Structure-activity relations of the cardiac gap junction channel

Cardiac gap junction channels play the important roles of synchronizing pacemaker cells and allowing impulse propagation along the conduction system and throughout the ventricular myocardium. These channels, which support current flow in both longitudinal and tranverse directions, are permeable to anions and cations with radii less than approximately 0.5 nm and in rat heart have unitary conductances on the order of 50 pS. This unitary conductance is consistent with channel geometry described by a right cylindrical pore with diameter large enough for the brilliantly fluorescent dye molecule lucifer yellow to pass between cells. These channels, like others in biological systems, are opened and closed by various treatments, a process termed gating. Cytoplasmic acidification reduces junctional conductance (gj), an effect that is apparently potentiated by elevated myoplasmic Ca ions. Reduced gj also occurs in response to a variety of lipophilic molecules, including halothane, heptanol, and unsaturated fatty acids; the mechanism of action may involve disruption of the protein-lipid microenvironment of the gap junction channel. Arachidonic acid uncouples, and this effect is partially, but incompletely, blocked by an inhibitor of the lipoxygenase metabolic pathways. Cyclooxygenase inhibitors have no protective effects. Certain cyclic nucleotides can rapidly increase gj [adenosine 3',5'-cyclic monophosphate (cAMP)] or slightly decrease it [guanosine 3',5'-cyclic monophosphate (cGMP)], and agents that use these cyclic nucleotides as second messengers (isoproterenol and perhaps carbachol, respectively) produce consistent effects. Agents expected to cause protein kinase C activation (tumor-promoting phorbol esters and diacylglycerol) increase gj rapidly. The gap junction protein from rat heart has been cloned and sequenced. From the primary sequence for the protein, plausible sites of action within the putative cytoplasmic domains are proposed for each of these treatments. In response to gating stimuli that close the channel (halothane, CO2, heptanol), unitary channel conductance is unchanged, suggesting that these agents act by reducing open time probability. Together, these properties constitute the beginnings of our endeavor to define pharmacological agents that are potentially useful in therapeutic manipulation of synchronous discharge, conduction velocity, and isochronous wavefront propagation in cardiac tissue.

Bimodal regulation of adrenal opiate peptides by cholinergic mechanisms

Physiologic stressors increase trans-synaptic impulse activity and result in adrenal catecholamine release and biosynthesis. To determine the effects of stress on the co-localized opiate peptide system, rats were cold stressed at 4 degrees C. While cold stress slightly decreased enkephalin levels, a more severe stress (wetting and cold) increased enkephalin levels by 95%. Examining trans-synaptic-cholinergic mechanisms, treatment with either nicotinic or muscarinic agonists alone resulted in no change in adrenal enkephalin content. However, treatment with both nicotinic and muscarinic agonists together resulted in a three-fold rise in enkephalin levels. To further examine cellular mechanisms, medullae were explanted in the presence of agents that increase second messenger cyclic nucleotide levels. Treatments that increase the levels of cAMP, the cyclic nucleotide associated with nicotinic receptor activation, prevented the rise in medullary enkephalin relative to control explants. In contrast, treatments that increased cGMP levels, the cyclic nucleotide associated with muscarinic receptor activation, had no effect on enkephalin content compared to control explants. However, in the presence of both forskolin (10 microM) plus db-cGMP (5 mM), enkephalin content rose three-fold over control explants. These data suggest that, distinct from catecholamine pathways, enkephalin levels can be positively or negatively regulated by the severity of a stressful stimulus, by cholinergic receptor mechanisms and by an interaction of cyclic nucleotide second-messenger pathways.

Pharmacological control of muscular activity in the sea urchin larva. III. Role of cyclic nucleotides

1. The muscular activity of the sea urchin pluteus is strongly affected by dibutyryl-c-AMP, in a stimulatory or inhibitory manner depending on the concentration, the time of exposure, and the spontaneous level of activity. 2. Dibutyryl-c-GMP, like muscarinic agents and the guanylate-cyclase activators biotin and nitrite, keeps the activity low. 3. It is suggested that the effects of muscarinic agents is mediated by c-GMP, the effects of certain monoamines by c-AMP. 4. The two cyclic nucleotides appear to control the cellular influx of Ca2+ in opposite directions. They therefore interfere with the stimulatory and paralytic effects of nicotinic agents.

Direct modulation of Aplysia S-K+ channels by a 12-lipoxygenase metabolite of arachidonic acid

Lipoxygenase metabolites of arachidonic acid have recently been shown to modulate the activity of ion channels in nerve and muscle cells. The mechanism of action of these metabolites is, however, unknown. In sensory neurons of Aplysia, the S-K- channel is under the dual modulatory control of 5-hydroxytryptamine (5-HT), which decreases the number of active S channels through cyclic AMP-dependent phosphorylation, and the neuropeptide FMRFamide, which increases the probability of S-channel opening through the 12-lipoxygenase metabolite 12-hydroperoxyeicosatetraenoic acid (12-HPETE). Here we report that the increase in the probability of S-channel opening with FMRFamide is mimicked by application of 12-HPETE to cell-free membrane patches that lack ATP and GTP. Thus, 12-HPETE can act directly to modulate S-channel activity, independently of protein phosphorylation or dephosphorylation, G-protein activation or cyclic nucleotides.

Purification of a vasodilator-regulated phosphoprotein from human platelets

Cyclic-nucleotide-elevating vasodilators such as prostaglandin E1, prostacyclin, sodium nitroprusside and endothelium-derived relaxing factor inhibit both contraction of vascular smooth muscle cells and the aggregation of platelets at an early step of the activation cascade. Previous studies from this laboratory [Waldmann, R., Nieberding, M. and Walter, U. (1987) Eur. J. Biochem. 167, 441-448) established that in human platelets cyclic-nucleotide-elevating vasodilators stimulated a pattern of protein phosphorylation which was mediated by both cAMP- and cGMP-dependent protein kinases. Of particular interest was a membrane-bound 50-kDa protein whose phosphorylation was increased both by cAMP- and cGMP-elevating vasodilators in intact platelets and by endogenous cAMP- and cGMP-dependent protein kinase in platelet membranes. Since the molecular mechanism of action of cyclic-nucleotide-elevating vasodilators is unknown, this 50-kDa phosphoprotein from human platelets was purified to apparent homogeneity by salt extraction, anion, cation and dye-ligand chromatography. The purified protein migrated as a 46-kDa protein in SDS/PAGE, was an excellent substrate for both cAMP- and cGMP-dependent protein kinases and migrated in SDS/PAGE as a 50-kDa protein after phosphorylation by these protein kinases. Analysis by limited proteolysis, tryptic fingerprinting and of phosphoamino acids established that the purified protein is identical with the 50-kDa protein phosphorylated by both cAMP- and cGMP-dependent protein kinases in platelet membranes and in response to cAMP- and cGMP-elevating vasodilators with intact platelets. Evidence is presented that the purified protein contains at least two phosphorylation sites, each of which is preferentially phosphorylated by either cAMP- or cGMP-dependent protein kinase. The availability of this vasodilator-regulated phosphoprotein as a purified protein should now allow new approaches for investigating the function of this protein and its possible role in the mechanism of action of cyclic-nucleotide-elevating vasodilators.

Cyclic nucleotide modulation of herpes simplex virus latency and reactivation

Clinical observations and experimental studies suggested that the relative proportions of ganglionic neuronal intracellular cyclic adenosine monophosphate (c-AMP) and cyclic guanosine monophosphate (c-GMP) concentrations may influence the state or activity of herpes simplex viral DNA in its relationship with the host cell DNA. We studied the effects of putative modulators of intracellular cyclic nucleotide levels on herpes simplex virus (HSV) reactivation from latency in murine trigeminal ganglion cells. We also investigated the effects of these same mediators on the c-GMP and/or c-AMP concentrations in HSV-latently infected trigeminal ganglion cells and in acyclovir-suppressed, HSV-infected neuroblastoma cells. Cholera toxin and theophylline increased c-AMP levels (2-fold and 5-fold at 1 min and 30 sec, respectively for cholera toxin and 2-fold and 1.5-fold at 1 min and 30 sec for theophylline) and enhanced the rapidity of HSV reactivation from latency (P less than 0.005). Exogenous dibutyryl c-AMP also stimulated viral reactivation (P less than 0.005). Carbamylcholine increased c-GMP levels (7-fold and 6-fold at 15 sec and 30 sec, respectively), produced no significant change in c-AMP levels, and delayed HSV reactivation from latency (P less than 0.005). None of these mediators had a demonstrable effect on HSV replication.

Cyclic nucleotides and polyamines in prolactin stimulation of lactose biosynthesis

The possible roles of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) and of polyamines on the early effect of prolactin (PRL) on lactose biosynthesis have been investigated in cultured mammary gland explants derived from mice 12-14 days pregnant. Elevated cAMP concentrations impaired the PRL stimulation of [3H]glucose incorporation into lactose. Adding dibutyryl cAMP (0.1-0.5 mM) or phosphodiesterase inhibitors [methyl isobutylxanthine (0.1-0.5 mM) or theophylline (0.5-5.0 mM)] to the culture medium abolished the PRL response. The addition of 8-bromo cGMP (0.5 mM) with or without 1.0 mM spermidine had no effect on the PRL stimulation of lactose synthesis. By itself, 1.0 mM spermidine consistently produces a small but significant PRL-like stimulation of lactose synthesis in this system. Ongoing polyamine metabolism appears to be necessary for the PRL effect on lactose synthesis because 100 microM methylglyoxal bis(guanyl hydrazone), an inhibitor of S-adenosyl methionine decarboxylase, abolished the PRL response. alpha-Difluoromethylornithine, an inhibitor of ornithine decarboxylase activity, at concentrations from 1.0 to 10 mM had no effect on the PRL stimulation of lactose synthesis.

The Escherichia coli primosome can translocate actively in either direction along a DNA strand

The primosome is a mobile multiprotein DNA replication-priming apparatus that requires seven Escherichia coli proteins (replication factor Y (protein n'), proteins n and n", and the products of the dnaB, dnaC, dnaT, and dnaG genes) for assembly at a specific site (termed a primosome assembly site) on single-stranded DNA binding protein-coated single-stranded DNA. Two of the protein components of the primosome have intrinsic DNA helicase activity. The DNA B protein acts in the 5'----3' direction, whereas factor Y acts in the 3'----5' direction. The primosome complex has DNA helicase activity when present at a replication fork in conjunction with the DNA polymerase III holoenzyme. In this report, evidence is presented that the multiprotein primosome per se can act as a DNA helicase in the absence of the DNA polymerase III holoenzyme. The primosome DNA helicase activity can be manifested in either direction along the DNA strand. The directionality of the primosome DNA helicase activity is modulated by the concentration and type of nucleoside triphosphate present in the reaction mixture. This DNA helicase activity requires all the preprimosomal proteins (the primosomal proteins minus the dnaG-encoded primase). Preprimosome complexes must assemble at a primosome assembly site in order to be loaded onto the single-stranded DNA and act subsequently as a DNA helicase. The 5'----3' primosome DNA helicase activity requires a 3' single-stranded tail on the fragment to be displaced, while the 3'----5' activity does not require a 5' single-stranded tail on the fragment to be displaced. Multienzyme preprimosomes moving in either direction are capable of associating with the primase to form complete primosomes that can synthesize RNA primers.

Cyclic nucleotide regulation of neurotransmission in guinea pig mesenteric artery

The possible involvement of three different second-messenger systems, namely cyclic AMP/protein kinase (PK)-A, cyclic GMP/PK-G, and diacylglycerol (DG)/PK-C systems, in the perivascular nerve terminals of guinea pig mesenteric artery was examined by intracellular microelectrode recording. Excitatory junction potentials (EJPs) were evoked by perivascular nerve stimulation. Isoproterenol (0.1 microM) enhanced the EJP amplitude without modifying the passive membrane properties of the vascular smooth muscle (VSM) cells. The facilitatory effect of isoproterenol on EJP amplitude was completely abolished by beta-adrenergic blockade (0.3 microM propranolol). Forskolin (activator of adenylate cyclase) also augmented the EJP amplitude in a concentration-dependent manner (EC50 congruent to 10 microM), without affecting the passive membrane properties of the VSM cells. In addition, forskolin (1-10 mM) markedly potentiated the isoproterenol-induced stimulation of EJP amplitude (EC50 congruent to 2 microM). A permeant analogue of cyclic AMP, 8-bromo-cyclic AMP (0.1 and 1 mM), enhanced the EJP amplitude, thus mimicking the effects of isoproterenol and forskolin. 8-Bromo-cyclic AMP had no effect on the resting potential or current-voltage relationship of the VSM cells, thus suggesting that the membrane properties of the VSM cells were not altered. 8-Bromo-cyclic GMP (1 mM) also augmented the EJP amplitude, but its facilitatory effect was weaker than that of 8-bromo-cyclic AMP. 8-Bromo-cyclic GMP hyperpolarized the VSM membrane by 4 mV and decreased the input resistance, presumably due to an increase in K+ conductance. Phorbol-12-myristate-13-acetate (PMA, 30-300 nM), a direct activator of PK-C, significantly enhanced the EJP amplitude after 40 min in a concentration-dependent manner, without affecting the resting potential of the VSM cells. From these results, we suggest that cyclic AMP/PK-A, cyclic GMP/PK-G, and DG/PK-C systems might be involved in regulation of the release of neurotransmitter in the perivascular nerve terminals. However, the possibility of some action on the postsynaptic VSM cell cannot be excluded.

Elevated intracellular Ca2+ acts through protein kinase C to regulate rabbit ileal NaCl absorption. Evidence for sequential control by Ca2+/calmodulin and protein kinase C

Calcium/calmodulin is involved in the regulation of basal rabbit ileal active Na and Cl absorption, but the mechanism by which elevated intracellular Ca2+ affects Na and Cl transport is unknown. To investigate the roles of the Ca2+/calmodulin and protein kinase C systems in ileal NaCl transport, two drugs, the isoquinolenesulfonamide, H-7, and the naphthalenesulfonamide, W13, were used in concentrations that conferred specificity in the antagonism of protein kinase C (60 microM H-7) and Ca2+/calmodulin (45 microM W13), respectively, as determined using phosphorylation assays in ileal villus cells. W13 but not H-7 stimulated basal active NaCl absorption. H-7 inhibited changes in Na and Cl absorption caused by maximal concentrations of Ca2+ ionophore A23187 and carbachol and serotonin, secretagogues that act by increasing cytosol Ca2+, while W13 had no effect. In contrast, neither H-7 nor W13 altered the change in NaCl transport caused by the cyclic nucleotides 8-Br-cAMP and 8-Br-cGMP. These data suggest that: (a) basal rabbit ileal NaCl absorption is regulated by the Ca2+/calmodulin complex and not by protein kinase C; (b) the effect of elevating intracellular Ca2+ to decrease NaCl absorption is mediated via protein kinase C but not by Ca2+/calmodulin; (c) the effects of protein kinase C are not overlapping or synergistic with those of Ca2+/calmodulin on either basal absorption or on the effects of increased Ca2+; and (d) neither Ca2+/calmodulin nor protein kinase C are involved in the effects of cAMP and cGMP on ileal active NaCl transport.

Augmentation of lymphocyte responses by monoclonal antibodies to the gangliosides GD3 and GD2: the role of protein kinase C, cyclic nucleotides, and intracellular calcium

Previous studies have shown that MAb's against the gangliosides GD3 and GD2 may augment T cell responses to a variety of stimuli. We present evidence that antiganglioside MAb's, like PHA, increase intracellular cGMP and protein kinase C yet have no effect on intracellular Ca2+. Stimulation of T cells with MAb's to GD3 was associated with increased cGMP levels, particularly in the CD8+ T cell subset which showed the highest degree of potentiation by the MAb's. Augmentation of T cell responses by the MAb's to GD3 and GD2 was also mimicked by activation of PKC with phorbol esters but both agents together produced marked synergistic effects on cell division, suggesting they had different but complementary modes of action. Furthermore, use of neomycin to inhibit PKC activation only partially reversed the augmentation of proliferative responses by the antiganglioside MAb's. It did however inhibit the MAb-induced increase in IL2 production and IL2 receptor (Tac) expression. These studies suggest therefore that the potentiation of IL2 production by the MAb's against GD2 and GD3 was due to enhanced activation of PKC whereas their augmentation of proliferative responses appeared to be due to effects on late events in T cell activation and was associated with both increased cGMP levels and activation of PKC.

Cyclic nucleotides and the rapid inhibitions of bone 45Ca uptake in response to bovine parathyroid hormone and 16,16-dimethyl prostaglandin E2 in chicks

Intravenous injection of chicks with bovine parathyroid hormone (1-34) (3.3 micrograms/100 g body wt.) or 16,16-dimethyl PGE2 (5 micrograms/100 g body wt.) caused rapid (3 minute) net inhibition of 45Ca uptake into femur and calvarium. These agents also elevated bone adenosine 3',5'-cyclic monophosphate (cAMP) but not guanosine 3',5'-cyclic monophosphate (cGMP) levels at this time. Methylxanthine phosphodiesterase inhibitors (MXPI), caffeine, theophylline, and 3-isobutyl-1-methylxanthine (IBMX) (0.3-5 mg/100 g body wt.) similarly inhibited net 45Ca uptake into femur and to a lesser extent calvarium. Plasma 45Ca and total Ca levels were unaltered or showed a slight tendency to be increased over control values 3 minutes after injection. However, the effects of the non-MXPI, dibutyryl-cAMP (0.5-5 mg/100 g body wt.) on bone 45Ca uptake were negligible. Of the MXPI, only IBMX elevated total cAMP levels in chick bone at 3 minutes. These data implicate but do not confirm a mediatory role for cAMP in the rapid inhibitory actions of PTH and PGEs on bone net 45Ca uptake in chicks.

Characterization of cyclic nucleotide and inositol 1,4,5-trisphosphate-sensitive calcium-exchange activity of smooth muscle cells cultured from the human corpora cavernosa

Smooth muscle-mediated expansion and contraction of the vascular sinusoids of the corpora cavernosa may modulate male erectile function. To elucidate the biochemical events that control erection by promoting or inhibiting contraction of cavernosal smooth muscle, tissue from a potent man was grown in cell culture. The cells grew as noncontractile cultures, but had the following smooth muscle cell properties: These cells expressed desmin, the muscle cell-specific intermediate filament protein. They accumulated 45Ca2+ from the medium, which was released by exposure to the ionophore A23187, to cyclic nucleotides (cyclic guanosine 5'-monophosphate [GMP] much greater than cyclic adenosine 3',5'-monophosphate [AMP]), and to the phosphodiesterase inhibitor, papaverine; and; they accumulated Ca2+ in an ATP-dependent manner when the cultured cells were permeabilized by digitonin extraction. ATP-dependent Ca2+ uptake was inhibited approximately 80% by ruthenium red and simulated by cyclic GMP much greater than cyclic AMP. Inositol 1,4,5-trisphosphate (IP3), which is thought to mediate the release of Ca2+ by the smooth muscle cell sarcoplasmic reticulum in vivo, released approximately 0.85 pmol Ca2+/million cells from the digitonin-extracted cells. IP3-dependent release occurred in the presence of ruthenium red and was not affected by cyclic GMP or cyclic AMP. These results indicate that smooth muscle from this human source can be grown successfully in cell culture and that the biochemical pathways that regulate tension in vivo may be perpetuated in vitro. Moreover, some of the clinical responses to drugs administered in situ for erectile dysfunction (e.g. papaverine) may be the result of altered cavernosal smooth muscle cell Ca2+ exchange and may be mediated by cyclic GMP.

[The H1 and H2 histamine receptors]

Histamine is a major mediator of the allergic reactions. Histamine have different actions: contraction of smooth muscles, vascular action, increase in gastric and adrenal medulla secretion. Effects on central or peripheral nervous system are discussed. The specific H1 or H2 activity explains the different configurations of histamine. The specificity of H1 receptors agonists is now well known: H1 activities have a positive charge on the side chain with an imidazole ring able to rotate around the axis of side chain. The contraction of smooth muscles is due to the action of H1 receptors agonists. Many doubts remain about the exact structures of the H2 receptors, and their agonists. Trough the H2 receptors occur dilatation of small arteries and capillaries, as well as an increase in gastric secretion. Subdivisions of H2 receptors have been, suggested. Recently H3 receptors have been described in the brain and in some peripheral tissues. Interrelations between H1 and H2 histamine receptors have been described as well as a feedback of synthesis and of histamine release.

Release of tumor necrosis factor-alpha from macrophages. Enhancement and suppression are dose-dependently regulated by prostaglandin E2 and cyclic nucleotides

PGE2 has previously been shown to suppress various leukocyte functions. In this study, we examined whether PGE2 would affect release of TNF-alpha from rat resident peritoneal macrophages. Two different, dose-dependent effects were observed: low PGE2 concentrations (0.1 to 10 ng/ml) stimulated, whereas higher concentrations (greater than 10 ng/ml) suppressed TNF-alpha release. PGE2-stimulated TNF-alpha production was dependent on de novo protein synthesis and was associated with an intracellular rise of cGMP. The importance of cGMP as an intracellular messenger for PGE2 was confirmed by the following evidence: (1) low PGE2 concentrations preferentially increased cGMP and not cAMP and (2) cGMP, either exogenously added or endogenously generated by sodium nitroprusside, were efficient stimulators of TNF-alpha production. In contrast, agents increasing intracellular cAMP concentrations such as PGE1, higher PGE2 doses, isoproterenol, and theophylline, all suppressed TNF-alpha synthesis. Only resident, but not casein-elicited or Corynebacterium parvum-activated macrophages, were stimulated by low PGE2 concentrations to increase TNF-alpha production. In tumor cytotoxicity assays, PGE2-activated macrophages were active only against TNF-alpha-sensitive target cells. These findings demonstrate that TNF-alpha synthesis in macrophages is up-regulated by cGMP and down-regulated by cAMP, which indicates that cyclic nucleotides act as intracellular messengers for extracellular signals of macrophage activation.

Cyclic nucleotide interactions involved in endothelium-dependent dilatation in rat aortic rings

The role of cyclic nucleotides in the dilation of rat aortic rings induced by isoprenaline was studied by measuring changes in isometric tension and in cyclic nucleotide levels in rings preconstricted with phenylephrine or U46619. Isoprenaline produced significantly greater relaxation in endothelium-containing than in endothelium-denuded rings. Relaxation of endothelium-containing rings to isoprenaline was markedly inhibited by pretreating the rings with methylene blue (30 microM) or haemoglobin (10 microM). Basal cyclic-AMP levels were not affected by the presence of endothelium in the rings, but basal cyclic-GMP levels were 5-6-fold higher in endothelium-containing rings. Isoprenaline (1 microM) elevated levels of cyclic-AMP to an extent which was not affected by the presence of endothelium. In contrast, isoprenaline had no effect on cyclic-GMP levels in rings with or without endothelium. Pretreatment of de-endothelialized rings with sodium nitroprusside or 8-bromo-cyclic-GMP, in concentrations that by themselves caused minor alterations in vascular tone, markedly amplified the relaxant responses to isoprenaline and mimicked the relaxant responses of endothelium-containing rings to isoprenaline. These data indicate that isoprenaline does not stimulate release of endothelium-derived relaxing factor, but suggest a major synergistic interaction between cyclic-GMP-mediated and cyclic-AMP-mediated relaxation in rat aorta.

Regulation of gap junctional channels

Regulation of gap junctional channels by voltage, calcium, pH and cyclic nucleotides is discussed, with emphasis on the physiological significance of each regulatory mechanism. The range of calcium and hydrogen ion concentrations that shut the junctional channels makes it unlikely that these ions may act as physiological modulators of intercellular communication. On the other hand, voltage and cyclic nucleotides in particular seem to play a physiologically significant role in the modulation of intercellular communication by way of gap junctions.

The glomerular mesangial cell: an expanding role for a specialized pericyte

The mesangial cell occupies a central position in the renal glomerulus. It has characteristics of a modified smooth muscle cell, but is also capable of a number of other functions. Among these are generation of prostaglandins (PGs) and mediators of inflammation; production and breakdown of basement membrane and other biomatrix material; synthesis of cytokines; and uptake of macromolecules, including immune complexes. In terms of its smooth muscle activity, the mesangial cell contracts or relaxes in response to a number of vasoactive agents. This ability allows the cells to modify glomerular filtration locally. The cellular mechanism of action of many agents influencing mesangial cells involves activation of phospholipase C for phosphatidylinositol 4,5-bisphosphate. This results in generation of inositol trisphosphate and release of intracellular calcium. Mesangial cell relaxation can be mediated by enhanced cAMP or cGMP generation. Many vasoactive substances also stimulate PG production by mesangial cells. This involves activation of both phospholipase C and A2, the latter being responsible for the release of arachidonic acid. Mesangial cells are also capable of endocytosis of macromolecules, including immune complexes. This is initiated by binding to a specific receptor, resulting in formation of PG, platelet-activating factor, and reactive oxygen species. Mesangial cells can generate interleukin 1 and platelet-derived growth factor and respond to these in an autocrine manner. Thus, the mesangial cell not only can control glomerular filtration, but may also be involved in the response to local injury, including cell proliferation and basement membrane remodeling.

Involvement of cyclic nucleotides in prejunctional modulation of noradrenaline release in mouse atria

1 In mouse isolated atria previously incubated with [3H]-noradrenaline, 8-bromo-cyclic AMP (3-270 microM) produced a concentration-dependent increase in the fractional stimulation-induced outflow of radioactivity. 8-Bromo-cyclic GMP induced a lesser increase in the stimulation-induced outflow. 2 The phosphodiesterase inhibitors: M&B 22948 (90 microM); ICI 63197 (30 and 90 microM) and 3-isobutyl-1-methylxanthine (90 microM) increased the fractional stimulation-induced outflow. Together these results indicate that cyclic AMP may have a modulatory effect on noradrenaline release. 3 The inhibition of the stimulation-induced outflow produced by clonidine (0.03 microM) and its facilitation produced by phentolamine (1 microM) were unaltered in the presence of 8-bromo-cyclic AMP (90 microM). However, in the presence of 8-bromo-cyclic AMP (270 microM), the facilitatory effect of phentolamine was enhanced, but the inhibitory effect of clonidine (0.03 microM) was unaltered. In the presence of ICI 63197 (30 microM) the inhibitory effect of clonidine (0.03 microM) was unaltered, but the facilitatory effect of phentolamine (1 microM) was slightly enhanced. 4 Isoprenaline (0.003-0.1 microM) enhanced the fractional stimulation-induced outflow, an effect blocked by propranolol (0.1 microM). In the presence of 8-bromo-cyclic AMP (90 microM), the facilitatory effect of isoprenaline (0.01 microM) was blocked. In the presence of ICI 63197 (30 microM) the facilitatory effect of isoprenaline (0.003 microM) was potentiated. 5 These results suggest that whereas beta-adrenoceptor-mediated enhancement of noradrenaline release is linked to the stimulation of adenylate cyclase and enhanced formation of cyclic AMP, alpha-adrenoceptor-mediated inhibition of noradrenaline release is not linked to inhibition of adenylate cyclase activity.

Molecular signaling mechanisms in T-lymphocyte activation pathways: a review and future prospects

Understanding of the molecular mechanisms which drive the complex activation responses of T lymphocytes was previously limited. However, current studies in lymphocytes, and in other cells, have indicated the involvement of several secondary messenger or signal systems, and recent progress in elucidating the relevant pathways has been extraordinarily rapid. This review therefore attempts to provide an overview of these processes--including the effects of Ca2+, hydrolysis of phospholipids, arachidonic acid, Ca2+/phospholipid-dependent and tyrosine kinases, calmodulin, cyclic nucleotides, ion channels, and adherent cells--and their roles in weaving a subtle and highly responsive web of regulatory signals.