The role of cyclic nucleotides in the action of peripheral-type benzodiazepine receptor ligands in rat aorta

1. Peripheral-type benzodiazepine ligands (Ro 5-4864, AHN-086, PK 11195 and PK 14105) inhibit, in a concentration-dependent and non-competitive manner, noradrenaline-induced contractions in isolated rat aortic rings (IC50 values: 24 +/- 1.8, 49 +/- 2.5, 15 +/- 1.2, 49 +/- 3.2 microM, respectively). 2. This effect is probably not mediated by peripheral-type benzodiazepine receptors and is not related to the presence of endothelium. 3. All compounds inhibited phosphodiesterase activity in vitro. 4. From the results obtained with nucleotide analogs, calcium antagonists and specific inhibitors of PDE isoenzymes, it can be concluded that the actions of AHN-086 and PK 11195 are related to effects on PDE I, III and IV.

Molecular mechanism of cyclic-nucleotide-gated channel activation

Studies on the activation of ligand- and voltage-gated ion channels have identified regions involved in both ligand binding and voltage sensing, but relatively little is known about how such domains are coupled to channel opening. Here we investigate the structural basis for the activation of cyclic-nucleotide-gated channels, which are directly opened by cytoplasmic cyclic nucleotides but are structurally homologous to voltage-gated channels. By constructing chimaeras between cyclic-nucleotide-gated channels cloned from bovine retinal photoreceptors and catfish olfactory neurons, we identify two distinct domains that are important for ligand binding and channel gating. A putative alpha-helix in the carboxy-terminal binding domain determines the selectivity of the channel for activation by cGMP relative to cAMP. A domain in the amino-terminal region determines the ease with which channels open and thus influences agonist efficacy. We propose that channel opening is coupled to an allosteric conformational change in the binding site which enhances agonist binding. Thus, cyclic nucleotides activate the channel by binding tightly to the open state and stabilizing it.

Ca2+ influx in platelets: activation by thrombin and by the depletion of the stores. Effect of cyclic nucleotides

In aspirin-treated platelets the thrombin-induced increase of cytosolic Ca2+ ([Ca2+]i) associated with the release from the intracellular stores is followed by a decrease to the baseline which is largely dependent on the re-uptake into the stores. This is shown by the further increase of [Ca2+]i upon inhibition of the endomembrane Ca(2+)-ATPase with thapsigargin. The re-uptake of Ca2+ into the stores is accelerated by sodium nitroprusside (SNP) or prostacyclin (PGI2). In all cases, after store depletion with thapsigargin the influx of external Ca2+ is maximal. After a thrombin-induced cycle of Ca(2+)-release re-uptake the stores are partly full: in these conditions the addition of external Ca2+ elicits a significant increment of [Ca2+]i and a further filling of the stores. Both are strongly reduced if Ca2+ addition is preceded by SNP or PGI2. Similar results are obtained also if (by supplementing and then cheleting Ca2+) the stores are as full as in native platelets at the moment of adding Ca2+. The thrombin-activated Ca2+ influx is reversed by hirudin. A PGI2- and SNP-sensitive Mn2+ influx is observed if Mn2+ is added in place of Ca2+. It is concluded that thrombin activates a cyclic nucleotide-sensitive Ca2+ (and Mn2+) influx pathway dependent on the occupancy of the thrombin receptor and independent of the filling state of the stores. In the absence of thrombin, thapsigargin releases Ca2+ relatively rapidly from a fraction of the stores; the remaining deposits are discharged much more slowly. This may indicate that platelets contain two distinct classes of agonist-sensitive stores. The addition of external Ca2+ (or Mn2+) at short or long incubation times with thapsigargin monitors the influx of Ca2+ activated by the depletion of one or both types of stores. The depletion of each type of store activates Ca2+ (Mn2+) influx. This type of cation influx is not inhibited by the cyclic nucleotides.

A second subunit of the olfactory cyclic nucleotide-gated channel confers high sensitivity to cAMP

Sensory transduction in olfactory neurons is mediated by intracellular cAMP, which directly gates a nonselective cation channel. A cDNA encoding a cyclic nucleotide-gated (CNG) ion channel subunit (rOCNC1) has been cloned previously from rat olfactory epithelium. However, differences between the functional properties of rOCNC1 and the native olfactory CNG channel suggest that the native channel could be composed of several distinct subunit types. Here, we report the cloning and characterization of a cDNA encoding a second olfactory CNG channel subunit (rOCNC2) that is 52% identical to rOCNC1 and that is expressed specifically in olfactory sensory neurons. Expression of rOCNC2 alone in Xenopus oocytes does not lead to detectable CNG currents. However, coexpression of rOCNC2 with rOCNC1 results in a CNG conductance that differs from that detected upon expression of rOCNC1 alone and more closely resembles the native conductance in several respects, including its sensitivity to cAMP. This suggests that the native olfactory CNG channel is a hetero-oligomer composed of rOCNC1 and rOCNC2 subunits.

Transduction diversity in olfaction

Odors are powerful stimuli that can focus the attention, elicit behaviors (or misbehaviors) and even resurrect forgotten memories. These actions are directed by the central nervous system, but they depend upon the initial transduction of chemical signals by olfactory receptor neurons. Electrophysiological recordings suggest that the responses of olfactory receptor neurons to odors are more diverse than was initially believed, being mediated by effects on several different conductances. Both excitatory and inhibitory responses are produced by these effects and some, if not all, odors can affect more than one component of the membrane conductance. The extent of this diversity is reviewed here, and its impact on our understanding of odor discrimination is discussed.

Cyclic ADP-ribose: a new member of a super family of signalling cyclic nucleotides

Cyclic nucleotides are second messengers exerting their cellular effects mainly through protein phosphorylation. A new member of this family, cyclic ADP-ribose, is involved, instead, in mediating mobilization of Ca+2 from internal stores. The structure of this nucleotide has now been determined by X-ray crystallography and accumulating evidence indicates it may be an endogenous modulator of the Ca+2 induced Ca+2 release mechanism. This article summarizes the current knowledge of the structure, the mechanism of action and the metabolic enzymes of this novel nucleotide. With this new addition, the signalling functions of the cyclic nucleotide family are now extended from protein phosphorylation to Ca+2 signalling.

[Role of calcium ions and cyclic nucleotides in the regulation of somatotroph functional activity in rats as a function of age]

Somatotropic hormone (STH) biosynthesis and secretion were studied in primary adenohypophyseal cultures of neonatal, prepubertal, and adult rats. It was shown by disc PAAG electrophoresis of products synthesized in incubation of neonatal rat hypophyseal cells that L-14C leucin incorporates predominantly in the STH containing fraction. The share of prelabeled STH secreted into the culture virtually did not depend on the age of animals, this indicating the maturity of mechanisms of basal somatotroph secretion as early as in the neonatal period of development. Ca-regulating agents (ionophore A23187, EGTA) caused quantitatively similar changes of somatotroph secretory activity in cultures of hypophyseal cells from rats of different ages. At the same time, clear-cut age-specific features of stimulating effect of dibutyryl derivatives of cAMP and cGMP on secretion of immunoreactive STH were detected. The results indicate that somatotropic function in neonatal hypophysis is a dominant one and demonstrates increased reactivity to secretogenic action of cyclic nucleotide analogs.

Structural basis of ion channel permeation and selectivity

There has been rapid progress in understanding the structural basis of ion selectivity and permeation in both ligand- and voltage-gated channels. Recognition of similarities in overall architecture within a channel class has led to an increasing focus on the specific molecular determinants that endow a channel with its own distinctive character. It has been possible in some cases to identify individual amino acids essential for ion selectivity.

Modulation of the inhibitory action of ATP on acetylcholine-activated current by protein phosphorylation in rat sympathetic neurons

Modulation by protein phosphorylation of the relation between acetylcholine (ACh)-activated current (IACh) and adenosine triphosphate-(ATP)-activated current (IATP) was investigated with the whole-cell voltage-clamp technique in rat sympathetic neurons. During simultaneous activation by 100 microM ATP of an inward current, the current evoked by 100 microM ACh was reduced to 60-70% of that in the absence of ATP. Effects of compounds that are known to modulate protein phosphorylation were tested by including them in the intracellular solution. The reduction of IACh by ATP was not observed when K252a (1 microM), a non-selective protein kinase inhibitor, adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma S], 1 mM) or alpha, beta-methylene ATP (1 mM) were included in the intracellular solution. Activators of protein kinases, adenosine 3',5'-cyclic monophosphate (cAMP, 100 microM), guanosine 3',5'-cyclic monophosphate (cGMP, 100 microM), phorbol 12-myristate 13-acetate (PMA, 1 microM), also abolished the reduction by ATP of IACh. The effects of okadaic acid, a protein phosphatase inhibitor, were paradoxical: okadaic acid (2 microM) itself abolished the reduction by ATP of IACh but it "antagonized" the abolishment by cAMP or cGMP of the reduction of IACh. Okadaic acid did not affect the disappearance of the reduction of IACh by ATP in the presence of intracellular PMA. The results suggest that the interaction between IACh and IATP is regulated by protein phosphorylation/dephosphorylation. Possible mechanisms underlying the effects of these modulators of protein phosphorylation are discussed.

Another member of the cyclic nucleotide-gated channel family, expressed in testis, kidney, and heart

Cyclic nucleotide-gated cation channels are essential in visual and olfactory signal transduction. An additional member of the cGMP-gated channel family, termed CNG-3, has been cloned from bovine kidney. Its deduced amino acid sequence is 60% and 62% identical with the CNG-channel proteins from bovine rod outer segment and bovine olfactory epithelium, respectively. Northern analysis and sequences amplified by the PCR showed that the CNG-3 mRNA is present in testis, kidney, and heart. Calcium permeated the expressed channel in the presence of extracellular Mg2+ and Na+ at membrane potentials from -100 to +45 mV. It is likely that CNG-3 protein is responsible for cGMP-induced Ca2+ entry in cells other than sensory cells.

CO2 and cerebral circulation in newborn pigs: cyclic nucleotides and prostanoids in vascular regulation

The role of cyclic nucleotides and prostanoids in cerebrovascular reactivity to increased carbon dioxide was investigated in anesthetized and artificially ventilated newborn pigs equipped with closed cranial windows. Pial arteriolar diameter was measured, and cortical periarachnoid cerebrospinal fluid (CSF) was collected from beneath the cranial window for determination of adenosine 3',5'-cyclic monophosphate (cAMP), guanosine 3',5'-cyclic monophosphate (cGMP), and prostanoids. Progressively increasing arterial PCO2 (PaCO2) from normocapnia (33 +/- 1 mmHg) to hypercapnia (final PaCO2, 83 +/- 2 mmHg) resulted in dose-dependent pial arteriolar dilation and concomitant increases in cAMP, cGMP, and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) in cortical CSF. N omega-methyl-L-arginine, N omega-nitro-L-arginine, N omega-nitro-L-arginine methyl ester, methylene blue, and LY 83583 did not inhibit cerebral vasodilation or the increases in cortical cAMP/cGMP induced by hypercapnia. Indomethacin abolished the vasodilatory response to hypercapnia and attenuated the hypercapnia-induced increases in cAMP and cGMP. Prostacyclin analogues increased both cAMP and cGMP levels in cortical CSF and induced pial arteriolar dilation (iloprost > carbaprostacyclin). The present data suggest that in newborn pigs cyclic nucleotides are involved in cerebral vasodilation in response to hypercapnia via a prostanoid-dependent mechanism.

The signal transduction mechanism responsible for gamma interferon-induced indoleamine 2,3-dioxygenase gene expression

We examined the signal transduction mechanism responsible for the gamma interferon-induced indoleamine 2,3-dioxygenase (IDO) gene expression in a human monocytic cell line, THP-1. Our results suggest that gamma interferon-induced activation of protein tyrosine kinase is a prerequisite for gene expression and that activation of protein kinase C and another unknown signal(s), both of which are transduced by the protein tyrosine kinase, synergistically induce IDO gene expression. Neither Ca2+ influx nor cyclic nucleotide-dependent kinases were suggested to be involved in the signaling pathway.

Role of cyclic nucleotides in the regulation of endothelin-1 production by human endothelial cells

The regulation of endothelin-1 (ET-1) production by endothelial cells is likely of crucial physiological importance in the maintenance of vascular homeostasis. The aim of the present study was to explore the possible role of cyclic nucleotides in the control of ET-1 production in human umbilical vein endothelial cells (HUVEC). ET-1 release was determined by measuring levels of immunoreactive ET-1 in HUVEC-conditioned media after 6-h incubations. In the presence of 10% fetal calf serum (FCS) there was a threefold increase in ET-1 release compared with serum-free conditions (1.96 +/- 0.17 vs. 0.56 +/- 0.06 pg/micrograms protein), respectively. Inhibition of protein kinase (PK) C using staurosporine (10 nM) reduced basal ET-1 release by approximately 50% and completely prevented the response to FCS. In contrast, the addition of other PK inhibitors had little effect on basal or serum-stimulated ET-1 release at the concentrations used. N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP) produced significant alterations in ET-1 release depending on the basal level of production. Under serum-free conditions of low basal ET-1 production, DBcAMP increased ET-1 release by 68 +/- 22% but only at the highest concentration studied (1 mM). The dose-response relationship for DBcAMP was potentiated by KT-5720 (0.1 microM), an inhibitor of PKA, with a significant shift to 10-fold lower concentrations, whereas it was blocked by KT-5823 (4 microM), which can inhibit PKG.(ABSTRACT TRUNCATED AT 250 WORDS)

[Role of lipid peroxidation, prostanoids and cyclic nucleotides in the individual resistance of animals to exogenous catecholamines]

The role of the lipid peroxidation, power of the antioxidants, peculiarities of synthesis of some prostanoids and cyclic nucleotides in determination of the rabbit individual sensitivity to exogenous catecholamines was studied. Some peculiarities of the initial state of homeostasis both of the survived and lost rabbits are demonstrated. It is determined that the low stability to izadrine's administration of the lost rabbits may be associated with the higher basal activity of the lipid peroxidation and the smaller antioxidative power. The peculiarities of the arachidonic acid's metabolism, which are associated with this, modulate the activity of the cyclic nucleotide enzymes. The latter may mobilize the cAMP-dependent mechanisms of the intracellular hormone signal transduction.

Sweet taste transduction in hamster taste cells: evidence for the role of cyclic nucleotides

1. Physiological and behavioral responses to artificial sweeteners, natural sweeteners, and cyclic nucleotides were assessed using two techniques. An extracellular "in situ" technique recorded action potentials from fungiform taste buds and the two-bottle preference test measured behavioral preferences for the different sweeteners. 2. Two high-potency sweeteners, NC-00274-01 (NC01) and NC-00044-AA (NCAA), were preferred over water at micromolar concentrations. Saccharin and sucrose were likewise preferred, but at millimolar concentrations. 3. Bursts of action currents were elicited by sucrose at 200 mM, saccharin at 20 mM, and NCAA at 0.1 mM. A concentration-response curve for the high-potency sweetener NC01 revealed a threshold concentration of 1 microM and a saturation concentration of 100 microM. No responses were elicited by aspartame. 4. The responses to different sweeteners adapted rapidly at saturating concentrations. With NC01, adaptation was concentration dependent: at threshold the response adapted very slowly if at all. Adaptation increased with increasing concentration. 5. Membrane-permeant analogues of adenosine 3',5'-cyclic monophosphate and guanosine 3',5'-cyclic monophosphate mimicked sweeteners in their ability to elicit a response. This occurred with high fidelity: nearly every taste bud that responded to sweeteners also responded to the nucleotides and every sweet-unresponsive taste bud was nucleotide unresponsive. 6. The sweet responses and nucleotide responses occurred in the absence of permeant apical cations and were not enhanced nor diminished by the presence of such cations. Amiloride had no effect on the sweet response.

Glucocorticoid receptors in bovine adrenal medullary cells in culture: regulation by cyclic nucleotides

Glucocorticoid receptor levels within a given cell determine the glucocorticoid effect in the target tissue. Glucocorticoid receptors are present in adrenal medullary cells in culture where they are involved in the regulation of catecholamine biosynthesis. Modulation of glucocorticoid receptor protein and/or messenger RNA levels in response to cyclic nucleotides has been found in various cell types. In this study, we have investigated the effects of cyclic AMP and cyclic GMP on glucocorticoid receptor binding and glucocorticoid receptor-mediated function in Percoll-isolated bovine adrenal medullary cells in culture. Four-day treatment of cells with 8-bromo-cyclic AMP (10(-3) M) an analogue of cAMP, or forskolin (10(-5) M), an activator of adenylate cyclase, decreased soluble [3H]dexamethasone binding by 55 and 54%, respectively. 8-Bromo-cyclic GMP treatment decreased [3H]dexamethasone binding by 31 and 34% at 10(-5) and 10(-4) M, respectively. Treatment with 8-bromo-cyclic AMP or forskolin, but not 8-bromo-cyclic GMP, elevated cortisol levels in the medium of treated cells, presumably by elevating steroidogenesis in contaminating cortical cells. Cultures further purified to produce chromaffin-enriched cell cultures, also showed a loss (41%) in soluble [3H]dexamethasone binding when treated with 8-bromo-cyclic AMP (10(-3) M). Four-day treatment of standard Percoll-isolated cells with low concentrations of cortisol (10(-9) to 2 x 10(-7) M) similar to that found in the medium of 8-bromo-cyclic AMP-treated cells, did not decrease soluble [3H]dexamethasone binding, whereas higher cortisol concentrations (10(-6) M) produced a 62% loss in soluble binding. Adsorption of cortisol with bovine serum albumin (5 mg/ml) prevented a cortisol (10(-6) M)-induced loss in soluble [3H]dexamethasone binding with no effect on the 8-bromo-cyclic AMP-induced loss in binding, suggesting that the decrease in binding observed following 8-bromo-cyclic AMP treatment is not due to the release of cortisol from contaminating cortical cells. Finally, we report a loss in the ability of 8-bromo-cyclic AMP- or 8-bromo-cyclic GMP-treated cells to fully induce the activity of phenylethanolamine N-methyltransferase in response to cortisol, indicating that decreases in soluble [3H]dexamethasone binding translate into a decrease in the functional consequence of glucocorticoid receptor binding in adrenal medullary cells. In conclusion, these results indicate that long-term increases in cyclic nucleotide second messengers are able to decrease glucocorticoid receptor binding in bovine adrenal medullary cells, via a mechanism independent of released cortisol.(ABSTRACT TRUNCATED AT 400 WORDS)

Nitric oxide and endothelin secretion by brain microvessel endothelial cells: regulation by cyclic nucleotides

Endothelin (ET)-1 was originally characterized as a potent vasoconstrictor peptide secreted by vascular endothelial cells. It possesses a wide range of biological activities within the cardiovascular system and in other organs, including the brain. Also secreted by endothelial cells, nitric oxide (NO), has recently been identified as a relaxing factor, as well as a pleiotropic mediator, second messenger, immune defence molecule, and neurotransmitter. Most of the data concerning the secretion of these two agents in vitro has been collected from studies on macrovascular endothelial cells. Given the remarkable heterogeneity of endothelia in terms of morphology and function, we have analyzed the ability of brain microvessel endothelial cells in vitro to release ET-1 and NO, which, at the level of the blood-brain barrier, have perivascular astrocytes as potential targets. The present study was performed with immortalized rat brain microvessel endothelial cells, which display in culture a non transformed phenotype. Our data demonstrate that: (1) these cells release NO when induced by IFN gamma and TNF alpha, (2) they constitutively secrete ET-1, and (3) cAMP potentiates the cytokine-induced NO release and exerts a biphasic regulation on ET-1 secretion: micromolar concentrations of 8-Br-cAMP inhibit and higher doses stimulate ET-1 secretion. This stimulation is blocked by EGTA and the calmodulin antagonist W7, but not by protein kinase C inhibitors, suggesting the involvement of the calmodulin branch of the calcium messenger system. These results suggest that cerebral microvessel endothelial cells may participate in vivo to the regulation of glial activity in the brain through the release of NO and ET-1.

[Role of prostaglandins and cyclic nucleotides in the mechanism of development of drug-induced thrombocytopenia]

The experiments on dogs have shown that at the moment of maximal thrombocytopenia, induced by using of 0.7 mg/kg rubomycin intravenous daily during 5 days, there was a sharp increase in cAMP concentration, while the content of prostaglandin E group and F2 alpha in blood decreased. Concentration of cGMP began to increase by the 10th day. We can arrive at the opinion that the rubomycin acts on the receptors of hemopoietic precursor cells' membrane. This causes the activation of adenylate-cyclase, which stimulates the formation of cAMP, causing cells' proliferation depression. This mechanism may be regarded as the restoration reaction of megakaryocyte-thrombocyte system.

Respiratory physiology in pregnancy

In conclusion, it is important to appreciate the anatomic and physiologic adaptations in pregnancy in order to accurately diagnose and treat cardiopulmonary disease states in the gravid woman. Without knowing what constitutes normalcy in pregnancy, inappropriate diagnosis and interventions may occur. Furthermore, this knowledge is fundamental for understanding how disease states affect pregnancy and how pregnancy affects disease.

Regulation of contraction and relaxation in arterial smooth muscle

Intracellular calcium concentration ([Ca2+]i)-dependent activation of myosin light chain kinase and its phosphorylation of the 20-kd light chain of myosin is generally considered the primary mechanism responsible for regulation of contractile force in arterial smooth muscle. However, recent data suggest that the relation between [Ca2+]i and myosin light chain phosphorylation is variable and depends on the form of stimulation. The dependence of myosin phosphorylation on [Ca2+]i has been termed the "[Ca2+]i sensitivity of phosphorylation." The [Ca2+]i sensitivity of phosphorylation is "high" when relatively small increases in [Ca2+]i induce a large increase in myosin phosphorylation. Conversely, the [Ca2+]i sensitivity of phosphorylation is "low" when relatively large increases in [Ca2+]i are required to induce a small increase in myosin phosphorylation. There are two proposed mechanisms for changes in the [Ca2+]i sensitivity of phosphorylation: Ca(2+)-dependent decreases in the [Ca2+]i sensitivity of phosphorylation induced by phosphorylation of myosin light chain kinase by Ca(2+)-calmodulin protein kinase II and agonist-dependent increases in the [Ca2+]i sensitivity of phosphorylation by inhibition of a myosin light chain phosphatase. I will review the proposed mechanisms responsible for the regulation of [Ca2+]i and the [Ca2+]i sensitivity of phosphorylation in arterial smooth muscle.

Cyclic nucleotide-dependent regulation of Mn2+ influx, [Ca2+]i, and arterial smooth muscle relaxation

Elevations in cyclic nucleotide levels can decrease myoplasmic [Ca2+] ([Ca2+]i) and thereby induce arterial smooth muscle relaxation. We evaluated whether cyclic nucleotide-induced reductions in [Ca2+]i are caused by 1) decreased Ca2+ influx or 2) increased Ca2+ sequestration or efflux. Swine carotid medial tissues were loaded with fura-2, and Ca2+ influx was estimated from the quenching rate of 360-nm fluorescence after addition of extracellular Mn2+. Histamine stimulation or high KCl depolarization increased Mn2+ influx, [Ca2+]i, and contractile force. The Ca2+ channel blocker diltiazem attenuated histamine- or KCl-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin (which increases cAMP) or nitroglycerin (which increases cGMP) attenuated histamine-induced increases in Mn2+ influx, [Ca2+]i, and force. Addition of forskolin or nitroglycerin also relaxed KCl depolarized tissues; however, Mn2+ influx and [Ca2+]i remained high. These results suggest that Mn(2+)-induced quenching of 360-nm fluorescence is an estimate of Ca2+ influx in the intact swine carotid artery. These results also suggest that cyclic nucleotides can relax swine arterial smooth muscle by at least two mechanisms: 1) reduction of [Ca2+]i primarily induced by decreases in Ca2+ influx and 2) uncoupling force from [Ca2+]i without changing Ca2+ influx or [Ca2+]i.