Endothelin stimulates phosphatidylinositol turnover in rat right and left atria

IP3 receptor-dependent Ca2+ release modulates excitation-contraction coupling in rabbit ventricular myocytes

Inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-dependent Ca(2+) signaling exerts positive inotropic, but also arrhythmogenic, effects on excitation-contraction coupling (ECC) in the atrial myocardium. The role of IP(3)R-dependent sarcoplasmic reticulum (SR) Ca(2+) release in ECC in the ventricular myocardium remains controversial. Here we investigated the role of this signaling pathway during ECC in isolated rabbit ventricular myocytes. Immunoblotting of proteins from ventricular myocytes showed expression of both type 2 and type 3 IP(3)R at levels approximately 3.5-fold less than in atrial myocytes. In permeabilized myocytes, direct application of IP(3) (10 microM) produced a transient 21% increase in the frequency of Ca(2+) sparks (P < 0.05). This increase was accompanied by a 13% decrease in spark amplitude (P < 0.05) and a 7% decrease in SR Ca(2+) load (P < 0.05) and was inhibited by IP(3)R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 microM) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP(3) production and caused a 38% (P < 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca(2+) transients. This effect was abolished by the IP(3)R antagonist 2-APB (2 microM) or by using adenoviral expression of an IP(3) affinity trap that buffers cellular IP(3). Together, these data suggest that in rabbit ventricular myocytes IP(3)R-dependent Ca(2+) release has positive inotropic effects on ECC by facilitating Ca(2+) release through ryanodine receptor clusters.

Ins(1,4,5)P3 receptors and inositol phosphates in the heart-evolutionary artefacts or active signal transducers?

The generation of the second messenger inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and its associated release of Ca(2+) from internal stores is a highly conserved module in intracellular signaling from Drosophila to mammals. Many cell types, often nonexcitable cells, depend on this pathway to couple external signals to intracellular Ca(2+) release. However, despite the presence of the requisite Ins(1,4,5)P(3) signaling machinery, excitable cells such as cardiac myocytes employ a robust alternate system of intracellular Ca(2+) release, namely, a coupled system of Ca(2+) influx, followed by Ca(2+) release via the IP(3)R-related ryanodine receptors. In these systems, Ins(1,4,5)P(3) signaling pathways appear to be largely dormant. In this review, we consider the general features of inositol phosphate (InsP) responses in cardiac myocytes and the molecules mediating these responses. The spatial localization of Ins(1,4,5)P(3) generation and Ins(1,4,5)P(3) receptor (IP(3)Rs) is likely of key importance, and we examine the state of knowledge in atrial, ventricular, and Purkinje myocytes. Several studies have implicated Ins(1,4,5)P(3) generation in both arrhythmogenic and hypertrophic responses, and possible mechanisms involving Ins(1,4,5)P(3) are discussed. While Ins(1,4,5)P(3) is unlikely to be a key player in cardiac excitation-contraction (EC) coupling, its potential role in an alternate Ca(2+) release system to signal changes in gene transcription warrants further investigation. Such studies will help to determine whether cardiac Ins(1,4,5)P(3) generation represents a vestigial pathway or plays an active role in cardiac signaling.

Phospholipase C activation by prostacyclin receptor agonist in cerebral microvascular smooth muscle cells

The mechanism through which iloprost permits cerebral vasodilation induced by specific stimuli is incompletely understood. Previous study suggests there might be interplay between the adenylyl cyclase and phospholipase C (PLC) systems. Coupling of the prostacyclin receptor with the PLC pathway system was investigated. Iloprost, a stable prostacyclin analog, was used as a prostacyclin receptor agonist. We investigated the effects of iloprost (10-12-10-6 M) on inositol 1,4,5-trisphosphate (IP3) production by piglet cerebrovascular smooth muscle cells in primary culture. Iloprost caused concentration- and time-dependent increases in IP3 production in control cells and in cells pretreated with LiCl (to prevent further IP3 metabolism). Iloprost treatment (10-12 M) of cerebrovascular smooth muscle cells, in the absence and presence of 20 mM LiCl, resulted in 2-fold and 4-fold increases in the formation of IP3, respectively. In contrast, 10-10 M to 10-6 M iloprost, either in the presence or absence of LiCl, induced moderate or no increase in IP3 formation. Iloprost (10-10-10-12 M) strongly stimulated diacylglycerol (DAG) generation, whereas higher concentrations (10-8 M) did not induce an increase. In conclusion, the results suggest that prostacyclin receptors on cerebromicrovascular smooth muscle can couple to PLC, generating the second messengers, IP3 and DAG.

The inositol phosphate response to thrombin in rat right atria differs from the response to noradrenaline

Addition of thrombin to isolated [3H]inositol-labelled rat right atria stimulated the release of 3H-labelled inositol phosphates. The thrombin response was smaller than the response to noradrenaline and generated a different spectrum of inositol phosphates. Unlike the inositol phosphate response to noradrenaline, the thrombin response was inhibited by pertussis toxin treatment and by the phospholipase C inhibitor U-73122 (1-(6-((17 beta-3- methoxyestra-1,3,5(10)-trien-17-yl)amino)amino)hexyl)-1H- pyrrole-2, 5-dione). The data indicate that the thrombin stimulation involves different G-proteins and phospholipase C isoforms from those which couple alpha 1-adrenoceptors in the myocardium.

Suppression of ventricular arrhythmias during ischemia-reperfusion by agents inhibiting Ins(1,4,5)P3 release

BACKGROUND

Reperfusion following myocardial ischemia causes a rapid and transient release of inositol (1,4,5)triphosphate [Ins(1,4,5)P3]. The aim of this study was to test whether this increased Ins(1,4,5)P3 release was important for the development of ventricular arrhythmias and whether agents that inhibit this signal transduction pathway, such as aminoglycoside antibiotics, suppress arrhythmias.

METHODS AND RESULTS

In perfused rat hearts, ventricular tachycardia (VT), ventricular fibrillation (VF), and accumulation of Ins(1,4,5)P3 were measured during early reperfusion. A number of different compounds, including neomycin, gentamicin, streptomycin, spermine, reserpine, and prazosin, were effective in inhibiting the reperfusion-induced Ins(1,4,5)P3 release and the onset of VT and VF in parallel. A strong correlation existed between Ins(1,4,5)P3 content, measured at 2 minutes of reperfusion, and the incidence of reperfusion VT and VF. In addition, intravenous gentamicin suppressed the onset of arrhythmias under ischemic and reperfusion conditions in vivo.

CONCLUSIONS

Our results are consistent with the view that Ins(1,4,5)P3 release plays a pivotal role in mediating arrhythmias during early reperfusion. Agents inhibiting Ins(1,4,5)P3 release are antiarrhythmic and may have potential use clinically.

Inositol phosphate release and metabolism during myocardial ischemia and reperfusion in rat heart

A detailed study of the effects of global myocardial ischemia and reperfusion on inositol phosphate release and metabolism has been undertaken by using isolated perfused rat hearts. Ischemia for longer than 5 minutes caused a cessation of inositol phosphate production, with inositol phosphates initially present accumulating as isomers of inositol monophosphate. This inhibition was independent of norepinephrine. In contrast, 2-minute reperfusion following 20-minute ischemia produced a rapid and transient release of inositol phosphates that was dependent on the release of norepinephrine and mediated by alpha 1-adrenergic receptors. By a number of criteria, this reperfusion response was different from the norepinephrine response in normoxic tissue. First, total release of inositol phosphates was greater (466 +/- 37 compared with 345 +/- 29 cpm/mg protein, P < .05). Second, inositol 1,4,5-trisphosphate was released with postischemic reperfusion (103 +/- 18 to 207 +/- 11 pmol/mg protein), whereas release was not detected in normoxic myocardium. In agreement with this, neomycin (0.5 and 5 mmol/L) inhibited inositol phosphate release only under reperfusion conditions. Third, the reperfusion response, unlike the response in nonischemic tissue, required extracellular Ca2+. Longer periods of reperfusion resulted in a return to a pattern of inositol phosphate release that was not different from that seen in normoxic tissue. The rapid and transient release of inositol 1,4,5-trisphosphate at 2-minute postischemic reperfusion provides an explanation for the enhanced role of alpha 1-adrenergic receptors under these conditions and suggests an important role for this compound in initiating reperfusion-induced pathological events.

A low-affinity, low-molecular-mass endothelin-A receptor in neonatal rat heart

Endothelin receptors with endothelin-A (ETa) specificity were present in neonatal rat ventricle. However, in both receptor-binding studies and studies of inositol phosphate accumulation, these receptors had lower affinity for endothelin-1 than ETa receptors on isolated neonatal cardiomyocytes or adult left atria. Receptors in the three myocardial preparations were cross-linked to 125I-endothelin-1 and their molecular masses measured using SDS/PAGE. Receptors on left atria and neonatal cardiomyocytes had the expected molecular mass of 48 kDa, whereas the receptors in neonatal ventricle were smaller (38 kDa). Despite this, neonatal ventricles contained ETa receptor mRNA which was not different in size from that in the isolated cells (4.5 kb). Thus the 38 kDa ETa receptor present in neonatal ventricle appears to be transcribed from full-length ETa receptor mRNA and is possibly formed by processing of the 48 kDa receptor.

Changes in the responsiveness to endothelin-1 in isolated atria from diabetic rats

This study investigates the influence of diabetes on the cardiac responsiveness to endothelin-1. The effects of endothelin-1 on rate and force of contraction were examined in isolated right and left atria, respectively, obtained from either streptozotocin (65 mg/kg)-treated rats (diabetic) or vehicle (0.02 M citric acid)-treated rats (control). The positive chronotropic and inotropic effects of endothelin-1 did not change in atria from diabetic rats at 2 and 4 weeks, but were reduced at 8 and 12 weeks. The positive chronotropic response to noradrenaline, but not to sympathetic nerve stimulation, was also reduced in 12-week diabetic rats. Endothelin-1 caused a decrease in the positive chronotropic and inotropic responses to sympathetic nerve stimulation and to noradrenaline; these inhibitory effects of endothelin-1 were not altered in 2-, 4-, 8- or 12-week diabetic rats. The study demonstrates that atrial responses to endothelin-1 and to noradrenaline are reduced by streptozotocin-induced diabetes, but the alteration depends on the duration of diabetes.

A low affinity, low molecular weight endothelin-A receptor present in neonatal rat heart

1. Addition of endothelin-1 (ET-1) to [3H]-inositol-labelled neonatal rat hearts stimulated the accumulation of [3H]-labelled inositol phosphates (InsP), but only at high concentrations; concentration at half maximum stimulation (EC50) > 0.1 mumol/L). When similar experiments were performed using isolated myocytes, the potency of endothelin-1 was higher and the EC50 value averaged 3.2 +/- 0.5 nmol/L (mean +/- s.e.m., n = 4). 2. The binding affinity of [125I]-endothelin-1 was higher for receptors on isolated cells than for receptors on membranes prepared from intact heart (72 +/- 16 pmol/L compared with 3.9 +/- 0.7 nmol/L, mean +/- s.e.m., n = 4, P < 0.01; Students' t test). 3. Receptors from both sources were cross-linked to [125I]-endothelin-1 and their molecular weights measured using sodium dodecylsulfate gradient polyacrylamide gel electrophoresis (SDS-PAGE). The receptors present on the isolated cells had a higher molecular weight (48 kD) than the receptor on the heart membranes (38 kD).

Interaction between vasopressin and endothelin in renal papillary tubules: uncoupling following cell isolation and culture

1. In freshly prepared rat renal papillary tubules, endothelin-related peptides inhibited the vasopressin-stimulated accumulation of cAMP. No inhibition was observed when tubules were cultured overnight ex vivo. 2. Endothelin-1, endothelin-3 and sarafatoxin S6b had similar potencies as inhibitors of cAMP accumulation, indicating an endothelin (ETb) receptor. 3. These results demonstrate an interaction between ETb receptors and vasopressin receptors at the level of their signal transduction pathways, and show that this relationship is lost following cell culture.

Isolation of adult cardiomyocytes initiates a return of inositol trisphosphate phosphorylating activity

1. We have previously reported that the addition of noradrenaline to [3H]-inositol-labelled adult rat atria or isolated perfused hearts caused the release of inositol-1,4,5-trisphosphate, which was metabolized by dephosphorylation to inositol-4-monophosphate. Inositol-1,3,4,5-tetrakisphosphate and its dephosphorylation products were not detected. 2. In the current study, the addition of noradrenaline to [3H]-inositol-labelled adult rat cardiomyocytes caused the release of inositol-1,4,5-trisphosphate, which was metabolized in part by phosphorylation to inositol-1,3,4,5-tetrakisphosphate. 3. These results demonstrate that the isolation and culture of rat adult cardiomyocytes initiates enhanced generation of inositol-1,3,4,5-tetrakisphosphate. This change would be expected to enhance the calcium response of the cells to stimulation of alpha 1-adrenoceptors.

Different pathways of inositol phosphate metabolism in intact neonatal rat hearts and isolated cardiomyocytes

In most tissues stimulation of the phosphatidylinositol turnover pathway causes release of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], which is subsequently metabolized to a wide range of inositol phosphate isomers deriving from both phosphorylation and dephosphorylation reactions. However, addition of noradrenaline to isolated intact neonatal-rat hearts generated only those inositol phosphates produced by dephosphorylation of Ins(1,4,5)P3. Products of the InsP3 kinase pathway were absent from the profiles, except after prolonged stimulation. In contrast, addition of noradrenaline to isolated cultured neonatal-rat cardiomyocytes caused the release of Ins(1,4,5)P3, which was metabolized by both phosphorylation and dephosphorylation pathways to yield a complex range of inositol phosphate isomers, as observed in many other cell types. These differences between the responses in intact tissues and in isolated cell preparations were not caused by the different conditions used for [3H]inositol labelling. Furthermore, results could not be explained by overgrowth of other cell types in the isolated cell preparations. Thus the results demonstrate that the isolation and culture of rat neonatal cardiomyocytes produces alterations in the nature of the phosphatidylinositol turnover pathway.

Endothelin receptors in rat renal papilla with a high affinity for endothelin-3

A high density of binding sites for endothelin has been described in rat renal papilla but the nature and significance of papillary endothelin receptors have not yet been evaluated. In the current study, the effect of endothelin peptides on phosphatidylinositol turnover in papillary tubules has been investigated. Endothelin-1, endothelin-3 and the endothelin-related peptide sarafatoxin S6b all stimulated the accumulation of inositol phosphates in [3H]inositol-labelled papillary tubule preparations. However, at these papillary receptors endothelin-3 was more potent than endothelin-1. In other tissues, endothelin-1 is more potent than endothelin-3 at endothelin receptors coupled to phosphatidylinositol turnover. The EC50 value for endothelin-3 expressed as the negative logarithm was 9.3 +/- 0.13 compared with 8.42 +/- 0.11 for endothelin-1 (mean +/- S.E.M., n = 5 in each case, P less than 0.01). The affinity of sarafatoxin S6b was similar to that for endothelin-3 (9.2 +/- 0.15, n = 3). These findings raise the possibility of a direct tubular function of endothelin and suggest that endothelin-3 rather than endothelin-1 may be the natural agonist for these papillary receptors.

Culturing rat neonatal myocytes causes changes in the phosphatidylinositol turnover pathway

1. Cultured neonatal myocytes are commonly used as a model system for the study of cardiac phosphatidylinositol (PI) turnover. 2. In neonatal myocytes stimulation with noradrenaline causes the release of the Ca2(+)-releasing compound inositol-1,4,5-trisphosphate and the generation of the Ca2(+)-regulatory compound inositol-1,3,4,5-tetrakisphosphate. 3. Addition of noradrenaline to intact, neonatal rat hearts stimulates the release of inositol-1,4,5-trisphosphate, but not inositol-1,3,4,5-tetrakisphosphate. 4. These findings show that the isolation and culture of the neonatal myocyte causes changes in the PI turnover pathway so that it becomes similar to that described in other cell types and different from that in intact myocardial tissue. 5. The neonatal myocyte is not a useful model for the study of cardiac PI turnover.