Volume overload cardiac hypertrophy exhibits decreased expression of g(s)alpha and not of g(i)alpha in heart

Role of Gi proteins in the regulation of blood pressure and vascular remodeling

Heterotrimeric guanine nucleotide regulatory proteins (G-proteins) through the activation of several signaling mechanisms including adenylyl cyclase/cAMP and phospholipase C (PLC)/phosphatidyl inositol (PI) turnover. regulate a variety of cellular functions, including vascular reactivity, proliferation and hypertrophy of VSMC. Activity of adenylyl cyclase is regulated by two G proteins, stimulatory (Gsα) and inhibitory (Giα). Gsα stimulates adenylyl cyclase activity and increases the levels of cAMP, whereas Giα inhibits the activity of adenylyl cyclase and results in the reduction of cAMP levels. Abnormalities in Giα protein expression and associated adenylyl cyclase\cAMP levels result in the impaired cellular functions and contribute to various pathological states including hypertension. The expression of Giα proteins is enhanced in various tissues including heart, kidney, aorta and vascular smooth muscle cells (VSMC) from genetic (spontaneously hypertensive rats (SHR)) and experimentally - induced hypertensive rats and contribute to the pathogenesis of hypertension. In addition, the enhanced expression of Giα proteins exhibited by VSMC from SHR is also implicated in the hyperproliferation and hypertrophy, the two key players contributing to vascular remodelling in hypertension. The enhanced levels of endogenous vasoactive peptides including angiotensin II (Ang II), endothelin-1 (ET-1) and growth factors contribute to the overexpression of Giα proteins in VSMC from SHR. In addition, enhanced oxidative stress, activation of c-Src, growth factor receptor transactivation and MAP kinase/PI3kinase signaling also contribute to the augmented expression of Giα proteins in VSMC from SHR. This review summarizes the role of Giα proteins, and the underlying molecular mechanisms implicated in the regulation of high blood pressure and vascular remodelling.

Resveratrol prevents the development of high blood pressure in spontaneously hypertensive rats through the inhibition of enhanced expression of Giα proteins

Resveratrol (RV), a polyphenolic component of red wine, has been shown to attenuate high blood pressure (BP) in spontaneously hypertensive rats (SHRs). We previously found that the enhanced expression of Giα proteins plays a role in the pathogenesis of hypertension in SHRs. In the present study, we investigated whether this RV-induced decrease in BP in SHRs can be attributed to the ability of RV to inhibit the enhanced expression of Giα proteins and the upstream signaling molecules implicated in the overexpression of Giα proteins. Administration of RV (50 mg/kg per day) to prehypertensive 2-week-old SHRs for 6 weeks prevented the development of high BP and inhibited the enhanced expression of Giα proteins, the enhanced levels of superoxide anion (O2−) and NADPH oxidase activity, the enhanced activation (phosphorylation) of c-Src and growth factor receptors, as well as the enhanced levels of extracellular signal-regulated kinase 1/2 (ERK1/2) and protein kinase B (Akt) exhibited by vascular smooth muscle cells isolated from SHRs. In conclusion, these results indicate that RV attenuates the development of high BP in SHRs through the inhibition of enhanced levels of Giα proteins, oxidative stress, and the upstream signaling molecules that contribute to the overexpression of Giα proteins. These findings suggest that RV could potentially be used as a therapeutic agent in the treatment of cardiovascular complications including hypertension.

Increased ubiquitination and the crosstalk of G protein signaling in cardiac myocytes: involvement of Ric-8B in Gs suppression by Gq signal

Hyperactivation of Gq signaling causes cardiac hypertrophy, and β-adrenergic receptor-mediated Gs signaling is attenuated in hypertrophic cardiomyocytes. Here, we found the increase in a global ubiquitination in hypertrophic mouse heart. The activation of Gq signaling resulted in the ubiquitination of Gαs in neonatal rat cardiomyocytes, reduced Gαs expression, and suppressed cAMP response to β-adrenergic receptor stimulation. Ectopic expression of Gαq induced a similar suppression, which is due to the degradation of Gαs by a ubiquitin-proteasome pathway. Co-expression of Ric-8B, a positive regulator of Gαs, effectively canceled the Gαq-induced ubiquitination of Gαs and recovered the cAMP accumulation. In vitro, Gαq competes for the binding of Gαs to Ric-8B. These data show a new role of Ric-8B in the crosstalk of two distinct G protein signaling pathways, which are possibly involved in a part of mechanisms of chronic heart failure.

Chronic sympathetic activation promotes downregulation of β-adrenoceptor-mediated effects in the guinea pig heart independently of structural remodeling and systolic dysfunction

It is uncertain if downregulation of β-adrenoceptor signaling pathway is promoted by an enhanced adrenergic tone at an early stage of cardiac disease, or it develops secondary to detrimental local myocardial changes in advanced heart failure. We examined the integrity of β-adrenoceptor signaling pathway upon chronic infusion of isoproterenol, a β-adrenoceptor agonist, at a dose producing no structural left ventricular (LV) remodeling and systolic dysfunction. Subcutaneous isoproterenol infusion (400 μg kg(-1) h(-1) over 16 days) to guinea pigs using osmotic minipumps produced no change in cardiac weights, LV internal dimensions, myocyte cross-sectional area, extent of interstitial fibrosis, and basal contractile function. Isolated, perfused heart preparations from isoproterenol-treated guinea pigs exhibited attenuated responsiveness to acute β-adrenoceptor stimulation, as evidenced by reduced LV developed pressure increase, less shortening of LV epicardial monophasic action potential and effective refractory period, and less myocardial cyclic adenosine monophosphate elevation, in response to isoproterenol exposure, when compared to saline-treated controls. Pharmacological responses to forskolin, an activator of the adenylate cyclase catalytic subunit, were well preserved in isoproterenol-treated hearts. Downregulation of β-adrenoceptor-mediated effects upon chronic isoproterenol infusion was associated with markedly reduced stimulatory G-protein α-subunit (G(sα)) myocardial expression levels. No change in expression levels of β-adrenoceptors, G-protein-coupled receptor kinase 2, inhibitory G-protein α-subunit (G(iα2)), and Ca(v)1.2 and K(v)7.1 ion channels was determined in isoproterenol-treated hearts. We therefore conclude that sustained adrenergic overstimulation may promote downregulation of myocardial β-adrenoceptor-mediated effects independently of structural LV remodeling and systolic failure, an effect attributed to β-adrenoceptor uncoupling from adenylate cyclase due to reduced G(sα)-protein expression.

Transactivation of epidermal growth factor receptor by enhanced levels of endogenous angiotensin II contributes to the overexpression of Giα proteins in vascular smooth muscle cells from SHR

We earlier showed that the increased expression of Gi proteins exhibited by vascular smooth muscle cells (VSMC) from spontaneously hypertensive rats (SHR) was attributed to the enhanced levels of endogenous endothelin. Since the levels of angiotensin II (Ang II) are also enhanced in VSMC from SHR, the present study was undertaken to examine the role of enhanced levels of endogenous Ang II in the overexpression of Giα proteins in VSMC from SHR and to further explore the underlying mechanisms responsible for this increase. The enhanced expression of Giα-2 and Giα-3 proteins in VSMC from SHR compared to WKY was attenuated by the captopril, losartan and AG1478, inhibitors of angiotensin converting enzyme, AT(1) receptor and epidermal growth factor receptor (EGFR) respectively as well as by the siRNAs of AT1, cSrc and EGFR. The enhanced inhibition of forskolin-stimulated adenylyl cyclase activity by low concentrations of GTPγS (receptor-independent functions) and of inhibitory responses of hormones on adenylyl cyclase activity (receptor-dependent functions) in VSMC from SHR was also attenuated by losartan. Furthermore, the enhanced phosphorylation of EGFR in VSMC from SHR was also restored to control levels by captopril, losartan, PP2, a c-Src inhibitor and N-acetyl-L-cysteine (NAC), superoxide anion (O(2)(-)) scavenger, whereas enhanced ERK1/2 phosphorylation was attenuated by captopril and losartan. Furthermore, NAC also restored the enhanced phosphorylation of c-Src in SHR to control levels. These results suggest that the enhanced levels of endogenous Ang II in VSMC from SHR, transactivate EGFR, which through MAP kinase signaling, enhance the expression of Giα proteins and associated adenylyl cyclase signaling.

Weaving hypothesis of cardiomyocyte sarcomeres: discovery of periodic broadening and narrowing of intercalated disk during volume-load change

To investigate how cardiomyocytes change their length, echocardiographic and morphological studies were performed on rabbit hearts that were subjected to volume overload, overload removal, and repeated cycles of overload and overload removal. These conditions were created by arterio-venous fistula between the carotid artery and jugular vein, closure of the fistula, and cycles of repeatedly forming and closing fistula, respectively. After overload, hearts dilated and myocytes elongated. Intercalated disks repeatedly broadened and narrowed with a 2-day cycle, which continued for 8 weeks in many animals. The cycle consisted of shifts between five modes characterized by two interdigitation elongation-and-shortenings as follows: (I) flat with short ( approximately 1/4 to approximately 1/3 sarcomere long) interdigitations; (II) flat with long (one sarcomere long) interdigitations; (III) grooved with short interdigitations; (IV) grooved with long interdigitations; (V) flat with short interdigitations intermingled by sporadic long interdigitations; and return to (I). After overload removal, hearts contracted and myocytes shortened with similar 2-day broadening and narrowing cycle of intercalated disks, in which the five modes were reversed. Repeated overload and overload removal resulted in the repetition of myocyte elongation and shortening. We hypothesize that a single elongation-and-shortening event creates or disposes one sarcomere layer, and the two consecutive elongation-and-shortenings occur complementarily to each other so that the disks return to their original state after each cycle. Our hypothesis predicts that intercalated disks weave and unravel one sarcomere per myocyte per day.

Defective intracellular Ca2+ homeostasis contributes to myocyte dysfunction during ventricular remodelling induced by chronic volume overload in rats

1. Previous studies have demonstrated progressive ventricular hypertrophy, dilatation and contractile depression in response to chronic volume overload. Whether this decompensation was related to intrinsic myocyte dysfunction was not clear. The present study evaluated ventricular myocyte function at critical times during the progression of ventricular remodelling induced by volume overload. 2. Chronic volume overload was induced with an infrarenal aortocaval fistula in rats. Myocyte contraction and intracellular Ca(2+) concentrations ([Ca(2+)](i)) were evaluated using a fura-2 fluorescence and edge detection system. Protein levels of sarcoplasmic reticulum (SR) Ca(2+) transporters were determined by western blots. Progressive ventricular dilatation developed following creation of the fistula. Although myocyte function in 5 week fistula rats was comparable to that of the control group, myocytes from rats 10 weeks post-fistula demonstrated significant depression of cell shortening and peak [Ca(2+)](i). Application of isoproterenol (0.1 micromol/L) was not able to compensate for the functional deficiency in myocytes from 10 week fistula rats. Caffeine (10 mmol/L) induced SR Ca(2+) release, as well as protein expression of SR Ca(2+)-ATPase, and ryanodine receptors were reduced in myocytes obtained from the same group of 10 week fistula rats. 3. These data indicate that the transition to heart failure secondary to chronic volume overload is related to depressed myocyte contractility secondary to altered intracellular Ca(2+) homeostasis.

Effects of forskolin on inotropic performance and phospholamban phosphorylation in exercise-trained hypertensive myocardium

Beta-adrenergic receptor (beta-AR) responsiveness is downregulated in left ventricular (LV) hypertrophy induced by chronic hypertension. While exercise training in hypertension enhances beta-AR responsiveness, the role of adenylyl cyclase remains unclear. The purpose of the present study was to test whether treadmill running in the spontaneously hypertensive rat (SHR) model improves LV responsiveness to forskolin (FOR) or the combination of FOR + isoproterenol (FOR+ISO). Female SHR (16-wk) were randomly placed into sedentary (SHR-SED; n = 7) or treadmill-trained (SHR-TRD; n = 8) groups. Wistar-Kyoto (WKY; n = 7) animals acted as normotensive controls. Langendorff, isovolumic LV performance was established at baseline and during incremental FOR infusion (1 and 5 micromol/l) and FOR+ISO (5 micromol/l + 1x10(-8) mol/l). Heart rate, systolic blood pressure, and heart-to-body weight ratio were lower in WKY relative to both SHR groups (P < 0.05). LV performance and heart rate significantly increased in all groups to a similar extent with incremental FOR infusion. However, in the presence of 5 micromol/l FOR, ISO increased LV developed pressure, positive change in LV pressure, and negative change in LV pressure to a greater extent in SHR-TRD relative to SHR-SED (P < 0.05). Phospholamban phosphorylation at the Thr17 was greater in SHR-TRD relative to SHR-SED and WKY (P < 0.05). Absolute LV developed pressure was moderately correlated with phospholamban phosphorylation at both the Ser16 (r = 0.64; P < 0.05) and Thr17 (r = 0.52; P < 0.05). Our data suggest that the adenylyl cyclase step in the beta-AR cascade is not downregulated in the early course of hypertension and that the enhanced beta-AR responsiveness with training is likely mediated at levels other than adenylyl cyclase. Our data also suggest that beta-AR inotropic responsiveness in the presence of direct adenylyl cyclase agonism is improved in trained compared with sedentary SHR hearts.

The effect of modafinil on counter-regulatory and cognitive responses to hypoglycaemia

AIMS/HYPOTHESIS

Our hypothesis is that reducing release of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) with modafinil will enhance symptomatic and hormonal responses to hypoglycaemia.

METHODS

Nine healthy men received, in random order, two 100-mg doses of modafinil or placebo, followed by an insulin clamp in which plasma glucose was either reduced stepwise to 2.4 mmol/l or was sustained at euglycaemia (four studies). Catecholamines, symptom scores and cognitive function were measured.

RESULTS

Modafinil had no effect on the measured parameters during euglycaemia. During hypoglycaemia, autonomic symptom scores were significantly higher with modafinil (increase at lowest plasma glucose concentration 271.3+/-118.9 vs 211.2+/-80.4/40 min, p=0.019), and the heart rate response was increased (12,928+/-184 vs 6773+/-148 bpm/140 min, p=0.016). Deterioration in performance of two cognitive tasks was reduced: Stroop colour-word test (613+/-204 vs 2375+/-161/65 min, p=0.009) and accuracy of a simple reaction task (11.3+/-1.8 vs 9.4+/-3.7, p=0.039).

CONCLUSIONS/INTERPRETATION

We conclude that modafinil improves adrenergic sensitivity and some aspects of cognitive function at hypoglycaemia, possibly by reducing neuronal central GABA concentrations.

Alterations of adenylyl cyclase and G proteins in aortocaval shunt-induced heart failure

Unlike most other experimental models of congestive heart failure, the volume overload model induced by aortocaval shunt (AVS) in rats was found to exhibit enhanced β-adrenoceptor (β-AR) signaling. To study whether the adenylyl cyclase (AC)-G protein system is involved in such a change, we examined cardiac AC activity and protein content as well as Gsα and Giα activities, protein contents, and mRNA levels in both left (LV) and right (RV) ventricles at the failing stage (16 wk after surgery). Basal and forskolin-stimulated AC activities were significantly increased in both LV and RV from the failing hearts; this change was associated with an upregulation of type V/VI AC protein. In contrast to 5′-guanylyl imidodiphosphate and NaF, the stimulatory effect of isoproterenol on AC was increased in the failing heart. Although Gsα and Giα protein contents in the failing hearts were not altered, the mRNA level for Gsα was decreased by 20% and that for Giα was increased by 20%. In addition, the activity of Gsα, but not Giα, as assessed by toxin-catalyzed ADP ribosylation, was significantly decreased in the failing heart. Losartan and imidapril treatments improved cardiac function and attenuated alterations in mRNA levels for Gsα and Giα proteins, as well as Gsα activity, without affecting changes in AC protein content or activities in heart failure due to volume overload. These data suggest that increased AC activity may contribute to the enhanced β-AR signaling in the AVS model of heart failure, whereas alterations in gene expression for G proteins may be of an adaptive nature at this stage of heart failure.

The role of glucosensing neurons in the detection of hypoglycemia

Hypoglycemia is a life-threatening side effect of intensive insulin therapy in Type 1 diabetic patients. The ability to detect hypoglycemia and restore blood glucose levels to normal is of critical concern to the brain since glucose is its preferred fuel. When plasma glucose levels fall, powerful hormonal and sympathoadrenal mechanisms respond to restore blood glucose levels to normal. These mechanisms are believed to be initiated by diverse populations of glucose sensors, which are located centrally as well as peripherally. The exact contribution of each of these individual glucose sensors to the regulation of glucose homeostasis is not known at this time. This review focuses on the diversity of central and peripheral glucose sensors and the mechanisms by which they sense glucose.

Polymorphism of the Type 6 Adenylyl Cyclase Gene and Cardiac Hypertrophy

We investigated whether polymorphism of the type 6 adenylyl cyclase gene influences the occurrence of left ventricular hypertrophy in a Japanese population. Type 6 adenylyl cyclase is a major cardiac adenylyl cyclase isoform and plays an important role in regulating cardiac function. We examined the type 6 adenylyl cyclase gene for single nucleotide polymorphism by heteroduplex analysis and found a mutation (T11215A) in intron 17. We genotyped the single nucleotide polymorphism (TT/TA/AA groups) by the mutagenically separated polymerase chain reaction method in 2068 subjects who underwent health screening for cardiovascular disease. Genetic variation was in the Hardy-Weinberg equilibrium. We found no significant association between the frequency of left ventricular hypertrophy and any of the genotype groups. In the TT and the TA genotype group, however, left ventricular hypertrophy was associated with increased blood pressure, while no association with increased blood pressure was found in the AA genotype group. It was concluded that the AA group may be at risk of developing left ventricular hypertrophy independent of increased blood pressure.

Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) and hypoglycemia-associated autonomic failure (HAAF)

Hypoglycemia is a profound threat to the brain since glucose is its preferred fuel. Thus, decreases in plasma glucose must be sensed and appropriate hormonal and neuroendocrine responses generated to restore glucose to safe levels (i.e. counterregulatory responses (CRR) to hypoglycemia). Recurrent hypoglycemia impairs these protective mechanisms, resulting in a potentially life-threatening condition known as hypoglycemia-associated autonomic failure (HAAF). During HAAF, the glycemic threshold is reset so that glucose levels must fall further before the CRR is initiated. The brain plays a critical role in sensing hypoglycemia and initiating the CRR. Additionally, many neurons may sense changes in plasma and extracellular glucose. However, the way in which central glucose sensing is integrated to lead to effective initiation of the CRR is unknown. Furthermore, the mechanisms by which this system becomes impaired during HAAF are also unknown. Glucosensing neurons in the ventromedial hypothalamic nucleus (VMN) are poised to serve an integrative function in glucose homeostasis. First, they sense glucose. Second, the VMN receives input from other glucose-sensing areas. Finally, the VMN projects to areas linked to the regulation of the sympathoadrenal system that mediates the CRR. This review discusses VMN glucosensing neurons relative to their capacity to play a role in the regulation of the CRR and the generation of HAAF. Glucosensing neurons in the hindbrain as well as peripheral glucosensors are also considered.

Cardiomegaly induced by pressure overload in newborn rats is accompanied by altered expression of the long isoform of G(s)alpha protein and deranged signaling of adenylyl cyclase

G proteins-coupled signaling pathways appear to play a role in the development of cardiac hypertrophy and its progression to heart failure. The present study aimed to investigate trimeric G proteins and adenylyl cyclase signaling in immature as well as in adult rat myocardium during this process caused by pressure overload. Pressure overload was induced in newborn (2-day-old) rats by abdominal aortic banding and myocardial preparations from left ventricular myocardium of immature (10-day-old) and adult (90-day-old) animals were analyzed for the relative content of different G protein subunits and adenylyl cyclase (AC) by immunoblotting with specific antibodies. A functional status of the AC signaling system was also evaluated. Normal maturation of rat heart was accompanied by increased expression of AC type V/VI and VII and of the long isoform (G(s)alphaL) of G(s)alpha protein. In parallel, the amounts of myocardial G(i)alpha/G(o)alpha proteins tended to decrease, and G(q)alpha/G(11)alpha and Gbeta did not change. Interestingly, whereas fluoride-stimulated AC activity increased in the course of maturation, activity of AC measured under other experimental conditions (stimulation by Mn2+, forskolin or isoproterenol) was lower in adult than in young rat myocardium. Pressure overload did not influence distribution of G proteins in immature myocardium, but considerably decreased the content of G(s)alphaL and increased G(o)alpha proteins in hearts of 90-day-old rats. These hearts exhibited worsened functional reserve as compared to age-matched controls and activity of AC was also markedly lower. A considerable reduction in Mn(2+)-stimulated AC activity together with similar decrease in AC activity determined under other stimulation conditions suggests that it is a function of AC catalytic subunit that is primarily impaired in this model of pressure overload.

Volume overload influence on hypertrophied myocardium function

The aim of this study was to demonstrate that hypertrophied cardiac muscle is more sensitive to volume-overload than normal cardiac muscle. We assessed the mechanical function of isolated left ventricular papillary muscle from male spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY) submitted to volume overload caused by aortocaval fistula (ACF) for 30 days. Muscles were perfused with Krebs-Henseleit solution at 28 degrees C and studied isometrically at a stimulation rate of 0.2 Hz. The ACF increased the right and left ventricular weight-to-body weight ratio in WKY rats; it also promoted right ventricular hypertrophy and further increased the basal hypertrophy in the left ventricle from SHR. The arterial systolic pressure was greater in SHR than in WKY rats, and decreased with ACF in both groups. Developed tension (DT) and maximum rate of DT (+dT/dt) were greater in the SHR-control than in the WKY-control (P < 0.05); the time from peak tension to 50% relaxation (RT 1/2) was similar in these animals. ACF did not change any parameters in the SHR group and increased the resting tension in the WKY group. However, the significant difference observed between myocardial contraction performance in WKY-controls and SHR-controls disappeared when the SHR-ACF and WKY-controls were compared. Furthermore, RT 1/2 increased significantly in the SHR-ACF in relation to the WKY-controls. In conclusion, the data lead us to infer that volume-overload for 30 days promotes more mechanical functional changes in hypertrophied muscle than in normal cardiac muscle.