that statins might exert an antiarrhythmic effect on the heart. The mechanism of this effect is unclear. Parasympathetic stimulation of the heart has been shown to protect against ventricular arrhythmias. The goal of this study was to determine the effect of statins on ventricular arrhythmias and its correlation with changes in parasympathetic responsiveness and Gαi2 expression. Patients were randomized to pravastatin and simvastatin in a double-blind crossover design. Ventricular arrhythmias were determined by analysis of 24-hour Holter recordings. Spectral RR interval analysis of Holter studies determined peak high-frequency power fraction, which reflects parasympathetic modulation of heart rate. Expression of Gαi2, a molecular component of the parasympathetic response pathway, was determined by Western blots of patients'' lymphocytes. Pravastatin treatment decreased the incidence of ventricular premature complexes by 22.5 ± 3.4% (n = 20, p <0.05), couplets, and runs of 3 to 6 beats of nonsustained ventricular tachycardia from 9.8 ± 2.67 to 3.9 ± 1.25 events/patient/24 hours (n = 12, p <0.05). Pravastatin increased peak high-frequency fraction by 29.8 ± 4.3% (n = 33, p <0.001), while Gαi2 expression increased by 51.3 ± 22.5% (n = 21, p <0.05). Effects of simvastatin on ventricular premature complexes and nonsustained ventricular tachycardia were not significant. Relative changes in couplets and nonsustained ventricular tachycardia in pravastatin-treated patients correlated negatively with changes in Gαi2 and high-frequency fraction (ρ = −0.588 and ρ = −0.763, respectively, n = 12, p <0.05). In conclusion, these data suggest that pravastatin might decrease cardiac irritability via an increase in parasympathetic responsiveness and that changes in Gαi2 expression might serve as a molecular marker for this effect, which might play a role in the molecular mechanism of the antiarrhythmic effect of statins. [Copyright &y& Elsevier]

Parasympathetic stimulation of the heart, which provides protection from arrhythmias and sudden death, involves activation of the G protein-coupled inward rectifying K+ channel GIRK1/4 and results in an acetylcholine-sensitive K+ current, I KACh. We describe a unique relationship between lipid homeostasis, the lipid-sensitive transcription factor SREBP-1, regulation of the cardiac parasympathetic response, and the development of ventricular arrhythmia. In embryonic chick atrial myocytes, lipid lowering by culture in lipoprotein-depleted serum increased SREBP-1 levels, GIRK1 expression, and I KACh activation. Regulation of the GIRK1 promoter by SREBP-1 and lipid lowering was dependent on interaction with 2 tandem sterol response elements and an upstream E-box motif. Expression of dominant negative SREBP-1 (DN-SREBP-1) reversed the effect of lipid lowering on I KACh and GIRK1. In SREBP-1 knockout mice, both the response of the heart to parasympathetic stimulation and the expression of GIRK1 were reduced compared with WT. I KACh, attenuated in atrial myocytes from SREBP-1 knockout mice, was stimulated by SREBP-1 expression. Following myocardial infarction, SREBP-1 knockout mice were twice as likely as WT mice to develop ventricular tachycardia in response to programmed ventricular stimulation. These results demonstrate a relationship between lipid metabolism and parasympathetic response that may play a role in arrhythmogenesis. [ABSTRACT FROM AUTHOR]

Parasympathetic stimulation of the heart, which provides protection from arrhythmias and sudden death, involves activation of the G protein-coupled inward rectifying K+ channel GIRK1/4 and results in an acetylcholine-sensitive K+ current, IKACh. We describe a unique relationship between lipid homeostasis, the lipid-sensitive transcription factor SREBP-1, regulation of the cardiac parasympathetic response, and the development of ventricular arrhythmia. In embryonic chick atrial myocytes, lipid lowering by culture in lipoprotein-depleted serum increased SREBP-1 levels, GIRK1 expression, and IKACh activation. Regulation of the GIRK1 promoter by SREBP-1 and lipid lowering was dependent on interaction with 2 tandem sterol response elements and an upstream E-box motif. Expression of dominant negative SREBP-1 (DN-SREBP-1) reversed the effect of lipid lowering on IKACh and GIRK1. In SREBP-1 knockout mice, both the response of the heart to parasympathetic stimulation and the expression of GIRK1 were reduced compared with WT. IKACh, attenuated in atrial myocytes from SREBP-1 knockout mice, was stimulated by SREBP-1 expression. Following myocardial infarction, SREBP-1 knockout mice were twice as likely as WT mice to develop ventricular tachycardia in response to programmed ventricular stimulation. These results demonstrate a relationship between lipid metabolism and parasympathetic response that may play a role in arrhythmogenesis. [ABSTRACT FROM AUTHOR]

Abstract: HMG-CoA reductase inhibitors (statins) are widely used in the treatment and prevention of atherosclerosis. Here we demonstrate that the HMG-CoA reductase inhibitor simvastatin potentiates TNFα-mediated apoptosis and TNFα signaling in human umbilical vein endothelial cells (HUVECs). While 2.5 μM simvastatin or 40 ng/ml TNFα alone had only a small effect on apoptosis in HUVECs, co-incubation with simvastatin and TNFα markedly increased apoptosis in a time- and dose-dependent manner as measured by FACS analysis of propidium iodide-stained cells. Geranylgeraniol, which serves as a substrate for the geranylgeranylation of small GTP binding proteins such as RhoA, which is required for the function and membrane localization of Rho, reversed the effect of simvastatin on apoptosis. GGTI, an inhibitor of protein geranylgeranylation, mimicked the effect of simvastatin on apoptosis and interfered with the membrane localization of RhoA. Furthermore, simvastatin increased the expression of the TNFα type I receptor (TNFαRI) with a dose dependence and a dependence on geranylgeranylation similar to that demonstrated for the potentiation of TNFα-mediated apoptosis. Adenoviral expression of a dominant-negative RhoA mimicked the effect of simvastatin on the expression of TNFαRI, while adenoviral expression of a dominant-activating RhoA mutant reversed the effect of simvastatin on the expression of TNFαRI. Simvastatin also potentiated TNFα signaling as determined by increased TNFα-mediated E-selectin expression. These data support the conclusion that TNFα signaling is under the negative control of RhoA and that statins potentiate TNFα signaling at least in part via interference with RhoA inhibition of TNFα type I receptor expression. [Copyright &y& Elsevier]

Transforming growth factor β (TGFβ) signaling is involved in the development and regulation of multiple organ systems and cellular signaling pathways. We recently demonstrated that TGFβ regulates the response of atrial myocytes to parasympathetic stimulation. Here, TGFβ1 is shown to inhibit expression of the M2 muscarinic receptor (M2), which plays a critical role in the parasympathetic response of the heart. This effect is mimicked by overexpression of a dominant negative mutant of RhoA and by the RhoA kinase inhibitor Y27632, whereas adenoviral expression of a dominant activating-RhoA reverses TGFβ inhibition of M2 expression. TGFβ1 also mediates a decrease in GTP-bound RhoA and a reciprocal increase in the expression of the RhoA GTPase-activating protein, p190RhoGAP, whereas total RhoA is unchanged. Inhibition of M2 promoter activity by TGFβ1 is mimicked by overexpression of p190RhoGAP, whereas a dominant negative mutant of p190RhoGAP reverses this effect of TGFβ1. In contrast to atrial myocytes, in mink lung epithelial cells, in which TGFI8 signaling through activation of RhoA has been previously identified, TGFβ1 stimulated an increase in GTP-bound RhoA in association with a reciprocal decrease in the expression of p190RhoGAP. Both effects demonstrated a similar dose dependence on TGFβ1. Thus TGFβ regulation of M2 muscarinic receptor expression is dependent on RhoA, and TGFβ regulation of p190RhoGAP expression may be a cell type-specific mechanism for TGFβ signaling through RhoA. [ABSTRACT FROM AUTHOR]

S. M. Ward, A. P. Gadbut, D. Tang, A. G. Papageorge, L. Wu, G. LI, J. V. Barnett and J. B. Galper. TGFβ Regulates the Expression of Gαi2 via an Effect on the Localization of Ras. Journal of Molecular and Cellular Cardiology (2002) 34, 1217–1226. The negative chronotropic response of the heart to parasympathetic stimulation is mediated via the interaction of M2 muscarinic receptors, Gαi2 and the G-protein coupled inward rectifying K+ channel, GIRK1. Here TGFβ1 is shown to decrease the expression of Gαi2 in cultured chick atrial cells in parallel with attenuation of the negative chronotropic response to parasympathetic stimulation. The response to the acetylcholine analogue, carbamylcholine, decreased from a 95±2% (±SEM, n=8) inhibition of beat rate in control cells to 18±2% (±SEM,n =8) in TGFβ1 treated cells. Data support the conclusion that TGFβ regulation of Gαi2 expression was mediated via an effect on Ras. TGFβ1 inhibited Gαi2 promoter activity by 56±6% (±SEM, n=4) compared to control. A dominant activating Ras mutant reversed the effect of TGFβ on Gαi2 expression and stimulated Gαi2 promoter activity 1.7 fold above control. A dominant negative Ras mutant mimicked the effect of TGFβ1 on Gαi2 promoter activity. TGFβ had no effect on the ratio of GDP/GTP bound Ras, but markedly decreased the level of membrane associated Ras and increased the level of cytoplasmic Ras compared to control. Furthermore, farnesol, a precursor to farnesylpyrophosphate, the substrate for the farnesylation of Ras, not only reversed TGFβ1 inhibition of Ras localization to the membrane, but also reversed TGFβ1 inhibition of Gαi2promoter activity. FTI-277, a specific inhibitor of the farnesylation of Ras, mimicked the effect of TGFβ1 on Ras localization and Gαi2 promoter activity. These data suggest a novel relationship between TGFβ signaling, regulation of Ras function and the autonomic response of the heart. [Copyright &y& Elsevier]

We propose a novel mechanism for the regulation of the processing of Ras and demonstrate a new function for Ras in regulating the expression of cardiac auto- nomic receptors and their associated G proteins. We have demonstrated previously that induction of endogenous cholesterol synthesis in cultured cardiac myocytes resulted in a coordinated increase in expression of muscarinic receptors, the G protein α-subunit, G-αi2, and the inward rectifying K+ channel, GIRK1. These changes in gene expression were associated with a marked increase in the response of heart cells to parasympathetic stimulation. In this study, we demonstrate that the induction of the cholesterol metabolic pathway regulates Ras processing and that Ras regulates expression of G-αi2. We show that in primary cultured myocytes most of the RAS is localized to the cytoplasm in an unfarnesylated form. Induction of the cholesterol metabolic pathway results in increased farnesylation and membrane association of RAS. Studies of Ras mutants expressed in cultured heart cells demonstrate that activation of Ras by induction of the cholesterol metabolic pathway results in increased expression of G-α i2 mRNA. Hence farnesylation of Ras is a regulatable process that plays a novel role in the control of second messenger pathways. [ABSTRACT FROM AUTHOR]

Introduction: Diabetes increases ventricular tachycardia (VT) and sudden cardiac death risk in humans. We previously found the type I diabetic (DMI) heart displays: reduced responsiveness to parasympathetic stimulation; increased QRS/T Wave alternans (marker of proarrhythmic calcium dyshomeostasis); and hyperactive GSK3beta. Cardiac parasympathetic stimulation promotes production of intracellular cyclic GMP (cGMP). The effects of cGMP and its downstream effector PKG, on these indices of VT in type II diabetes (DMII) remain poorly understood. Hypothesis: PKGIa modulates inducibility of VT and QRS/TWA in the DMII heart and mediates the cGMP effect on VT through inhibition of myocardial GSK3beta. Methods: Using an established protocol of programmed ventricular stimulation we measured: VT incidence, duration, and QRS/T Wave alternans. We studied the following mice: wild type; Db/Db model of DMII; high fat high sucrose (HFHS) model of insulin resistance; and the PKGIa leucine zipper mutant (LZM) mouse, which has no DM but has PKGIa disrupting mutations. Mice were treated with or without the cGMP-augmenting phosphodiesterase inhibitor sildenafil 10 mg/kg, or the GSK3b inhibitor TWS119 20 mg/kg. Results: LVs of HFHS mice displayed 30 ± 8% reduction of cGMP compared with control (p<0.05, n=3 per group), while myocardial cGMP in LZM mice did not differ from control. In failing LVs from humans, there was also a 46% reduction in myocardial cGMP in DM compared with nondiabetic (p=.058, n=3 per group). Db/Db and HFHS mice had increased inducible VT (0.25 +/- 0.1 s in Db/Db, 2.2 +/- 1 s HFHS, vs 0 s in WT control, p <0.05), as well as TWA. Treatment with sildenafil reduced VT incidence, duration, and TWA in Db/Db and HFHS mice. LZM mice also had increased VT duration and TWA incidence (0 of 4 in WT vs 5 of 6 in LZM, p<0.05). Inhibition of GSK3β rescued inducibility of VT in Db/Db, HFHS, and in LZM mice. Conclusions: These findings support that in the DMII heart reduced cGMP promotes VT, whereas pharmacological augmentation of cGMP inhibits VT. Inducible VT and TWA in the PKG LZM mouse provides direct support that PKGIa opposes VT in the DMII heart. These support that cGMP activation of PKG inhibits GSK3beta, leading to reduced incidence of inducible VT. [ABSTRACT FROM AUTHOR]

The response of the heart to parasympathetic control involves stimulation of an acetylcholine sensitive current, IKAch. via activation of a G protein-coupled inward rectifying K channel, GIRK1/GIRK4. In an in vitro model for lipid lowering, embryonic chick atrial myocytes (EAM) cultured in absence of lipoproteins (LPDS) demonstrated both a markedly increased negative chronotropic response to parasympathetic stimulation and an increased level of the lipid-sensitive transcription factor, sterol response element binding protein (SREBP). Here we show that compared to cells cultured in FBS, growth of EAM in LPDS increased both the expression of GIRK1 protein and GIRK1 promoter activity. Overexpression of SREBP1a and 1c, but not SREBP2 mimicked the effect of LPDS on GIRK1 promoter activity and protein level. Analysis of deletion constructs and site-specific mutations of the GIRK1 promoter identified a regulatory complex consisting of an E-box and 2 tandem sterol response elements required for regulation by LPDS and/or SREBP. EAM cultured in LPDS demonstrated a 3 fold increased carbamylcholine-stimulated IKAch compared to cells grown in FBS. Expression of DN-SREBP reversed the effect of on GIRK1 promoter activity, GIRK1 protein level and IKAch. These data demonstrate a novel relationship between lipid metabolism and the regulation of the parasympathetic response of the heart. [ABSTRACT FROM AUTHOR]