Modulation of phosphatidylinositol-4,5-bisphosphate hydrolysis in rat aorta by guanine nucleotides, calcium and magnesium

High risk of coagulopathy among Type-2 Diabetes Mellitus clients at a municipal hospital in Ghana

Background

Persistent hyperglycaemia in diabetes mellitus causes coagulopathies due to glycation of haemoglobin, prothrombin, fibrinogen and other proteins involved in the clotting mechanism. Shortened activated partial thromboplastin time (APTT) and prothrombin time (PT) reflect hypercoagulable state, which is associated with an increased thrombotic risk and adverse cardiovascular effects. This study assessed the coagulation profile of type 2 diabetes mellitus (T2DM) clients at a municipal hospital in Ghana.

Methods

A hospital-based case-control study was conducted from January to April 2015 at the Agona Swedru Municipal Hospital. Sixty (60) persons with T2DM and 40 without were recruited and screened using appropriate protocols. Blood samples were collected for coagulation and biochemical tests. Demographic and clinical information were collected using pre-tested questionnaire. Data was analyzed with GraphPad Prism version 5.

Results

APTT and PT were significantly shorter among patients with T2DM compared to those without (20.88 ± 5.19 v 31.23 ± 5.41, P=0.0001; and 11.03 ± 2.06sec v 14.46 ± 1.86, P=0.0001 respectively). INR was decreased among patients with T2DM compared to those without (0.83 ± 0.18 v 1.13 ± 0.17, P=0.0001). No significant difference was found in platelet count between T2DM and non-diabetics (179.85 ± 66.15×10³ /mm³ v 168.55 ± 35.77×10³ /mm³, P=0.326). Serum magnesium was lower among the T2DM patients compared to the non-diabetics, while serum ionized calcium was significantly higher among the T2DM patients (P<0.05).

Conclusion

Clients with T2DM may have a high coagulation risk evidenced by shortened APTT, PT and a high ionized calcium compared with controls.

Funding

Study was funded by Lord Ampomah and Solomon Panford.

Potentiation of the intracellular Ca2+ response to arginine vasopressin by increased cytosolic-free Mg2+ in rat vascular smooth muscle cells

Although the inhibitory effects of extracellular Mg2+ on Ca2+ influx are well established, little is known about the effects of intracellular Mg2+ on Ca2+ handling. In the present study, the effects of cytosolic-free Mg2+ concentration in the physiological (submillimolar) range on Ca2+ handling were investigated after stimulation of rat vascular smooth muscle cells with arginine vasopressin. Cytosolic Mg2+ was manipulated by culturing cells in medium containing different Mg2+ concentrations. Peak cytosolic-free Ca2+ concentration responses to arginine vasopressin (1 mumol/1) were measured in the presence and absence of external Ca2+. The results suggest that an increase in cytosolic-free Mg2+ concentration increases both Ca2+ discharge from intracellular stores and Ca2+ influx, whereas a decrease in intracellular Mg2+ attenuates Ca2+ influx.

Magnesium deficiency in alcoholism: possible contribution to osteoporosis and cardiovascular disease in alcoholics

Magnesium (Mg) deficiency occurs frequently in chronic alcoholism and may contribute to the increased incidence of osteoporosis and cardiovascular disease seen in this population. Mg deficiency is primarily due to renal Mg-wasting and is exacerbated by dietary Mg deprivation, gastrointestinal losses with diarrhea or vomiting, as well as concomitant use of drugs such as diuretics and aminoglycosides. Osteoporosis is prevalent in the alcoholic population. Mg deficiency may contribute to increased bone loss by its effects on mineral homeostasis. In Mg depletion, there is often hypocalcemia due to impaired parathyroid hormone (PTH) secretion, as well as renal and skeletal resistance to PTH action. Serum concentrations of 1,25-vitamin D are also low. These changes are seen with even mild degrees of Mg deficiency and may contribute to the metabolic bone disease seen in chronic alcoholics. Hypomagnesemia in alcoholics may also contribute to increased cardiovascular disease by altering platelet function. Mg deficiency has been demonstrated to enhance platelet reactivity. In these studies, Mg was shown to inhibit platelet aggregation against various aggregation agents. Patients with Mg deficiency were shown to have increased platelet aggregation that was normalized with Mg therapy. The antiplatelet effect of Mg may be related to the finding that Mg inhibits the synthesis of thromboxane A2 and 12-hydroxyeicosatetraenoic acid, eicosanoids thought to be involved in platelet aggregation. Mg also inhibits the thrombin-induced Ca2+ influx in platelets, as well as stimulates synthesis of prostaglandin I2, the potent antiaggregatory eicosanoid. Therefore, Mg deficiency may increase platelet aggregation and cause increased hypertension and atherosclerotic cardiovascular disease in alcoholics.

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Cell signalling associated with fibrinolytic ligand binding to human colorectal carcinoma cells

Addition of purified plasmin or plasminogen (0.1 microM) to serum-free culture media elevated cellular D-myo-inositol 1,4,5-trisphosphate (InsP3) levels in human colorectal carcinoma cells within 1 h to double those of control cells. This was accompanied by decreases in cellular phosphatidylinositol bisphosphate by 40% in cells exposed to fibrinolytic ligands for up to 1 h. The effect was not due to opening of Ca2+ channels of the type blocked by 5 microM nifedipine, and 100 microM EGTA, a Ca2+ chelator, did not suppress plasmin's ability to elevate InsP3. Binding assays at 4 degrees C with 125I-labelled plasmin indicated maximum binding within 1 h suggesting that the effects of plasmin may be associated with its cell-binding function. These cells could convert exogenous plasminogen to plasmin with endogenous activation and this was accompanied by a decrease in radioactive phosphatidylinositol well below control levels (13% of control). Our results contribute to evidence for the association of plasmin-binding sites with a signalling system. A cell signalling system indirectly or directly associated with plasmin binding, would permit carcinoma cells to coordinate extracellular fibrinolysis with cell migration and motility through second messengers.

Assay of a phosphatidylinositol bisphosphate phospholipase C activity in postmortem human brain

The activity of a phospholipase C which hydrolyses exogenous phosphatidylinositol-4,5-bisphosphate [( 3H]PtdIns(4,5)P2) in membranes prepared from frozen postmortem human brain and rat brain was investigated. Enzyme characteristics were essentially similar in membranes prepared from frozen postmortem brain and fresh or frozen rat brain. The [3H]PtdIns(4,5)P2 solubilization and assay procedure employed resulted in an efficient availability of the substrate for the enzyme. The non-hydrolysable guanosine triphosphate analogue guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) stimulated hydrolysis rapidly with a half maximum activity of approximately 25 microM. This stimulation was not specific for guanine nucleotides as ATP, imidodiphosphate and pyrophosphate also caused enzyme activation. However these activation effects could be distinguished by the polyanion spermine. The non-hydrolysable guanine dinucleotide analogue guanosine 5'-[beta-thio]diphosphate acted as a partial agonist thereby inhibiting the stimulatory effect of Gpp[NH]p. Gpp[NH]p-stimulated enzyme activity showed a maximum response in the presence of 1 mM deoxycholate and displayed a pH optima in the range 7.0-7.5. PtdIns(4,5)P2 hydrolysis was observed in the absence of added calcium, but hydrolytic cleavage was inhibited in the presence of divalent ion chelators. Magnesium inhibited PtdIns(4,5)P2 hydrolysis in a concentration-dependent manner. Elucidation of these aspects of the phosphatidylinositol cycle in normal human postmortem brain will permit comparative studies in CNS disease states.

Multiple actions of fluoride ions upon the phosphoinositide cycle in the rat brain

The effects of sodium fluoride upon basal and agonist-stimulated inositol phospholipid breakdown have been investigated in rat brain miniprisms. NaF concentration dependently increased basal inositol phospholipid breakdown, with a maximum effect being seen at 20 mM. NaF reduced the inositol phospholipid breakdown responses to stimulation by carbachol, noradrenaline, serotonin and quisqualate, but not to the stimulation produced by raising the assay [K+] from 6 to 18 mM. More detailed study demonstrated NaF to have a 'levelling' effect, reducing all InsP/(Lipid + InsP) values greater than 0.15 (i.e. produced by carbachol at raised [K+], noradrenaline and by 50 mM K+) to about this value. Time-course experiments indicated that NaF treatment reduced the rate of carbachol-stimulated inositol phospholipid breakdown up to this InsP/(Lipid + InsP) level and thereafter blocked further breakdown. Inhibitory effects upon carbachol-stimulated inositol phospholipid breakdown were not seen with forskolin, sodium nitroprusside or 8BrcGMP. Under conditions where there is no de novo synthesis of phosphoinositides from [3H]myo-inositol, NaF reduced the total Lipid + InsP labelling by about 20%. NaF in addition inhibits the activity of Ins(1,4)P2-phosphatase in cerebral cortical homogenates. It is concluded that fluoride ions inhibit agonist-stimulated inositol phospholipid breakdown via actions not only on G-proteins but also on phosphoinositide-specific phospholipase C substrate availability.

Histamine receptors: subclasses and specific ligands

In this review the three main types of histamine receptors are discussed together with their specific ligands. For the classical H1-receptors much emphasis is put on the mechanism by which the receptor is stimulated. For the H1- and H2-receptor the review includes information on the several models available for establishing agonistic or antagonistic activity. In the section on the H3-receptor the ligands are discussed as well as the possible physiological role of this receptor. In the final paragraphs some less well defined activities are presented.

The differential effect of calcium antagonists on the positive inotropic effects induced by calcium and monensin in cardiac preparations of rats and guinea-pigs

It was the aim of the present study to gain more insight into the role of extracellular calcium and of calcium from intracellular sources in the development of contractile force in the mammalian heart. In rat Langendorff hearts the effect of nifedipine, verapamil, diltiazem, bepridil and lidoflazine as well as of the intracellularly acting calcium antagonists ryanodine and TMB-8 on the increase of the left ventricular pressure induced by calcium and the sodium ionophore monensin, respectively, was studied. In rat and guinea-pig papillary muscles the influence of nifedipine, ryanodine and lidoflazine on the effect of monensin on the force of contraction was evaluated. Calcium and monensin concentration-dependently increased the left ventricular pressure in rat Langendorff hearts. The calcium-induced effect was characterized by a sharp initial rise of the left ventricular pressure which stabilized at a lower level while monensin elicited a gradual rise of the left ventricular pressure. Nifedipine, verapamil and diltiazem, applied at the EC50 and the EC80 for the reduction of the left ventricular pressure under control conditions, shifted the concentration-response curves for calcium and monensin into the right. Ryanodine, TMB-8, lidoflazine and bepridil, applied at the EC50, displaced the concentration-response curves for calcium and monensin to the right but reduced the maximal increase of the left ventricular pressure. At the EC80, these drugs almost completely abolished the positive inotropic effects elicited by calcium and monensin, respectively. In rat papillary muscles monensin did not influence the basal force of contraction. A clear positive inotropic effect was only observed in the presence of nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of cerebral-cortical membranes with exogenously added phosphatidylinositol 4,5-bisphosphate. Effects on measured phospholipase C activity

Exogenously added phosphatidylinositol 4,5-bisphosphate (PtdInsP2) is rapidly associated with cerebral-cortical membranes. Substrate association with membranes was promoted by Mg2+, but inhibited by bivalent chelators. Once associated with the membrane, the PtdInsP2 was resistant to displacement by EDTA. The apparent phospholipase C activity was dependent on the degree of association of substrate with membranes. After preincubation of membranes with substrate, PtdInsP2 hydrolysis was independent of the incubation volume, indicating that substrate and membrane-associated phospholipase C were not independently diluted. Hydrolysis of the membrane-associated substrate was stimulated by Ca2+, guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG), guanosine 5'[gamma-thio]triphosphate and carbachol in the presence of p[NH]ppG. Carbachol in the absence of guanine nucleotides, GDP, GTP, ATP and pyrophosphate was ineffective. These results demonstrate that exogenously added PtdInsP2 substrate is rapidly associated with membranes and hydrolysed by a phospholipase C whose activity is regulated by guanine nucleotides and agonist in the presence of guanine nucleotides. Use of exogenously added substrate for studies on the regulation of membrane phospholipase C requires consideration as to possible effects of incubation conditions on the partitioning of substrate into membranes.

Enhancement by potassium of carbachol-stimulated inositol phospholipid breakdown in rat cerebral cortical miniprisms: comparison with other depolarising agents

Increasing the [K+] in the assay medium from 5.7 to 17.8 mM produces a large enhancement of the inositol phospholipid breakdown response to the muscarinic agonist carbachol in rat cerebral cortical miniprisms, with minor effects on basal inositol phospholipid breakdown. This effect is also found with Rb+. The enhancement by a raised [K+] is not accompanied by a change in the composition of the labelled polyphosphoinositides. The carbachol-stimulated inositol phospholipid breakdown at 17.8 and 42.7 mM K+ was antagonised by veratrine (5-80 microM), 4-aminopyridine (5 mM), and tetraethylammonium (20 mM). These compounds, however, also inhibited the binding of [3H]quinuclidinyl benzilate to cortical membranes. BRL 34915 (0.2-20 microM) was without significant effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+.Mg2+ (10 mM) considerably reduced the carbachol-stimulated inositol phospholipid breakdown at 17.8, but not 42.7, mM K+. Inositol phospholipid breakdown was also stimulated, albeit to a small extent, by L-glutamate (100-3,000 microM) and quisqualate (1-100 microM), with the stimulation being additive to that produced by carbachol at both 5.7 and 17.8 mM K+. N-Methyl-D-aspartate (10-1,000 microM in Mg2+-free medium) had no significant effect on basal inositol phospholipid breakdown and had little or no effect on carbachol-stimulated inositol phospholipid breakdown at either 5.7 or 17.8 mM K+. It is concluded that it may not be correct to ascribe wholly the enhancement by K+ of carbachol-stimulated inositol phospholipid breakdown to the tissue-depolarising actions of this ion and that other actions of K+ may be involved.

Characterization of phospholipase C-mediated polyphosphoinositide hydrolysis in rat heart ventricles

Phospholipase C (PLC)-mediated degradation of polyphosphoinositides (phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylinositol 4-phosphate (PIP] was found to be present in rat heart ventricular soluble and total membrane fractions (100,000g supernatant and pellet). Distribution of polyphosphoinositide-specific phospholipase C activity between the membrane and soluble fraction was approximately 63 and 33% of total activity, respectively, whereas, phosphatidylinositol (PI) degradation could be detected only in the soluble fraction. Optimal PIP2-PLC activity occurred at a pCa2+ of 4.5. A similar peak in PIP-PLC activity could be demonstrated in soluble and membrane preparations; however, the rate of PIP degradation in the soluble fraction continued to increase at the highest calcium level tested (pCa2+ 3). With the exception of Sr2+, other noncalcium polycations did not support homogenate PIP2-PLC activity. In the presence of Ca2+, addition of Mg2+, La3+, or Sr2+ (10(-3) M) inhibited PIP2-PLC while Mn2+ and Gd3+ stimulated activity. In both the total membrane and soluble fractions, maximal polyphosphoinositide degradation occurs at pH 5.5 and 6.8. The detergents deoxycholate, cholate, and saponin exert a biphasic effect on PIP2-PLC activity (stimulating at lower concentrations and inhibiting at higher concentrations). The deoxycholate effect is observed in both the cytosolic and membrane fractions. Neutral and cationic detergents inhibit PIP2-PLC activity in a concentration-dependent manner. Similar to cytosolic PI-PLC activity, PIP2-PLC appears to depend on intact sulfhydryl groups. In the presence of a mixture of all three inositol phospholipids or the three phosphoinositides plus noninositol phospholipids, polyphosphoinositides are preferentially degraded.

G-proteins, the inositol lipid signalling pathway, and secretion

The formation of the second messenger cyclic AMP (cAMP) is known to be coupled to its receptor via a guanine nucleotide regulatory protein, GS. Ca2+-mobilizing receptors stimulate the hydrolysis of phosphatidylinositol bisphosphate (PtdIns(4,5)P2), which generates two intracellular signals Ins(1,4,5)P3 and diacylglycerol. We review the evidence that this signalling system is also composed of three types of proteins: receptor, G-protein and effector. The G-protein that couples to the effector, polyphosphoinositide phosphodiesterase (PPI-PDE), is a novel G-protein, GP, which is a substrate for pertussis toxin in some cells (e.g. neutrophils and platelets) but not others (e.g. pancreatic acinar cells and GH3 cells). This implies that GP is not a single G-protein but encompasses a family of proteins that can activate PPI-PDE. We have also identified a role for another G-protein, GE, which is involved in the secretory process in mast cells and neutrophils. In this case, neither the receptor nor effector has been identified and the main evidence for proposing this second G-protein is based on the ability of guanine nucleotide analogues (e.g. GTP gamma S) to stimulate secretion independently of PPI-PDE activation.

Inositol trisphosphate, calcium and muscle contraction

The identity of organelles storing intracellular calcium and the role of Ins(1,4,5)P3 in muscle have been explored with, respectively, electron probe X-ray microanalysis (EPMA) and laser photolysis of 'caged' compounds. The participation of G-protein(s) in the release of intracellular Ca2+ was determined in saponin-permeabilized smooth muscle. The sarcoplasmic reticulum (SR) is identified as the major source of activator Ca2+ in both smooth and striated muscle; similar (EPMA) studies suggest that the endoplasmic reticulum is the major Ca2+ storage site in non-muscle cells. In none of the cell types did mitochondria play a significant, physiological role in the regulation of cytoplasmic Ca2+. The latency of guinea pig portal vein smooth muscle contraction following photolytic release of phenylephrine, an alpha 1-agonist, is 1.5 +/- 0.26 s at 20 degrees C and 0.6 +/- 0.18 s at 30 degrees C; the latency of contraction after photolytic release of Ins(1,4,5)P3 from caged Ins(1,4,5)P3 is 0.5 +/- 0.12 s at 20 degrees C. The long latency of alpha 1-adrenergic Ca2+ release and its temperature dependence are consistent with a process mediated by G-protein-coupled activation of phosphatidylinositol 4,5 bisphosphate (PtdIns(4,5)P2) hydrolysis. GTP gamma S, a non-hydrolysable analogue of GTP, causes Ca2+ release and contraction in permeabilized smooth muscle. Ins(1,4,5)P3 has an additive effect during the late, but not the early, phase of GTP gamma S action, and GTP gamma S can cause Ca2+ release and contraction of permeabilized smooth muscles refractory to Ins(1,4,5)P3. These results suggest that activation of G protein(s) can release Ca2+ by, at least, two G-protein-regulated mechanisms: one mediated by Ins(1,4,5)P3 and the other Ins(1,4,5)P3-independent. The low Ins(1,4,5)P3 5-phosphatase activity and the slow time-course (seconds) of the contractile response to Ins(1,4,5)P3 released with laser flash photolysis from caged Ins(1,4,5)P3 in frog skeletal muscle suggest that Ins(1,4,5)P3 is unlikely to be the physiological messenger of excitation-contraction coupling of striated muscle. In contrast, in smooth muscle the high Ins(1,4,5)P3-5-phosphatase activity and the rate of force development after photolytic release of Ins(1,4,5)P3 are compatible with a physiological role of Ins(1,4,5)P3 as a messenger of pharmacomechanical coupling.

Molecular model for receptor-stimulated calcium spiking

Many cells exhibit periodic transient increases in cytosolic calcium levels rather than a sustained rise when stimulated by a hormone or growth factor. We propose here a molecular model that accounts for periodic calcium spiking induced by a constant stimulus. Four elements give rise to repetitive calcium transients: cooperativity and positive feedback between a pair of reciprocally coupled (crosscoupled) messengers, followed by deactivation and then by reactivation. The crosscoupled messengers in our model are inositol 1,4,5-trisphosphate (InsP3) and cytosolic calcium ions. The opening of calcium channels in the endoplasmic reticulum by the binding of multiple molecules of InsP3 provides the required cooperativity. The stimulation of receptor-activated phospholipase C by released calcium ions leads to positive feedback. InsP3 is destroyed by a phosphatase, and calcium ion is pumped back into the endoplasmic reticulum. These processes generate bistability: the cytosolic calcium concentration abruptly increases from a basal level to a stimulated level at a threshold degree of activation of phospholipase C. Spiking further requires slow deactivation and subsequent reactivation. In our model, mitochondrial sequestration of calcium ion prevents the cytosolic level from increasing above several micromolar and enables the system to return to the basal state. When the endoplasmic reticulum calcium store is refilled to a critical level by the Ca2+-ATPase pump, cooperative positive feedback between the InsP3-gated channel and phospholipase C begins again to give the next calcium spike. The time required for the calcium level in the endoplasmic reticulum to reach a threshold sets the interval between spikes. The amplitude, shape, and period of calcium spikes calculated for this model are like those observed experimentally.

Effects of a phorbol ester on rat aortic contraction and calcium influx in the presence and absence of BAY k 8644

Phorbol esters like phorbol-12,13-dibutyrate (PDB) exert potent contractile effects on rat aorta. In the present study, we determined whether phorbol esters alter contraction by augmenting calcium influx through calcium channels. Isometric tensions and calcium influx (using 45Ca) were measured in rat aortic rings exposed to PDB in the presence and absence of BAY k 8644, a calcium channel agonist. We found that PDB alone caused significant increases in isometric tension. The addition of 100 nM BAY k 8644 enhanced the PDB-induced rat aortic contraction which was inhibited as well as reversed by the calcium channel antagonist, nitrendipine. In the presence of PDB alone, calcium influx was significantly increased only after an extended time (30 min) of isometric contraction. In the presence of BAY k 8644, however, PDB caused a significant increase in calcium influx during shorter periods (10 min) of isometric contraction. We also found that the biologically inactive phorbol, 4-alpha-phorbol, did not potentiate the effects of BAY k 8644 on either contraction or calcium influx. These results indicate that PDB may induce rat aortic contraction via activation of protein kinase C which in the presence of a calcium channel agonist can potentiate the mobilization of calcium through nitrendipine-sensitive calcium channels.