Ultracytochemical localization of adenylate cyclase activity in rat thymocytes

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Angiotensin Receptors Heterodimerization and Trafficking: How Much Do They Influence Their Biological Function?

G-protein–coupled receptors (GPCRs) are targets for around one third of currently approved and clinical prescribed drugs and represent the largest and most structurally diverse family of transmembrane signaling proteins, with almost 1000 members identified in the human genome. Upon agonist stimulation, GPCRs are internalized and trafficked inside the cell: they may be targeted to different organelles, recycled back to the plasma membrane or be degraded. Once inside the cell, the receptors may initiate other signaling pathways leading to different biological responses. GPCRs’ biological function may also be influenced by interaction with other receptors. Thus, the ultimate cellular response may depend not only on the activation of the receptor from the cell membrane, but also from receptor trafficking and/or the interaction with other receptors. This review is focused on angiotensin receptors and how their biological function is influenced by trafficking and interaction with others receptors.

Inducible nitric oxide synthase in the myocard

Recognition of significance of nitric oxide synthases (NOS) in cardiovascular regulations has led to intensive research and development of therapies focused on NOS as potential therapeutic targets. However, the NOS isoform profile of cardiac tissue and subcellular localization of NOS isoforms remain a matter of debate. The aim of this study was to investigate the localization of an inducible NOS isoform (NOS2) in cardiomyocytes. Employing a novel immunocytochemical technique of a catalyzed reporter deposition system with tyramide and electron microscopical immunocytochemistry complemented with Western blotting and RT-PCR, we detected NOS2 both in rat neonatal and adult cultured cardiomyocytes and in the normal myocard of adult rats as well as in the human myocard of patients with dilative cardiomyopathy. NOS2 was targeted predominantly to a particulate component of the cardiomyocyte--along contractile fibers, in the plasma membrane including T-tubules, as well as in the nuclear envelope, mitochondria and Golgi complex. Our results point to an involvement of NOS2 in maintaining cardiac homeostasis and contradict to the notion that NOS2 is expressed in cardiac tissue only in response to various physiological and pathogenic factors. NOS2 targeting to mitochondria and contractile fibers suggests a relationship of NO with contractile function and energy production in the cardiac muscle.

Adenylyl cyclase in the heart: an enzymocytochemical and immunocytochemical approach

This review provides a discussion of the localization of adenylyl cyclase (AC) in normal mammalian heart tissue employing enzymocytochemistry (detection of the catalytic activity of AC by a metal precipitation technique) and immunocytochemistry (immunolabeling of the enzyme protein with antibodies against AC subtypes). By the metal precipitation technique, AC activity was localized in adult guinea pig cardiomyocytes along the sarcolemma and the T-tubule membranes. This reaction can be enhanced by hormones and guanylyl imidodiphosphate, fluoride, and forskolin. With this technique, no precipitates were detected at the sarcoplasmic reticulum. However, under ischemic conditions, AC activity was also found in the junctional sarcoplasmic reticulum of rat cardiomyocytes. Immunocytochemistry revealed AC in the plasma membrane of rat cardiomyocytes. Detection of AC in the perinuclear space of cardiomyocytes might reflect initiation of synthesis and processing of the enzyme protein. Colocalization of AC with cytoskeleton fibers of non-cardiomyocytes emerging in the cell culture of neonatal rat cardiocytes imply a direct cytoskeletal-AC interaction. Finally, it can be stated that the immunolabeling pattern of AC in cryosections of adult and new-born rat hearts reveals a good correspondence with the localization of AC activity in cardiomyocytes demonstrated by enzymocytochemistry.

Intracellular localization of inducible nitric oxide synthase in neonatal rat cardiomyocytes in culture

Recognition of the role of nitric oxide (NO) in cardiovascular regulations raised an acute interest in NO-generating enzymes-nitric oxide synthases (NOS). Nevertheless, the subcellular localization of inducible isoform of NOS (NOS II) and regulation of its expression in the cardiomyocyte still remains to be elucidated. Therefore, we focused this study on the subcellular localization of NOS II in cultured neonatal rat cardiomyocytes using immunocytochemical techniques at the light and electron microscopic level as well as the demonstration of NADPH-diaphorase activity and the Griess assay for NO measurement. Cultivation of neonatal cardiomyocytes during 2 and more days induced a moderate increase in the NOS II immunolabeling in defined cytoplasmic structures and a nuclear NOS II staining in some cells. Exposure of the cell cultures to exogenous cAMP markedly stimulated NO production with a concomitant enhancement of NOS II immunolabeling of cardiomyocytes. cAMP-induced changes were significantly attenuated by dexamethasone. This report provides evidence for the localization of NOS II in the perinuclear space, Golgi complex, mitochondria, plasma membrane and along contractile fibers of cardiomyocytes, as well as for the appearance of NOS II staining of the cell nuclei in the course of cultivation. In non-cardiomyocytes contaminating the cell culture, positive immunoreaction was detected in the Golgi complex and endoplasmic reticulum. Our data point to a notable constitutive expression of NOS II in rat cardiomyocytes apparently dependent on the developmental stage.

Comparative immunocytochemical demonstration of G proteins in rat heart tissue

Localization of G proteins in the rat heart tissue was investigated using primary affinity-purified antibodies against synthetic peptides with amino acid sequences corresponding to alpha-subunits (alpha i common and alpha i 1, 2) of G proteins. Detection of immunoreactivity was performed with the peroxidase-anti-peroxidase complex (PAP), avidin-biotin complex (ABC) and fluorescein-labelled secondary antibodies for light microscopy and the protein A-gold technique for electron microscopy. In ventricles and atria, immunostaining for G proteins was detected in the sarcolemma and perinuclear space of cardiomyocytes. In endotheliocytes and fibroblasts, immunoreactivity was present also in the endoplasmic reticulum. All four immunocytochemical methods permit to demonstrate the same localization of G proteins in heart tissue. The ABC method and fluorescein labelled secondary antibodies technique showed the same sensitivity which is higher than that of the PAP method. Nomarski contrast microscopy enhanced the visualization of the final reaction product formed by the peroxidase reaction developed with diaminobenzidine in the ABC method. The results are discussed in terms of the role of G proteins in signal transduction via plasma membrane and membranes of the intracysternal space of the cell.

Cytochemical localization of adenylate cyclase activity in rat olfactory cells

Adenylate cyclase activity was demonstrated in the cilia, dendritic knob and axon of rat olfactory cells by using a strontium-based cytochemical method. The activity in the cilia and the dendritic knob was enhanced by non-hydrolyzable GTP (guanosine triphosphate) analogues and forskolin, and inhibited by Ca2+, all in agreement with biochemical reports of the odorant-sensitive adenylate cyclase. The results support the hypothesis of cyclic AMP working as a second messenger in olfactory transduction and imply that the transduction sites exist not only in the olfactory cilia but also in the dendritic knob. Enzymatic activity was also observed in the olfactory dendritic shaft by treating the tissue with 0.0002% Triton X-100, although the properties and role of the enzyme in this region are uncertain. The detergent inhibited the enzymatic activity in the cilia and the dendritic knob.

Specificity of cytochemical demonstration of adenylate cyclase in liver using adenylate-(beta, gamma-methylene) diphosphate as substrate

Adenylate cyclase activity was demonstrated cytochemically in rat liver for the first time under the light microscope using cryostat sections mounted on glass cover slips and fixed with 1% glutaraldehyde for 1 min. Adenylate-(beta, gamma-methylene)diphosphate (AMP-P(CH2)P) was introduced as a new substrate for adenylate cyclase. It was found that adenylate cyclase was distributed heterogenously within the liver lobule. The enzyme activity was stronger in the area surrounding the central vein. A more specific localization at the plasma membrane and less unspecific background was obtained with AMP-P(CH2)P as compared to adenylylimidodiphosphate (AMP-P(NH)P). The specificity of the enzyme reaction using AMP-P(CH2)P was proved by increased formation of reaction product in the presence of 0.05 mg/ml glucagon and 0.125 mg/ml cholera toxin, as well as by inhibition of the reaction with 0.05 mg/ml alloxan. These effects were also observed at the electron microscopic level. On the other hand, no increase in reaction was observed in the presence of glucagon with AMP-P(NH)P as a substrate for adenylate cyclase, and only a weak activation was observed after adding cholera toxin; alloxan-inhibition was not complete. These effects may be due to the presence of enzymes which hydrolyze AMP-P(NH)P nonspecifically, superimposing on the product of adenylate cyclase activity. We therefore suggest the use of AMP-P(CH2)P as substrate for histochemical adenylate cyclase demonstration in the liver.

Differences of adenylate cyclase localization in guinea-pig peritoneal macrophages under different physiological conditions: an ultracytochemical study

The ultrastructural localization of adenylate cyclase activity has been investigated in unfixed guinea-pig peritoneal macrophages in different physiological states (such as suspension, adhesion and phagocytosis) using a medium containing 5'-adenylyl-imidodiphosphate (AMP-PNP) as the substrate. Adenylate cyclase activity was observed in cytoplasmic vacuoles of macrophages in suspension; in the perinuclear space, endoplasmic reticulum, Golgi complex and pseudopods of adherent macrophages; and surrounding phagocytosed polystyrene particles. The activity was inhibited by Alloxan added to the incubation medium and no staining was observed when AMP-PNP was omitted from the medium. The segregation of this enzyme to phagocytic vacuoles and pseudopods may have significant implications in understanding cyclic nucleotide function in adhesion and phagocytosis.