Mechanisms involved in alpha-adrenergic phenomena

Hepatic noradrenergic innervation acts via CREB/CRTC2 to activate gluconeogenesis during cold

BACKGROUND AND AIM

Although it is well established that hormones like glucagon stimulates gluconeogenesis via the PKA-mediated phosphorylation of CREB and dephosphorylation of the cAMP-regulated CREB coactivators CRTC2, the role of neural signals in the regulation of gluconeogenesis remains uncertain.

METHODS AND RESULTS

Here, we characterize the noradrenergic bundle architecture in mouse liver; we show that the sympathoexcitation induced by acute cold exposure promotes hyperglycemia and upregulation of gluconeogenesis via triggering of the CREB/CRTC2 pathway. Following its induction by dephosphorylation, CRTC2 translocates to the nucleus and drives the transcription of key gluconeogenic genes. Rodents submitted to different models of sympathectomy or knockout of CRTC2 do not activate gluconeogenesis in response to cold. Norepinephrine directly acts in hepatocytes mainly through a Ca2+-dependent pathway that stimulates CREB/CRTC2, leading to activation of the gluconeogenic program.

CONCLUSION

Our data demonstrate the importance of the CREB/CRTC2 pathway in mediating effects of hepatic sympathetic inputs on glucose homeostasis, providing new insights into the role of norepinephrine in health and disease.

Functional Remodeling of the Contractile Smooth Muscle Cell Cortex, a Provocative Concept, Supported by Direct Visualization of Cortical Remodeling

Simple Summary

As a key element of the smooth muscle cell contractile apparatus, the actin cytoskeleton participates in the development of force by acting as a molecular track for the myosin cross bridge motor. At the same time, the actin cytoskeleton must transmit the force developed during contraction to the extracellular matrix and, thus, to neighboring cells. This propagation of force to the cell periphery and beyond is initiated in part on specifically localized cellular cortical actin filaments also involved in mechano-chemical transduction. During the contractile process itself and in response to extracellular structural and chemical alterations, the smooth muscle actin cytoskeletal remodels. This indicates that the cytoskeleton is a dynamic cellular organelle that adapts to the changes in cell shape and chemical cues. Current evidence connecting contractile function and mechano-transduction mechanisms to the plasticity of the vascular smooth muscle actin cytoskeleton is reviewed; we then describe new evidence for cytoskeletal remodeling in vascular smooth muscle cells. Here, using immunoelectron microscopy, we visualize the actin binding proteins filamin A, zyxin and talin in these cells and show that they participate in the cortical cell cytoskeletal alteration, thus supporting the premise that smooth muscle cell remodeling occurs during contraction.

Abstract

Considerable controversy has surrounded the functional anatomy of the cytoskeleton of the contractile vascular smooth muscle cell. Recent studies have suggested a dynamic nature of the cortical cytoskeleton of these cells, but direct proof has been lacking. Here, we review past studies in this area suggesting a plasticity of smooth muscle cells. We also present images testing these suggestions by using the technique of immunoelectron microscopy of metal replicas to directly visualize the cortical actin cytoskeleton of the contractile smooth muscle cell along with interactions by representative cytoskeletal binding proteins. We find the cortical cytoskeletal matrix to be a branched, interconnected network of linear actin bundles. Here, the focal adhesion proteins talin and zyxin were localized with nanometer accuracy. Talin is reported in past studies to span the integrin–cytoplasm distance in fibroblasts and zyxin is known to be an adaptor protein between alpha-actinin and VASP. In response to activation of signal transduction with the alpha-agonist phenylephrine, we found that no movement of talin was detectable but that the zyxin-zyxin spacing was statistically significantly decreased in the smooth muscle cells examined. Contractile smooth muscle is often assumed to have a fixed cytoskeletal structure. Thus, the results included here are important in that they directly support the concept at the electron microscopic level that the focal adhesion of the contractile smooth muscle cell has a dynamic nature and that the protein–protein interfaces showing plasticity are protein-specific.

Use of DREADD Technology to Identify Novel Targets for Antidiabetic Drugs

G protein-coupled receptors (GPCRs) form a superfamily of plasma membrane receptors that couple to four major families of heterotrimeric G proteins, G_s, G_i, G_q, and G₁₂. GPCRs represent excellent targets for drug therapy. Since the individual GPCRs are expressed by many different cell types, the in vivo metabolic roles of a specific GPCR expressed by a distinct cell type are not well understood. The development of designer GPCRs known as DREADDs (designer receptors exclusively activated by a designer drug) that selectively couple to distinct classes of heterotrimeric G proteins has greatly facilitated studies in this area. This review focuses on the use of DREADD technology to explore the physiological and pathophysiological roles of distinct GPCR/G protein cascades in several metabolically important cell types. The novel insights gained from these studies should stimulate the development of GPCR-based treatments for major metabolic diseases such as type 2 diabetes and obesity.

Decreased Blood Glucose and Lactate: Is a Useful Indicator of Recovery Ability in Athletes?

During low-intensity exercise stages of the lactate threshold test, blood lactate concentrations gradually diminish due to the predominant utilization of total fat oxidation. However, it is unclear why blood glucose is also reduced in well-trained athletes who also exhibit decreased lactate concentrations. This review focuses on decreased glucose and lactate concentrations at low-exercise intensity performed in well-trained athletes. During low-intensity exercise, the accrued resting lactate may predominantly be transported via blood from the muscle cell to the liver/kidney. Accordingly, there is increased hepatic blood flow with relatively more hepatic glucose output than skeletal muscle glucose output. Hepatic lactate uptake and lactate output of skeletal muscle during recovery time remained similar which may support a predominant Cori cycle (re-synthesis). However, this pathway may be insufficient to produce the necessary glucose level because of the low concentration of lactate and the large energy source from fat. Furthermore, fatty acid oxidation activates key enzymes and hormonal responses of gluconeogenesis while glycolysis-related enzymes such as pyruvate dehydrogenase are allosterically inhibited. Decreased blood lactate and glucose in low-intensity exercise stages may be an indicator of recovery ability in well-trained athletes. Athletes of intermittent sports may need this recovery ability to successfully perform during competition.

Phenylephrine induces relaxation of longitudinal strips from small arteries of goat legs

Alpha adrenergic stimulation is known to produce vasoconstriction. We have earlier shown that, in spiral strips of small arteries Phenylephrine (PE) caused vasorelaxation under high nitric oxide (NO) environment. However, on further experimentation it was realized that the PE-induced vasorelaxant response occurred only with longitudinal strips of small arteries even under normal NO environment while circular strips showed contraction with PE even under high NO environment. Such PE-induced vasorelaxation of longitudinal strips was blocked by Phentolamine, an alpha-adrenergic receptor blocker. On delineation of specific receptor subtype, PE-induced relaxation was found to be mediated through alpha 1D receptor. However, this phenomenon is specific to small artery, as longitudinal smooth muscle of aorta showed only contractile response to adrenergic stimulation. There is no prior report of longitudinal smooth muscle in small artery up to our knowledge. The results of this study and histological examination of vessel sections suggest the presence of longitudinal smooth muscle in small artery and their relaxant response to alpha adrenergic stimulation is a novel phenomenon.

Behavior and histopathology as biomarkers for evaluation of the effects of paracetamol and propranolol in the neotropical fish species Phalloceros harpagos

Pharmaceutical drugs in the aquatic environment can induce adverse effects on nontarget organisms. This study aimed to assess the short-term effects of sublethal concentrations of both paracetamol and propranolol on the fish Phalloceros harpagos, specifically light/dark preference, swimming patterns, skin pigmentation, histopathology, and liver glycogen levels. Fish were acutely exposed to sublethal concentrations of both paracetamol (0.008, 0.08, 0.8, 8, 80 mg L^-1) and propranolol (0.0001, 0.001, 0.01, 0.1, 1 mg L^-1) under controlled conditions. For scototaxis, a significant preference for the dark compartment was observed for the group exposed to the highest concentration of paracetamol (80 mg L^-1). Propranolol exposure significantly altered the swimming pattern, especially in fish exposed to the 0.001 mg L^-1 concentration. Pigmentation was reduced in propranolol-exposed fish (0.1, 1 mg L^-1). The lowest concentration of propranolol (0.0001 mg L^-1) induced a decrease of histochemical reaction for hepatic glycogen. These data demonstrate that pharmaceuticals can induce sublethal effects in nontarget organisms, even at low concentrations, compromising specific functions of the individual with ecological relevance, such as energy balance and behavior.

Osmoregulatory role of vasotocinergic and isotocinergic systems in the gilthead sea bream (Sparus aurata L)

Gilthead sea bream, Sparus aurata L., is an important fish species for the Mediterranean aquaculture and is considered a good model for studying the osmoregulatory process, due to its capacity to cope with great changes in environmental salinity (5-60‰). Our group studied the osmoregulatory role of different endocrine systems in this species, focusing on the vasotocinergic and isotocinergic systems over several years. For this purpose, the cDNAs coding for pro-vasotocin (pro-vt), pro-isotocin (pro-it), two arginine vasotocin (AVT) receptors (avtr v1a2- and v2-types) and one IT receptor (itr) were cloned. Acclimation to different environmental salinities induced a direct lineal relationship between plasma AVT levels and salinity, with no changes in plasma IT values. In addition, higher values in vasotocinergic, isotocinergic and stress pathways (pro-vt and pro-it gene expression, AVT and IT storage and plasma cortisol levels) in both hypo- and/or hyper-osmotic transfers, suggest an interaction between cortisol and AVT/IT pathways. Moreover, gene expression of specific receptors, as well as the use of different in vitro techniques, demonstrated an important osmoregulatory orchestration in different organs. In addition, individuals intraperitoneally injected with AVT and transferred to different environmental salinities enhanced plasma cortisol levels and/or gill Na⁺, K⁺-ATPase activity. These effects could be related to the energy repartitioning process occurring during osmotic adaptation of S. aurata to extreme environmental salinities, which could be mediated not only by plasma cortisol but also by AVT. Finally, our results indicated a very important role of the vasotocinergic and/or isotocinergic systems in both osmoregulatory and non-osmoregulatory organs.

Hepatic Gi signaling regulates whole-body glucose homeostasis

An increase in hepatic glucose production (HGP) is a key feature of type 2 diabetes. Excessive signaling through hepatic Gs-linked glucagon receptors critically contributes to pathologically elevated HGP. Here, we tested the hypothesis that this metabolic impairment can be counteracted by enhancing hepatic Gi signaling. Specifically, we used a chemogenetic approach to selectively activate Gi-type G proteins in mouse hepatocytes in vivo. Unexpectedly, activation of hepatic Gi signaling triggered a pronounced increase in HGP and severely impaired glucose homeostasis. Moreover, increased Gi signaling stimulated glucose release in human hepatocytes. A lack of functional Gi-type G proteins in hepatocytes reduced blood glucose levels and protected mice against the metabolic deficits caused by the consumption of a high-fat diet. Additionally, we delineated a signaling cascade that links hepatic Gi signaling to ROS production, JNK activation, and a subsequent increase in HGP. Taken together, our data support the concept that drugs able to block hepatic Gi-coupled GPCRs may prove beneficial as antidiabetic drugs.

G-protein Coupled Receptor Signaling in Pluripotent Stem Cell-derived Cardiovascular Cells: Implications for Disease Modeling

Human pluripotent stem cell derivatives show promise as an in vitro platform to study a range of human cardiovascular diseases. A better understanding of the biology of stem cells and their cardiovascular derivatives will help to understand the strengths and limitations of this new model system. G-protein coupled receptors (GPCRs) are key regulators of stem cell maintenance and differentiation and have an important role in cardiovascular cell signaling. In this review, we will therefore describe the state of knowledge concerning the regulatory role of GPCRs in both the generation and function of pluripotent stem cell derived-cardiomyocytes, -endothelial, and -vascular smooth muscle cells. We will consider how far the in vitro disease models recapitulate authentic GPCR signaling and provide a useful basis for discovery of disease mechanisms or design of therapeutic strategies.

The impact of age-related dysregulation of the angiotensin system on mitochondrial redox balance

Aging is associated with the accumulation of various deleterious changes in cells. According to the free radical and mitochondrial theory of aging, mitochondria initiate most of the deleterious changes in aging and govern life span. The failure of mitochondrial reduction-oxidation (redox) homeostasis and the formation of excessive free radicals are tightly linked to dysregulation in the Renin Angiotensin System (RAS). A main rate-controlling step in RAS is renin, an enzyme that hydrolyzes angiotensinogen to generate angiotensin I. Angiotensin I is further converted to Angiotensin II (Ang II) by angiotensin-converting enzyme (ACE). Ang II binds with equal affinity to two main angiotensin receptors—type 1 (AT₁R) and type 2 (AT₂R). The binding of Ang II to AT₁R activates NADPH oxidase, which leads to increased generation of cytoplasmic reactive oxygen species (ROS). This Ang II-AT₁R–NADPH-ROS signal triggers the opening of mitochondrial K_ATP channels and mitochondrial ROS production in a positive feedback loop. Furthermore, RAS has been implicated in the decrease of many of ROS scavenging enzymes, thereby leading to detrimental levels of free radicals in the cell. AT₂R is less understood, but evidence supports an anti-oxidative and mitochondria-protective function for AT₂R. The overlap between age related changes in RAS and mitochondria, and the consequences of this overlap on age-related diseases are quite complex. RAS dysregulation has been implicated in many pathological conditions due to its contribution to mitochondrial dysfunction. Decreased age-related, renal and cardiac mitochondrial dysfunction was seen in patients treated with angiotensin receptor blockers. The aim of this review is to: (a) report the most recent information elucidating the role of RAS in mitochondrial redox hemostasis and (b) discuss the effect of age-related activation of RAS on generation of free radicals.

Neural pathways that control the glucose counterregulatory response

Glucose is an essential metabolic substrate for all bodily tissues. The brain depends particularly on a constant supply of glucose to satisfy its energy demands. Fortunately, a complex physiological system has evolved to keep blood glucose at a constant level. The consequences of poor glucose homeostasis are well-known: hyperglycemia associated with uncontrolled diabetes can lead to cardiovascular disease, neuropathy and nephropathy, while hypoglycemia can lead to convulsions, loss of consciousness, coma, and even death. The glucose counterregulatory response involves detection of declining plasma glucose levels and secretion of several hormones including glucagon, adrenaline, cortisol, and growth hormone (GH) to orchestrate the recovery from hypoglycemia. Low blood glucose leads to a low brain glucose level that is detected by glucose-sensing neurons located in several brain regions such as the ventromedial hypothalamus, the perifornical region of the lateral hypothalamus, the arcuate nucleus (ARC), and in several hindbrain regions. This review will describe the importance of the glucose counterregulatory system and what is known of the neurocircuitry that underpins it.

Stress is a critical player in CYP3A, CYP2C, and CYP2D regulation: role of adrenergic receptor signaling pathways

Stress is a critical player in the regulation of the major cytochrome P-450s (CYPs) that metabolize the majority of the prescribed drugs. Early in life, maternal deprivation (MD) stress and repeated restraint stress (RS) modified CYP expression in a stress-specific manner. In particular, the expression of CYP3A1 and CYP2C11 was increased in the liver of MD rats, whereas RS had no significant effect. In contrast, hepatic CYP2D1/2 activity was increased by RS, whereas MD did not affect it. The primary effectors of the stress system, glucocorticoids and epinephrine, highly induced CYP3A1/2. Epinephrine also induced the expression of CYP2C11 and CYP2D1/2. Further investigation indicated that AR-agonists may modify CYP regulation. In vitro experiments using primary hepatocyte cultures treated with the AR-agonists phenylephrine, dexmedetomidine, and isoprenaline indicated an AR-induced upregulating effect on the above-mentioned CYPs mediated by the cAMP/protein kinase A and c-Jun NH₂-terminal kinase signaling pathways. Interestingly though, in vivo pharmacological manipulations of ARs using the same AR-agonists led to a suppressed hepatic CYP expression profile, indicating that the effect of the complex network of central and peripheral AR-linked pathways overrides that of the hepatic ARs. The AR-mediated alterations in CYP3A1/2, CYP2C11, and CYP2D1/2 expressions are potentially connected with those observed in the activation of signal transducer and activator of transcription 5b. In conclusion, stress and AR-agonists may modify the expression of the major CYP genes involved in the metabolism of drugs used in a wide range of diseases, thus affecting drug efficacy and toxicity.

Roles of the α1A-adrenergic receptor carboxyl tail in protein kinase C-induced phosphorylation and desensitization

Noradrenaline- and tetradecanoyl phorbol acetate (TPA)-induced phosphorylation and functional desensitization of the following receptors were studied: (1) wild-type bovine α(1A)- and hamster α(1B)-adrenergic receptors (ARs), (2) chimeric ARs in which the carboxyl terminus tails were exchanged (α(1AB)- and α(1BA)-ARs), and (3) carboxyl terminus-truncated α(1A)-ARs fussed to enhanced green fluorescent protein. Noradrenaline and TPA pronouncedly increased α(1B)-AR phosphorylation while TPA markedly desensitized these receptors. In contrast, TPA-induced desensitization and TPA- and noradrenaline-induced phosphorylation of α(1A)-ARs were clearly of lesser magnitude. Chimeric ARs with exchanged carboxyl terminus tails showed that the extent of phosphorylation reflected the carboxyl domain rather than the receptor core. Surprisingly, there was no correlation between phosphorylation and functional desensitization, i.e., activation of protein kinase C clearly desensitized both chimeric receptors to a similar extent. Interestingly, TPA and noradrenaline increased carboxyl terminus-truncated α(1A)-AR phosphorylation and TPA also induced receptor desensitization. We were unable to detect carboxyl terminus-truncated α(1A)-AR internalization after 5-min stimulations with noradrenaline or TPA. Our results suggest the following: (a) the α(1A)-AR carboxyl terminus tail was not essential for signaling or desensitization; (b) carboxyl terminus tail exchange "transplanted" the phosphorylation pattern of the receptors, but the functional consequences of such a transplant were very limited; (c) α(1A)-AR desensitization was not associated to receptor internalization.

Adrenergic signaling polymorphisms and their impact on cardiovascular disease

This review examines the impact of recent discoveries defining personal genetics of adrenergic signaling polymorphisms on scientific discovery and medical practice related to cardiovascular diseases. The adrenergic system is the major regulator of minute-by-minute cardiovascular function. Inhibition of adrenergic signaling with pharmacological beta-adrenergic receptor antagonists (beta-blockers) is first-line therapy for heart failure and hypertension. Advances in pharmacology, molecular biology, and genetics of adrenergic signaling pathways have brought us to the point where personal genetic differences in adrenergic signaling factors are being assessed as determinants of risk or progression of cardiovascular disease. For a few polymorphisms, functional data generated in cell-based systems, genetic mouse models, and pharmacological provocation of human subjects are concordant with population studies that suggest altered risk of cardiovascular disease or therapeutic response to beta-blockers. For the majority of adrenergic pathway polymorphisms however, published data conflict, and the clinical relevance of individual genotyping remains uncertain. Here, the current state of laboratory and clinical evidence that adrenergic pathway polymorphisms can affect cardiovascular pathophysiology is comprehensively reviewed and compared, with a goal of placing these data in the broad context of potential clinical applicability.

Cardiac GPCRs: GPCR signaling in healthy and failing hearts

G protein-coupled receptors (GPCRs) are widely implicated in human heart disease, making them an important target for cardiac drug therapy. The most commonly studied and clinically targeted cardiac GPCRs include the adrenergic, angiotensin, endothelin, and adenosine receptors. Treatment options focusing on the complex and integrated signaling pathways of these GPCRs are critical for the understanding and amelioration of heart disease. The focus of this review is to highlight the most commonly studied and clinically targeted cardiac GPCRs, placing emphasis on their common signaling components implicated in cardiac disease.

Nitric oxide synthase inhibition activates L- and T-type Ca2+channels in afferent and efferent arterioles

Previous studies have shown that L-type Ca2+channel (LCC) blockers primarily dilate resting and ANG II-constricted afferent arterioles (AA), but do not influence either resting or ANG II-constricted efferent arterioles (EA). In contrast, blockade of T-type Ca2+channels (TCC) dilate EA and prevent ANG II-mediated efferent constriction. The present study determined the role of LCC and TCC in mediating the AA and EA constriction following inhibition of nitric oxide synthase (NOS) and tested the hypothesis that inhibition of NOS increases the influence of LCC on EA. With the use of an isolated blood-perfused rat juxtamedullary nephron preparation, single AA or EA were visualized and superfused with a NOS inhibitor, N-nitro-l-arginine (l-NNA), with or without concomitant treatment with an LCC blocker, diltiazem, or a TCC blocker, pimozide. In response to l-NNA (1, 10, and 100 μmol/l), AA and EA diameters decreased significantly by 6.0 ± 0.3, 13.7 ± 1.7, and 19.9 ± 1.4%, and by 6.2 ± 0.5, 13.3 ± 1.1, and 19.0 ± 1.9%, respectively. During TCC blockade with pimozide (10 μmol/l), l-NNA did not significantly constrict afferent (0.9 ± 0.6, 1.5 ± 0.5, and 1.7 ± 0.5%) or efferent (0.4 ± 0.1, 2.1 ± 0.7, and 2.5 ± 1.0%) arterioles. In contrast to the responses with other vasoconstictors, the l-NNA-induced constriction of EA, as well as AA, was reversed by diltiazem (10 μmol/l). The effects were overlapping as pimozide superimposed on diltiazem did not elicit further dilation. When the effects of l-NNA were reversed by superfusion with an NO donor, SNAP (10 μmol/l), diltiazem did not cause significant efferent dilation. As a further test of LCC activity, 55 mmol/l KCl, which depolarizes and constricts AA, caused only a modest constriction in resting EA (8.7 ± 1.3%), but a stronger EA constriction during concurrent treatment with l-NNA (23.8 ± 4.8%). In contrast, norepinephrine caused similar constrictions in both l-NNA-treated and nontreated arterioles. These results provide evidence that NO inhibits LCC and TCC activity and that NOS inhibition-mediated arteriolar constriction involves activation of LCC and TCC in both AA and EA. The difference in responses to high KCl between resting and l-NNA-constricted EA and the ability of diltiazem to block EA constriction caused by l-NNA contrasts with the lack of efferent effects in resting and SNAP-treated l-NNA-preconstricted arterioles and during ANG II-mediated vasoconstriction, suggesting a recruitment of LCC in EA when NOS is inhibited. These data help explain how endothelial dysfunction associated with hypertension may lead to enhanced activity of LCC in postglomerular arterioles and increased postglomerular resistance.

Sympathetic neural inhibition of conducted vasodilatation along hamster feed arteries: complementary effects of alpha1- and alpha2-adrenoreceptor activation

Vasodilatation initiated on arterioles of skeletal muscle ascends into the proximal feed arteries through cell-to-cell conduction along the endothelium and into smooth muscle. Whereas perivascular sympathetic nerve activity (SNA) can inhibit conducted vasodilatation and restrict muscle blood flow, the signalling events mediating this interaction are poorly defined. Therefore, using isolated pressurized (75 mmHg) feed arteries (diameter (microm) at rest = 53 +/- 3; maximum = 99 +/- 2; n = 86) of the hamster retractor muscle, we tested the hypothesis that distinct yet complementary signalling pathways underlie the ability of SNA to inhibit conduction. Conducted vasodilatation was initiated using ACh microiontophoresis (1 microA; 250, 500 and 1000 ms) and SNA was initiated using local field stimulation (30-50 V; 1 ms at 2, 8 and 16 Hz). With vasodilatations of 5-20 microM, conduction increased with ACh pulse duration and was inhibited progressively as the frequency of SNA increased. During SNA, conduction was partially restored with inhibition of alpha1- (0.1 microM prazosin) or alpha2- (0.1 microM RX821002) adrenoreceptors and fully restored with both antagonists present. Activating alpha1- (50 nM phenylephrine) or alpha2- (1 microM UK 14,304) adrenoreceptors inhibited conduction partially and their simultaneous activation inhibited conduction cumulatively (P < 0.05). Elevated [K+]o (30 or 40 mM) or phorbol esters (0.5 microM) also inhibited conduction yet similar constriction with l-NNA (50 microM) or Bay K 8644 (10 nM) did not. Thus, the activation of alpha1- and alpha2-adrenoreceptors inhibits conducted vasodilatation through complementary signalling events. With robust coupling along the endothelium, our modelling predicts that the inhibition of conduction by SNA can be explained by reduced electrical coupling through myoendothelial gap junctions or greater current leak across smooth muscle cell membranes.

Sympathetic nerves inhibit conducted vasodilatation along feed arteries during passive stretch of hamster skeletal muscle

Ascending vasodilatation is integral to blood flow control in exercising skeletal muscle and is attributable to conduction from intramuscular arterioles into proximal feed arteries. Passive stretch of skeletal muscle can impair muscle blood flow but the mechanism is not well understood. We hypothesized that the conduction of vasodilatation along feed arteries can be modulated by changes in muscle length. In anaesthetized hamsters, acetylcholine (ACh) microiontophoresis triggered conducted vasodilatation along feed arteries (diameter, 50-70 microm) of the retractor muscle secured at 100 % resting length or stretched by 30 %. At 100 % length, ACh evoked local dilatation (> 30 microm) and this response conducted rapidly along the feed artery (14 +/- 1 microm dilatation at 1600 microm upstream). During muscle stretch, feed arteries constricted approximately 10 microm (P < 0.05) and local vasodilatation to ACh was maintained while conducted vasodilatation was reduced by half (P < 0.01). Resting diameter and conduction recovered upon restoring 100 % length. Sympathetic nerve stimulation (4-8 Hz) produced vasoconstriction and attenuated conduction in the manner observed during muscle stretch, as did noradrenaline or phenylephrine (10 nM). Inhibiting nitric oxide production (Nomega-nitro-L-arginine, 50 microM) produced similar vasoconstriction yet had no effect on conduction. Phentolamine, prazosin, or tetrodotoxin (1 microM) during muscle stretch abolished vasoconstriction and restored conduction. Inactivation of sensory nerves with capsaicin had no effect on vasomotor responses. Thus, muscle stretch can attenuate conducted vasodilatation by activating alpha-adrenoreceptors on feed arteries through noradrenaline released from perivascular sympathetic nerves. This autonomic feedback mechanism can restrict muscle blood flow during passive stretch.

Hemorrhage increases cytokine expression in lung mononuclear cells in mice: involvement of catecholamines in nuclear factor-kappaB regulation and cytokine expression

The expression of proinflammatory and immunoregulatory cytokines rapidly increases in the lungs after hemorrhage, and such alterations contribute to the frequent development of acute inflammatory lung injury in this setting. Blood loss also produces elevations in catecholamine concentrations in the pulmonary and systemic circulation. In the present experiments, we used alpha- and beta-adrenergic receptor blockade to examine in vivo interactions between hemorrhage-induced adrenergic stimulation and pulmonary cytokine expression. Treatment of mice with the alpha-adrenergic receptor antagonist phentolamine prevented not only the elevation in mRNA levels of IL-1beta, TNF-alpha, and TGF-beta1, the increase in IL-1beta protein, but also the activation of nuclear factor (NF)-KB and cyclic AMP response element binding protein, which occurred in lung cells of untreated animals during the first hour after hemorrhage. In contrast, treatment before hemorrhage with the beta-adrenergic receptor antagonist propranolol was associated with increases in mRNA levels for IL-1beta, TNF-alpha, and TGF-beta1, which were greater than those present in untreated hemorrhaged mice, and did not prevent hemorrhage-associated increases in lung IL-1beta protein. Treatment with propranolol prevented hemorrhage-induced phosphorylation of cyclic AMP response element binding protein, but increased hemorrhage-associated activation of NF-KB. These results demonstrate that hemorrhage initially increases pulmonary cytokine expression through alpha- but not beta-adrenergic stimulation, and suggest that such alpha-adrenergic-mediated effects occur through activation of the transcriptional regulatory factor NF-kappaB.

Intracellular calcium and cAMP regulate directional pigment movements in teleost erythrophores

Teleost pigment cells (erythrophores and melanophores) are useful models for studying the regulation of rapid, microtubule-dependent organelle transport. Previous studies suggest that melanophores regulate the direction of pigment movements via changes in intracellular cAMP (Rozdzial and Haimo, 1986a; Sammak et al., 1992), whereas erythrophores may use calcium- (Ca(2+)-) based regulation (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). Despite these observations, there have been no direct measurements in intact erythrophores or any cell type correlating changes of intracellular free Ca2+ ([Ca2+]i) with organelle movements. Here we demonstrate that extracellular Ca2+ is necessary and that a Ca2+ influx via microinjection is sufficient to induce pigment aggregation in erythrophores, but not melanophores of squirrel fish. Using the Ca(2+)-sensitive indicator, Fura-2, we demonstrate that [Ca2+]i rises dramatically concomitant with aggregation of pigment granules in erythrophores, but not melanophores. In addition, we find that an erythrophore stimulated to aggregate pigment will immediately transmit a rise in [Ca2+]i to neighboring cells, suggesting that these cells are electrically coupled. Surprisingly, we find that a fall in [Ca2+]i is not sufficient to induce pigment dispersion in erythrophores, contrary to the findings obtained with the ionophore and lysed-cell models (Luby-Phelps and Porter, 1982; McNiven and Ward, 1988). We find that a rise in intracellular cAMP ([cAMP]i) induces pigment dispersion, and that this dispersive stimulus can be overridden by an aggregation stimulus, suggesting that both high [cAMP]i and low [Ca2+]i are necessary to produce pigment dispersion in erythrophores.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

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Electrogenic bicarbonate secretion in gallbladder: induction by barium via neuronal, possibly VIP-ergic pathways

In guinea-pig gallbladder epithelium, cAMP converts electroneutral HCO3- secretion into an electrogenic process. The effects of blood side Ba2+ (5 mmol/l) on HCO3- transport were investigated in vitro, using pH-stat and voltage clamp techniques to determine unidirectional fluxes of HCO3- and transepithelial electrical characteristics. Serosal, not mucosal addition of Ba2+ elevated short-circuit current (Isc), transepithelial potential difference, and tissue conductance; it inhibited the absorptive HCO3- flux while leaving the secretory flux unchanged. The Isc effect of Ba2+ was inhibited or prevented by tetrodotoxin; D- and L-propranolol; the Cl- channel blocker 4-N-methyl-N-phenylaminothiophene-3-carboxylic acid; the intracellular Ca2+ antagonist, 3,4,5-trimethoxybenzoic acid 8-(diethylamino)ocytl ester; noradrenaline, by a yohimbine-sensitive action; somatostatin; HCO3(-)-free solutions. Thus Ba2+ appeared to release a neurotransmitter that gives rise to cAMP synthesis sufficient to turn part of electroneutral HCO3- secretion electrogenic. In a search for the involved signalling pathways, the H1-receptor antagonist, cetirizine, largely and hexamethonium, atropine, atenolol, indomethacin, and trifluoperazine entirely failed to antagonize the Isc effect of Ba2+. Similarly, carbachol, dobutamine, salbutamol, and serotonin were unable to mimic the action of Ba2+ and Isc effects of histamine were small and short-lived. By contrast, vasoactive intestinal peptide (VIP; 3 x 10(-7) mol/l) completely transformed HCO3- secretion into an electrogenic process. The VIP receptor antagonist (4Cl-DPhe6, Leu17) VIP, delayed and reduced the Isc responses to Ba2+ and VIP. As guinea-pig gallbladder epithelial cells possess cAMP-coupled VIP receptors close to VIPergic neurons, Ba2+ is likely to act by releasing VIP from neural terminals.

Inhibition by verapamil of hepatocarcinogenesis induced by N-nitrosomorpholine in Sprague-Dawley rats

The effect of verapamil on hepatocarcinogenesis induced by N-nitrosomorpholine (NNM) was investigated in male Sprague-Dawley rats. Rats were given drinking water containing NNM for 8 weeks and received i.p. injections of verapamil or vehicle every other day for 16 weeks from the start of the experiment. Pre-neoplastic and neoplastic lesions staining positive for gamma-glutamyl transpeptidase (GGT) or the placental type of glutathione-S-transferase (GST-P) were examined histochemically at week 16. Prolonged administration of verapamil resulted in a significant decrease in the number of GGT-positive and GST-P-positive lesions. The incidence and volume as a percentage of parenchyma of hepatocellular carcinomas were also significantly less in rats treated with verapamil than in controls. Administration of verapamil significantly decreased the labelling indices of pre-neoplastic lesions and adjacent liver. These findings indicate that verapamil inhibits hepatocarcinogenesis and that this may be related to its inhibitory effect on cell proliferation in neoplastic lesions and surrounding hepatocytes.

Characterization of adrenoceptors involved in the electrogenic chloride secretion by cultured rat epididymal epithelium

1. Short-circuit current (SCC) technique was used to study the adrenoceptors involved in the electrogenic chloride secretion by cultured cauda epididymal epithelium of rats. Stimulation of the epithelium with noradrenaline (primarily beta 1-adrenoceptor selective agonist), salbutamol (beta 2-adrenoceptor selective agonist) and adrenaline (non-selective beta-adrenoceptor agonist) led to a rise in SCC. At a low chart-speed (2 mm min-1), the response profile to these agonists consisted of a peak followed by a sustained response considerably higher than the basal SCC. 2. The EC50s (doses of agonist producing 50% maximum response) of noradrenaline, salbutamol and adrenaline were 300, 115 and 10 nM respectively. Pretreating the tissues with 1 microM atenolol (beta 1-selective antagonist) and 10 microM butoxamine (beta 2-selective antagonist) shifted the dose-response curves of noradrenaline (shifted EC50 = 4000 nM) and salbutamol (shifted EC50 = 1050 nM) to the right. Atenolol (1 microM) and butoxamine (10 microM) shifted the dose-response curve of adrenaline to the right with new EC50s of 30 nM and 115 nM, respectively. 3. The rapidly rising phase of the SCC response to noradrenaline and adrenaline observed at low chart-speed consisted of a brief and transient retraction followed by a rebound increase in SCC. At a high chart-speed (1 mm s-1), the retraction and rebound phenomenon manifested as a fast initial spike which could be blocked by phentolamine (non-specific alpha-adrenoceptor antagonist) in a dose-dependent fashion. Similar initial spikes were observed when the tissues were stimulated with phenylephrine (alpha-selective agonist) but not with isoprenaline (non-selective beta-agonist) or forskolin (activator of adenylate cyclase). The response of the initial spike triggered by noradrenaline was dose-dependent and the EC50 was 2000 nM.4. The present study showed that the electrogenic chloride secretion by rat epididymis could be stimulated by (alphaxi-, beta131- and beta2-adrenoceptor agonists. The al-mediated response had a faster onset and more transient action than the 3-counterpart. It is postulated that epididymal chloride secretion might be regulated by neural (noradrenaline-mediated) and humoral (adrenaline-mediated) controls and that the stimulus-secretion coupling mechanisms might involve both Ca2+(alpha-mediated response) and adenosine 3':5'-cyclic monophosphate (beta-mediated response) as intracellular second messengers.

Dihydroxyphenylalanine and dopamine are released from portal vein together with noradrenaline and dihydroxyphenylglycol during nerve stimulation

The overflows of 3,4-dihydroxyphenylalanine, dopamine, noradrenaline, and 3,4-dihydroxyphenylglycol in canine portal vein superfused in vitro were studied before, during, and after depolarization of sympathetic nerve endings. The four compounds were separated from superfusate and from tissue on Sep-Pak C-18 cartridges and quantified by HPLC with electrochemical detection. Physiological and biochemical methods were used to show that the compound released was most probably 3,4-dihydroxyphenylalanine; the identity of the other endogenous compounds has been established previously. Release of 3,4-dihydroxyphenylalanine was calcium and frequency dependent, inhibited by a-m-L-p-tyrosine (an inhibitor of tyrosine hydroxylase) and augmented by 3-hydroxybenzylhydrazine (an inhibitor of aromatic amino acid decarboxylase). The overflows of dopamine, noradrenaline, and 3,4-dihydroxyphenylglycol from the vein were calcium and frequency dependent. It was estimated that under control conditions, approximately 80% of the total 3,4-dihydroxyphenylalanine that was synthesized was directed to catecholamine biosynthesis, approximately 8% overflowed from the vein, and approximately 14% remained unchanged within the tissue. It is concluded that 3,4-dihydroxyphenylalanine and dopamine are released together with noradrenaline and 3,4-dihydroxyphenylglycol from portal vein upon nerve depolarization.

Intracellular cyclic AMP not calcium, determines the direction of vesicle movement in melanophores: direct measurement by fluorescence ratio imaging

Intracellular movement of vesiculated pigment granules in angelfish melanophores is regulated by a signalling pathway that triggers kinesin and dyneinlike microtubule motor proteins. We have tested the relative importance of intracellular Ca2+ ([Ca2+]i) vs cAMP ([cAMP]i) in the control of such motility by adrenergic agonists, using fluorescence ratio imaging and many ways to artificially stimulate or suppress signals in these pathways. Fura-2 imaging reported a [Ca2+]i elevation accompanying pigment aggregation, but this increase was not essential since movement was not induced with the calcium ionophore, ionomycin, nor was movement blocked when the increases were suppressed by withdrawal of extracellular Ca2+ or loading of intracellular BAPTA. The phosphatase inhibitor, okadaic acid, blocked aggregation and induced dispersion at concentrations that suggested that the protein phosphatase PP-1 or PP-2A was continuously turning phosphate over during intracellular motility. cAMP was monitored dynamically in single living cells by microinjecting cAMP-dependent kinase in which the catalytic and regulatory subunits were labeled with fluorescein and rhodamine respectively (Adams et al., 1991. Nature (Lond.). 349:694- 697). Ratio imaging of F1CRhR showed that the alpha 2-adrenergic receptor-mediated aggregation was accompanied by a dose-dependent decrease in [cAMP]i. The decrease in [cAMP]i was both necessary and sufficient for aggregation, since cAMP analogs or microinjected free catalytic subunit of A kinase-blocked aggregation or caused dispersal, whereas the cAMP antagonist RpcAMPs or the microinjection of the specific kinase inhibitor PKI5-24 amide induced aggregation. Our conclusion that cAMP, not calcium, controls bidirectional microtubule dependent motility in melanophores might be relevant to other instances of non-muscle cell motility.

Characterization of the biological effects of 2-methylthio-ATP on rat hepatocytes: clear-cut differences with ATP

1. In several tissues, 2-methylthio adenosine triphosphate (2MeSATP) is a very potent P2y-purine agonist. In rat hepatocytes, 2MeSATP half-maximally activated glycogen phosphorylase at 20 nM and was therefore about 25 times more effective than ATP (Ka 0.5-0.8 microM). This strong glycogenolytic potency of 2MeSATP suggests on its own the presence of P2Y-purinoceptors in liver. 2. Displacement of the radioligand ATP alpha[35S] from its receptor however occurred at much higher concentrations of 2MeSATP than was anticipated on the basis of its glycogenolytic potency. 3. The interaction of 2MeSATP with the receptor, characterized with ATP alpha[35S] as radioligand, cannot be considered as a pure competitive interaction. 4. 2MeSATP did not share the ability of ATP to counteract the effect of glucagon on the adenosine 3':5'-cyclic monophosphate levels. 5. 2MeSATP barely increased the levels of inositol trisphosphate (IP3). 6. The glycogenolytic effect of 2MeSATP was completely abolished by pretreatment of the hepatocytes with phorbol myristic acetate. 7. It is tentatively concluded that 2MeSATP and ATP are interacting with different P2 purinoceptors.

Flosequinan, a vasodilator with a novel mechanism of action

1. The mechanism of action of flosequinan was investigated in ferret aortic smooth muscle by the simultaneous measurement of aequorin luminescence and isometric force. 2. The control calcium-force curve was obtained by plotting the calibrated aequorin luminescence against the force from potassium-depolarized muscles. Flosequinan relaxed potassium-depolarized muscles by causing parallel changes in [Ca2+]i and force with no shift in the control [Ca2+]i-force relationship. 3. The [Ca2+]i-force relationship in the presence of a maximally effective concentration of phenylephrine was significantly shifted to the left of that for the control, potassium-depolarized muscle. Flosequinan relaxed the phenylephrine-contracted muscle by causing a large decrease in force with only a minimal decrease in [Ca2+]i, resulting in an apparent rightward shift of the [Ca2+]i-force relationship, toward the control curve. 4. In comparison, sodium nitroprusside caused relaxation of either the potassium- or phenylephrine-induced contraction solely by a decrease in [Ca2+]i with no shift in either calcium-force relationship. 5. Milrinone caused no significant rightward shift of the calcium-force relationship during phenylephrine- or potassium-induced contractions, but when milrinone was added in the absence of vasoconstrictors, relaxation was obtained with no significant decrease in [Ca2+]i. 6. Flosequinan appears to differ in mechanism of action from both nitroprusside and milrinone. It relaxes depolarization-mediated contractions solely by decreasing [Ca2+]i but also appears to be capable of reversing the apparent calcium sensitizing action of phenylephrine.

Frequency and amplitude enhancement of calcium transients by cyclic AMP in hepatocytes

Interactions between signalling pathways such as the cyclic AMP and the Ca2+/phosphatidylinositol pathway may occur and be of major relevance in the regulation of cell function. We demonstrate here that cyclic-AMP-dependent mechanisms cause a marked increase in frequency and peak free Ca2+ of alpha 1-receptor-induced Ca2+ transients in single hepatocytes and lower the threshold for alpha 1-receptor agonists. Adrenaline at low physiological concentrations generates alpha 1-receptor-induced Ca2+ transients, which requires activation of the beta 2-receptor signalling pathway. We conclude that an interaction between the alpha 1-receptor signalling pathway and cyclic-AMP-dependent mechanisms activated by beta 2-receptor occupation is crucial to elicit a complete adrenergic response to adrenaline at physiological concentrations in rat hepatocytes.

Regulation of organelle transport in melanophores by calcineurin

Previous studies have shown that pigment granule dispersion and aggregation in melanophores of the African cichlid, Tilapia mossambica, are regulated by protein phosphorylation and dephosphorylation, respectively (Rozdzial, M. M., and L. T. Haimo. 1986. Cell. 47:1061-1070). The present studies suggest that calcineurin, a Ca2+/calmodulin-stimulated phosphatase, is the endogenous phosphatase that mediates pigment aggregation in melanophores. Aggregation, but not dispersion, is inhibited by okadaic acid at concentrations consistent with an inhibition of calcineurin activity. Inhibition of aggregation in melanophores that have been BAPTA loaded or treated with calmodulin antagonists implicate Ca2+ and calmodulin, respectively, in this process. Moreover, addition of calcineurin rescues aggregation in lysed melanophores which are otherwise incapable of aggregating pigment. Immunoblotting with an anticalcineurin IgG reveals that calcineurin is a component of the dermis, which contains the melanophores, and indirect immunofluorescence localizes calcineurin specifically to the melanophores. Finally, this antibody, which inhibits calcineurin's phosphatase activity (Tash, J. S., M. Krinks, J. Patel, R. L. Means, C. B. Klee, and A. R. Means. 1988. J. Cell Biol. 106:1625-1633), inhibits aggregation but has no effect on pigment granule dispersion. Together these studies indicate that retrograde transport of pigment granules to the melanophore cell center depends upon the participation of calcineurin.

Effect of neuropeptide Y on alpha 2-adrenoceptor-mediated cardiovascular responses in the pithed rat

1. The effects of neuropeptide Y (NPY) on the cardiovascular responses induced by stimulation of pre and postjunctional alpha 2-adrenoceptors were studied in the pithed normotensive rat. 2. The increase in diastolic blood pressure induced by cumulative injection of xylazine (1-1000 micrograms kg-1, i.v.) were potentiated by NPY (5 micrograms kg-1) but not affected by a lower dose (0.75 micrograms kg-1) of this peptide. 3. Xylazine (1-100 micrograms kg-1) inhibited in a dose-dependent manner the tachycardia induced by continuous electrical stimulation (0.2 Hz, 2 ms, 60 V) of the spinal cord (C7-Thl). 4. NPY (5 micrograms kg-1 but not 0.75 micrograms kg-1) enhanced the inhibitory effect of xylazine on the tachycardia induced by electrical stimulation without having any direct effect on heart rate. 5. These results suggest that there may be a positive interaction between NPY receptors, postjunctional alpha 2-adrenoceptors and between NPY receptors and postjunctional alpha 2-adrenoceptors in the cardiovascular system of the rat.

Complex regulation of calcium current in cardiac cells. Dependence on a pertussis toxin-sensitive substrate, adenosine triphosphate, and an alpha 1-adrenoceptor

We investigated regulation of the cardiac L-type calcium channel by intracellular ATP and by alpha 1-adrenergic agonism using single adult guinea pig ventricular cells and the whole-cell patch clamp method. Inclusion of 5 mM ATP in the patch clamp pipette prevented calcium current rundown but did not increase the maximal magnitude of the slow inward calcium current (ICa). During beta 1-adrenergic blockade with 10 microM (-)-propranolol, cells preincubated with 1 microgram/ml pertussis toxin for 2-5 h exhibited a rapid twofold increase in ICa after rupture of the membrane patch when 5 mM ATP was present in the patch clamp pipette. In the absence of ATP, the increase in ICa did not occur. In pertussis toxin-treated cells, 100 microM (-)-phenylephrine inhibited the augmentation of ICa. This inhibitory effect was blocked by 100 nM terazosin, a selective alpha 1-antagonist. The inhibitory effect of alpha 1-adrenergic agonism was not mediated by cAMP-dependent phosphodiesterase since incubation with 100 microM (-)-phenylephrine did not augment the activity of this enzyme. We conclude that regulation of the L-type calcium channel in cardiac cells is complex, and is dependent on a pertussis toxin-sensitive substrate, ATP, and an alpha 1-adrenergic receptor. The marked increase in ICa after pertussis toxin treatment in the presence of ATP indicates significant inhibition of ICa by a pertussis toxin substrate, presumably the guanine nucleotide inhibitory protein (Gi) in the basal state. The inhibitory action of (-)-phenylephrine in pertussis toxin-treated cells is consistent with modulation of ICa by an alpha 1-adrenergic receptor not coupled to Gi.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Elevation of intracellular cyclic AMP concentration fails to inhibit adrenaline-induced hyperpolarization in amphibian sympathetic neurons

1. The effect of drugs on the adenosine 3':5'-cyclic monophosphate (cyclic AMP) content of desmethylimipramine (DMI)-treated bullfrog paravertebral sympathetic ganglia was studied by radioimmunoassay. The adrenaline-induced hyperpolarization (Adh) in the tissue was recorded by means of the sucrose-gap technique. 2. In the presence of propranolol (1 microM) and DMI (0.5 microM), adrenaline (1 microM) significantly reduced the concentration of cyclic AMP in forskolin-treated ganglia. This effect was prevented by pertussis toxin (5 micrograms ml-1). 3. The relative potency for drugs which increased ganglionic cyclic AMP content was: 50 microM forskolin much greater than 5 mM fluoride greater than 2 mM fluoride greater than 2 mM isobutylmethylxanthine (IBMX) greater than 5 mM caffeine. In contrast, their relative potency for inhibition of the Adh was: 2 mM IBMX greater than 5 mM fluoride greater than 5 mM caffeine much greater than 2 mM fluoride greater than 50 microM forskolin. The Adh was unaffected by pertussis toxin (5 micrograms ml-1). 4. Although the Adh was slightly reduced by the extracellular application of 8-bromo (8-Br) cyclic AMP, the majority of the data suggest that the transduction mechanism underlying the Adh is independent of the intracellular cyclic AMP concentration and provide an example of an alpha 2-adrenoceptor-mediated response that occurs independently of inhibition of adenylate cyclase.

Effects of Bordetella pertussis toxin on catecholamine inhibition of insulin release from intact and electrically permeabilized rat islets

Noradrenaline- and clonidine-induced inhibition of insulin release from intact and electrically permeabilized rat islets was markedly relieved by prior exposure to 100 ng of Bordetella pertussis toxin/ml. The reversal of catecholamine inhibition of insulin secretion by this toxin was not associated with a decrease in specific binding of the alpha 2-adrenergic ligand [3H]yohimbine, and could not be fully explained by an increase in intracellular cyclic AMP. Exposure of intact islets to 1 microgram of pertussis toxin/ml for 2 h, followed by electrical permeabilization and incubation with 5 microCi of [alpha-32P]NAD+, resulted in the ADP-ribosylation in situ of a protein of molecular mass approx. 41 kDa. These results suggest that pertussis toxin alleviates catecholamine inhibition of beta-cell secretory responses by ADP-ribosylating at least one protein of molecular mass 41 kDa. In analogous systems the 41 kDa substrate of pertussis toxin has been shown to be the alpha subunit of Gi, but catecholamine-activated G proteins linked to effector systems other than adenylate cyclase might also be modified by this toxin in pancreatic beta-cells.

Stimulation of hepatic inositol 1,4,5-trisphosphate kinase activity by Ca2+-dependent and -independent mechanisms

A cytosolic fraction derived from rat hepatocytes was used to investigate the regulation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] kinase, the enzyme which converts Ins(1,4,5)P3 to inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4]. The activity was doubled by raising the free Ca2+ concentration of the assay medium from 0.1 microM to 1.0 microM. A 5 min preincubation of the hepatocytes with 100 microM-dibutyryl cyclic AMP (db.cAMP) plus 100 nM-tetradecanoylphorbol acetate (TPA) resulted in a 40% increase in Ins(1,4,5)P3 kinase activity when subsequently assayed at 0.1 microM-Ca2+. This effect was smaller at [Ca2+] greater than 0.5 microM, and absent at 1.0 microM-Ca2+. Similar results were obtained after preincubation with 100 microM-db.cAMP plus 300 nM-vasopressin (20% increase at 0.1 microM-Ca2+; no effect at 1.0 microM-Ca2+). Preincubation with vasopressin, db.cAMP or TPA alone did not alter Ins(1,4,5)P3 kinase activity. It is proposed that these results, together with recent evidence implicating Ins(1,3,4,5)P4 in the control of Ca2+ influx, could be relevant to earlier findings that hepatic Ca2+ uptake is synergistically stimulated by cyclic AMP analogues and vasopressin.

In vivo desensitization of glycogenolysis to Ca2+-mobilizing hormones in rat liver cells

Rat hepatocytes contain several types of Ca2+-linked receptors, all of which stimulate glycogen breakdown by increasing cytosolic free Ca2+ concentration [( Ca2+]c). In vivo desensitization of this Ca2+ messenger system was studied in hepatocytes isolated from either pheochromocytoma (PHEO)-harboring and chronically norepinephrine (NE)-infused rats. Homologous desensitization for alpha 1-adrenergic receptor-mediated phosphorylase activation developed in the early stage of PHEO rats (3-4 wk after implantation), whereas, in the later stage of tumor development or in the NE-infused rats, phosphorylase responses to all Ca2+-mobilizing stimulations were subsensitive (heterologous desensitization). In the homologous desensitization, the [Ca2+]c response to alpha 1-adrenergic stimulation was selectively reduced. We found, using the phenoxybenzamine inactivation method, that there was a linear relationship between alpha 1 receptor density and the [Ca2+]c response; consequently, the blunted [Ca2+]c response to alpha 1-adrenergic stimulation could not be explained by the 34% downregulation of alpha 1 receptors seen in these rats. These results indicated that uncoupling at a step proximal to alpha 1 receptor-stimulated [Ca2+]c increase is also of primary importance in homologous desensitization of phosphorylase activation. On the other hand, heterologous desensitization also involved alteration(s) at steps distal to the rise in [Ca2+]c. Our data demonstrate that prolonged exposure to catecholamines results in desensitization of the [Ca2+]c mobilization pathway and may involve multiple mechanisms.

An attempt at selective protection from phenoxybenzamine of postjunctional alpha-adrenoceptor subtypes mediating contractions to noradrenaline in the rabbit isolated saphenous vein

1. An attempt has been made, with the irreversible alpha-adrenoceptor antagonist phenoxybenzamine, to find the conditions under which postjunctional alpha 1-adrenoceptors in the rabbit isolated saphenous vein can be inactivated, such that postjunctional alpha 2-adrenoceptors can be studied in isolation. 2. Following exposure to various concentrations of phenoxybenzamine, no evidence was found for a selective inactivation of the postjunctional population of alpha 1-adrenoceptors: the "rauwolscine-resistant' (alpha 1-) and the "rauwolscine-sensitive' (alpha 2-) responses to (--)-noradrenaline were similarly affected. 3. However, in "receptor protection' experiments following exposure to a combination of phenoxybenzamine and the selective alpha 2-adrenoceptor antagonist rauwolscine, the remaining response to (--)-noradrenaline appeared to be mediated by a single population of postjunctional alpha 2-adrenoceptors: the response was insensitive to prazosin and rauwolscine was more potent than corynanthine. 4. Partial isolation of the alpha 1-adrenoceptor population was attempted by pre-exposure of the preparation to a combination of phenoxybenzamine and a selective alpha 1-adrenoceptor antagonist, i.e. prazosin or YM-12617. Following receptor protection, the inhibition produced by "selective' concentrations of either of these alpha 1-adrenoceptor antagonists were not significantly different from that observed in control preparations (no phenoxybenzamine). However, the selective alpha 2-adrenoceptor antagonists rauwolscine and CH-38083 were still able to inhibit part of the remaining responses to NA. This is interpreted as indicating that, in addition to protecting the putative postjunctional alpha 1-adrenoceptors, these procedures fail to produce complete inactivation of postjunctional alpha 2-adrenoceptors. 5. It is concluded that, although phenoxybenzamine appeared to be non-selective for the two populations of postjunctional alpha-adrenoceptors in the rabbit isolated saphenous vein, inclusion of a "selective' concentration of a competitive antagonist during the inactivation period results in differing degrees of functional protection of each subtype. Pharmacological isolation was possible for alpha 2-adrenoceptors but not convincingly for alpha 1-adrenoceptors.

Inorganic pyrophosphate is located primarily in the mitochondria of the hepatocyte and increases in parallel with the decrease in light-scattering induced by gluconeogenic hormones, butyrate and ionophore A23187

1. The effects of a variety of hormones on the PPi content and light-scattering of isolated rat liver cells was studied. 2. The basal PPi content was about 130 pmol/mg of cell protein, and increased after hormone addition, in parallel with a decrease in light-scattering which we have observed previously [Quinlan, Thomas, Armston & Halestrap (1983) Biochem. J. 214, 395-404]. 3. The mean increases in PPi content with the agonists shown (as pmol/mg of protein) were: 0.1 microM-glucagon, 25; 20 microM-phenylephrine, 30; 25 nM-vasopressin, 127; glucagon + phenylephrine, 115; glucagon + vasopressin, 382; 100 microM-ADP, 50; 15 microM-A23187, 72; 1 mM-butyrate, 80. 4. In the absence of extracellular Ca2+, vasopressin had little effect on either the PPi content or the light-scattering of hepatocytes. 5. The magnitude of the increase in PPi content correlated with that of the decrease in light-scattering irrespective of the stimulating agent, provided that the PPi did not exceed 300 pmol/mg of protein. Above this value little additional change in light-scattering was observed. 6. Subcellular fractionation showed that over 90% of the cellular PPi was intramitochondrial in both control and stimulated cells. 7. The data support the conclusions of previous experiments using isolated liver mitochondria [Davidson & Halestrap (1987) Biochem. J. 246, 715-723] that hormones increase the mitochondrial matrix volume through a Ca2+-induced rise in matrix [PPi]. 8. It is further proposed that this increase in mitochondrial [PPi] allows entry of ADP into the mitochondria in exchange for PPi and is therefore responsible for the increase in total mitochondrial adenine nucleotides observed after hormone treatment.

Bradykinin-activated membrane-associated phospholipase C in Madin-Darby canine kidney cells

Previous studies have demonstrated that bradykinin stimulates the rapid release of inositol 1,4,5 trisphosphate (IP3) from membrane phosphatidylinositol 4,5 bisphosphate (PIP2) in Madin-Darby canine kidney (MDCK) cells. Since current evidence would suggest that the activation of phospholipase C (PLC) is mediated through a guanine nucleotide-binding protein in receptor-mediated activation of PLC, we evaluated the role of guanine nucleotide proteins in receptor-mediated (bradykinin-stimulated) activation of PLC in MDCK cells. Bradykinin at 10(-7) M produced a marked increase in IP3 formation within 10 s increasing from a basal level of 46.2 to 686.6 pmol/mg cell protein a 15-fold increase. Pretreatment of MDCK cells in culture with 200 ng/ml of pertussis toxin for 4 h reduced the bradykinin-stimulated response to 205.8 pmol/mg protein. A 41-kD protein substrate in MDCK membranes was ADP ribosylated in vitro in the presence of pertussis toxin. The ADP ribosylation in vitro was inhibited by pretreatment of the cells in culture with pertussis toxin. Membranes from MDCK cells incubated in the presence of [3H]PIP2/phosphatidyl ethanolamine liposomes demonstrated hydrolysis of [3H]PIP2 with release of [3H]IP3 when GTP 100 microM or GTP gamma S 10 microM was added. Bradykinin 10(-7) M added with GTP 100 microM markedly increased the rate of hydrolysis within 10 s, thus demonstrating a similar time course of PLC activation as intact cells. These results demonstrate that bradykinin binds to its receptor and activates a membrane-associated PLC through a pertussis toxin-sensitive, guanine nucleotide protein.

Alpha-adrenergic suppression of very-low-density-lipoprotein triacylglycerol secretion by isolated rat hepatocytes

The effect of adrenaline on triacylglycerol synthesis and secretion was examined in isolated rat hepatocytes. Cells were incubated with 0.5 mM-[1-14C]oleate, and the accumulation of triacylglycerol and [14C]triacylglycerol was measured in the incubation medium. Triacylglycerol appearing in the medium was present in a form with properties similar to very-low-density lipoproteins. Triacylglycerol, [14C]triacylglycerol and [14C]phospholipid contents of hepatocytes were also determined. Addition of 10 microM-(-)adrenaline decreased accumulation of glycerolipid in the incubation medium and also decreased cellular [14C]phospholipid content. Prazosin abolished these effects, whereas propranolol did not. The hormone did not affect cellular triacylglycerol content or rates of incorporation of [1-14C]oleate into cell triacylglycerol. The effect of adrenaline on the removal of newly secreted triacylglycerol and the secretion of synthesized glycerolipid was also examined. The catecholamine did not affect rates of removal of newly secreted triacylglycerol. Adrenaline did inhibit the secretion of pre-synthesized lipid by the cells, as assessed by the appearance of radiolabelled triacylglycerol from hepatocytes that had been preincubated with [1,2,3-3H]-glycerol. Adrenaline did not affect rates of fatty acid uptake by hepatocytes, but did stimulate oxidation of [1-14C]oleate, principally to 14CO2.

Prostaglandin F2 alpha and the thromboxane A2 analogue ONO-11113 stimulate Ca2+ fluxes and other physiological responses in rat liver. Further evidence that prostanoids may be involved in the action of arachidonic acid and platelet-activating factor

The administration of prostaglandin F2 alpha (PGF2 alpha) and the thromboxane A2 analogue, ONO-11113, to rat livers perfused with media containing either 1.3 mM- or 10 microM-Ca2+ was followed by a stimulation of Ca2+ efflux, changes in O2 uptake and glucose output, and increase in portal pressure. The responses elicited by 5 microM-PGF2 alpha were similar to those induced by the alpha-adrenergic agonist phenylephrine. At both 1.3 mM and 10 microM extracellular Ca2+, PGF2 alpha induced Ca2+ efflux (70-90 nmol/g of liver), probably from the same source as that released by phenylephrine. Prostaglandin D2 (5 microM) and prostaglandin E2 (5 microM) also induced responses, but these were generally much smaller (less than 30%) than those induced by PGF2 alpha. Similarly to vasopressin and other Ca2+-mobilizing hormones, PGF2 alpha also interacted synergistically with glucagon (and cyclic AMP) in stimulating Ca2+ influx both in the perfused liver and in isolated hepatocytes. By comparison with phenylephrine and PGF2 alpha, ONO-11113 was much more potent in inducing vasoconstriction, and, at concentrations of 10-200 nM, induced a different pattern of changes in Ca2+ flux, respiration and glycogenolysis. There was first a rapid efflux of Ca2+ (45-60 nmol/g of liver), followed by a smaller Ca2+ influx, and a further release of Ca2+ (approx. 90 nmol/g of liver) when ONO-11113 was removed. Respiration was first stimulated but then markedly inhibited. At concentrations less than 5 nM, ONO-11113 induced a sustained stimulation of O2 uptake and a more prolonged efflux of Ca2+, with less Ca2+ efflux occurring upon the removal of the agent. Glycogenolysis followed a pattern which was similar to the Ca2+ response. Co-administration of glucagon did not potentiate Ca2+ influx by ONO-11113, but the action of ONO-11113 was inhibited (50%) by a few minutes' prior administration of 10 nM-vasopressin. The vasoconstrictive action of ONO-11113 was synergistically potentiated by the co-administration of phenylephrine. Since the actions of arachidonic acid, platelet-activating factor and lysophosphatidylcholine in liver were recently found to be cyclo-oxygenase-sensitive, the results provide strong evidence that at least PGF2 alpha and thromboxane A2 may be involved in mediating the action of these agents.

Involvement of alpha-adrenoceptors in myometrial responses in the pro-oestral rat

1. Myometrial responses to different agents acting on adrenoceptors were examined in vivo in the pro-oestrous rat. Changes in spontaneous uterine mechanical activity were recorded isometrically and evaluated in terms of amplitude and duration of uterine contractions. 2. Phenylephrine (10 micrograms kg-1) markedly increased the amplitude and duration of contractions and 40 micrograms kg-1 gave rise to tetanic contractions. 3. Administration of either nicergoline (400 micrograms kg-1) or phentolamine (1000 micrograms kg-1) to phenylephrine-primed rat uterus reduced the strength of contractions and phentolamine abolished the phenylephrine-induced uterine contracture. 4. Following blockade of alpha 2-adrenoceptors by yohimbine (1000 micrograms kg-1) and beta-adrenoceptors by propranolol (2400 micrograms kg-1), a single injection of phenylephrine (100 micrograms kg-1) increased the amplitude of uterine contractions by 30%. 5. Noradrenaline reduced the amplitude of contractions and caused elevation of the baseline level. The response of myometrium to the combination of both propranolol and noradrenaline was the establishment of uterine contracture with subsequent increase of the duration of contractions. 6. These results clearly demonstrate the involvement of alpha-adrenoceptors in the myometrial activity of the rat in vivo during pro-oestrus.

Phosphatidic acid and arachidonic acid each interact synergistically with glucagon to stimulate Ca2+ influx in the perfused rat liver

The administration of phosphatidic acid to rat livers perfused with media containing either 1.3 mM- or 10 microM-Ca2+ was followed by a stimulation of Ca2+ efflux, O2 uptake and glucose output. The responses elicited by 100 microM-phosphatidic acid were similar to those induced by the alpha-adrenergic agonist phenylephrine. Contrary to suggestions that phosphatidic acid acts like a Ca2+-ionophore, no net influx of Ca2+ was detected until the phosphatidic acid was removed. Sequential infusions of phenylephrine and phosphatidic acid indicate that the two agents release Ca2+ from the same intracellular source. The co-administration of glucagon (or cyclic AMP) and phosphatidic acid, and also of glucagon and arachidonic acid, led to a synergistic stimulation of Ca2+ uptake of the liver, a feature similar to that observed after the co-administration of glucagon and other Ca2+-mobilizing hormones [Altin & Bygrave (1986) Biochem. J. 238, 653-661]. A notable difference, however, is that the synergistic stimulation of Ca2+ uptake induced by the co-administration of glucagon and arachidonic acid was inhibited by indomethacin, whereas that induced by glucagon and phosphatidic acid, or glucagon and other Ca2+-mobilizing agents, was not. The results suggest that the synergistic action of glucagon and arachidonic acid in stimulating Ca2+ influx is mediated by prostanoids, but that of glucagon and phosphatidic acid is evoked by a mechanism similar to that of Ca2+-mobilizing agents.

Induction of the skeletal alpha-actin gene in alpha 1-adrenoceptor-mediated hypertrophy of rat cardiac myocytes

Myocardial hypertrophy in vivo is associated with reexpression of contractile protein isogenes characteristic of fetal and neonatal development. The molecular signals for hypertrophy and isogene switching are unknown. We studied alpha (sarcomeric)-actin messenger RNA (mRNA) expression in cultured cardiac myocytes from the neonatal rat. In the cultured cells, as in the adult heart in vivo, expression of cardiac alpha-actin (cACT) predominated over that of skeletal alpha-actin (sACT) mRNA, the fetal/neonatal isoform. alpha 1-Adrenergic receptor stimulation induced hypertrophy of these cells, increasing total RNA and cytoskeletal actin mRNA by 1.8-fold over control, and total alpha-actin mRNA by 4.3 fold. This disproportionate increase in total alpha-actin mRNA was produced by a preferential induction of sACT mRNA, which increased by 10.6-fold over control versus only 2.6-fold for cACT mRNA. The alpha 1-adrenoceptor is the first identified molecular mediator of early developmental isogene reexpression in cardiac myocyte hypertrophy.

Evidence that Ca2+ fluxes and respiratory, glycogenolytic and vasoconstrictive effects induced by the action of platelet-activating factor and L-alpha-lysophosphatidylcholine in the perfused rat liver are mediated by products of the cyclo-oxygenase pathway

The administration of 'acetylglyceryl ether phosphorylcholine' (AGEPC, also known as platelet-activating factor) and L-alpha-lysophosphatidylcholine (LPC) to rat livers perfused with media containing 1.3 mM-Ca2+ was followed by a concentration-dependent efflux of Ca2+ from the liver. Near-maximal response was observed at 100 nM-AGEPC and 50 microM-LPC, and resulted in a net efflux of approx. 130 nmol of Ca2+/g of liver. Onset of Ca2+ efflux occurred about 10 s after AGEPC and LPC administration, reached a maximum after about 50 s (the maximum rate of efflux was approx. 180 nmol/min per g) and thereafter decreased rapidly, and was sometimes followed by a much smaller influx of Ca2+. Sequential infusions of AGEPC or LPC, and phenylephrine, indicate that each of these agents mobilizes Ca2+ from the same intracellular source. The efflux of Ca2+ was not observed in the presence of indomethacin or bromophenacyl bromide, or when the liver was perfused with low-Ca2+-containing (25 microM) media. Other physiological responses, such as changes in respiration, glucose output and portal pressure, were also inhibited under these conditions. The results suggest that the Ca2+-flux changes and other responses are mediated by prostaglandins produced and released within the liver, possibly by cell types other than hepatocytes.

Atrial natriuretic peptide receptors along the rat and rabbit nephrons: [125I] alpha-rat atrial natriuretic peptide binding in microdissected glomeruli and tubules

Binding of [125I] alpha-rat atrial natriuretic peptide ([125I] alpha-RANP) was measured in glomeruli and pieces of tubule microdissected from rat and rabbit nephrons. High densities of specific ANP binding sites were found only in the glomeruli (10-30 X 10(-18) mol X glom-1), whereas no specific binding could be detected in the proximal tubule, the thin segments of the Henle's loop, the thick ascending limb, the distal tubule and the cortical and outer medullary collecting tubules. Rising the temperature from 4 degrees C to 35 degrees C resulted in biphasic kinetics of binding, suggesting a temperature-dependent inactivation of labelled hormone by glomeruli. At 4 degrees C, specific binding of [125I] alpha-RANP was time and dose-dependent and Scatchard analysis of data indicated an apparent equilibrium dissociation constant of 0.63 nM. Competition experiments revealed the following sequence of stereospecificity for binding to rat glomeruli: RANP 3-28 greater than [125I] alpha-RANP = [125I] alpha-HANP = alpha-RANP = antriopeptin III greater than antriopeptin II, whereas binding was unaffected by pharmacological doses of unrelated peptide hormones, prostaglandins, adrenergic agonists, dopamine, histamine and carbamylcholine. The results indicate that glomerular binding sites might be the physiological ANP receptors.

Effects of glucagon and Ca2+ on the metabolism of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate in isolated rat hepatocytes and plasma membranes

Rat hepatocytes whose phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP2) had been labelled for 60 min with 32P were treated with glucagon for 10 min or phenylephrine for 2 min. Glucagon caused a 20% increase in PIP but no change in PIP2 whereas phenylephrine caused a similar increase in PIP but a 15% decrease in PIP2. Addition of both hormones together for 10 min produced a 40% increase in PIP. A crude liver mitochondrial fraction incubated with [32P]Pi and ADP incorporated label into PIP, PIP2 and phosphatidic acid. The PIP2 was shown to be in contaminating plasma membranes and PIP in both lysosomal and plasma-membrane contamination. A minor but definitely mitochondrial phospholipid, more polar than PIP2, was shown to be labelled with 32P both in vitro and in hepatocytes. The rate of 32P incorporation into PIP was faster in mitochondrial/plasma-membrane preparations from rats treated with glucagon or if 3 microM-Ca2+ and Ruthenium Red were present in the incubation buffer. Loss of 32P from membranes labelled in vitro was shown to be accompanied by formation of inositol 1,4,5-trisphosphate (IP3) and inositol 1,4-bisphosphate, and was faster in preparations from glucagon-treated rats or in the presence of 3 microM-Ca2+. It is concluded that glucagon stimulates both PIP2 phosphodiesterase and phosphatidylinositol kinase activities, as does the presence of 3 microM-Ca2+. The resulting formation of IP3 may be responsible for the observed release of intracellular Ca2+ stores. The roles of a guanine nucleotide regulatory protein and phosphorylation in mediating these effects are discussed.

Alpha-adrenergic inhibition of rat cerebellar Purkinje cells in vitro: a voltage-clamp study

1. The effects of the alpha 2-adrenergic agonist clonidine on the membrane properties of Purkinje cells were analysed in sagittal slices of adult rat cerebellum by the use of intracellular recordings performed at a somatic level in the single-electrode voltage-clamp mode. 2. In preliminary current-clamp experiments, clonidine elicited in all cells a hyperpolarization 3-8 mV in amplitude, accompanied by a 15-35% increase of the input resistance when it was added to the bath at a concentration of 2-5 microM. 3. In voltage-clamped cells at a potential of -65 mV. the same concentration of clonidine always induced an outward shift of the holding current (0.2-0.5 nA in amplitude), thus corresponding to the hyperpolarization seen in current-clamp experiments, and this effect was accompanied by a clear increase of membrane resistance. Furthermore, clonidine markedly depressed the inward relaxations induced by hyperpolarizing commands of amplitude less than 10-20 mV whereas those induced by larger steps were much less affected. All these effects of clonidine were reversible when the drug was washed out. 4. When the slices were bathed in a medium containing 10 mM-Cs and 5 X 10(-6) M-tetrodotoxin, the inward relaxations induced by hyperpolarizing steps were abolished. However, a small inward current was still present when the membrane potential was stepped back to -65 mV, which was in turn blocked by the Ca-channel blocker Cd. This inward Ca current was also blocked by 2-5 microM-clonidine in the bath. 5. All these effects of clonidine were abolished by the alpha 1-adrenergic antagonists prazosin and phentolamine at concentrations of 0.5 and 40 microM respectively in the bath. In contrast, they were only weakly antagonized or unaffected by 2 microM of the alpha 2-adrenergic antagonist yohimbine. 6. On the basis of these results and of a previous work on the ionic basis of the inward rectification of Purkinje cells (Crepel & Penit-Soria, 1986), it appears that these neurones exhibit a well developed alpha (possibly alpha 1)-adrenergic inhibition of a low-threshold Ca conductance and a Ca-dependent K conductance operating near resting potential.

Cardiac myocyte hypertrophy is associated with c-myc protooncogene expression

The mechanism of hormonally induced cell hypertrophy is unknown. Stimulation of cardiac myocytes by alpha 1-adrenergic agents, phorbol esters, and serum induces an increase in the cell size of nondividing cardiac myocytes in primary culture. Expression of the c-myc gene, known to be increased in growth factor-induced cell division, was studied in this model of cell hypertrophy. The alpha-adrenergic agonist norepinephrine (0.002-20 microM) increased levels of c-myc-encoded mRNA to 10-fold over control levels. This increase was detectable at 30 min, peaked at 2 hr, and returned to baseline by 6 hr after stimulation. The norepinephrine response was abolished by the alpha 1-antagonist terazosin (2 microM) but was not affected by the beta-adrenergic antagonist propranolol (2 microM) and was only slightly (25%) attenuated by the alpha 2-adrenergic antagonist yohimbine (2 microM). Serum and the phorbol ester tumor promoter phorbol 12-myristate 13-acetate also enhanced c-myc expression in cardiac myocyte cultures. These findings show that the induction of cardiac myocyte hypertrophy is associated with enhanced expression of the c-myc gene and suggest that hormonally induced cell hypertrophy and cell division share common mechanistic pathways.