Different pathways of endothelin/sarafotoxin-stimulated phosphoinositide hydrolysis in myocytes

Essential fatty acid metabolism in long term primary cultures of rat cardiomyocytes: a beneficial effect of n-6:n-3 fatty acids supplementation

In long term (21 days) primary cultures of neonatal rat cardiomyocytes, utilized as a model of in vitro senescence, we investigated the dual effect of the time length in culture and of the supplementation with n-6:n-3 fatty acid mixtures on linoleic (LA) and alpha-linolenic acid (ALA) metabolism. Cardiomyocytes were divided into groups receiving: (1) control medium; (2) control medium plus n-3 fatty acids; (3) and (4) control medium plus n-6 and n-3 fatty acids in the ratio 1:2 or 2:1, respectively. In control cells. senescence caused a reduction in the conversion of LA and ALA, and the decrease in their metabolites was bypassed by the different supplementations. The fatty acid composition of cardiomyocyte lipids was therefore affected by both senescence and supplementation, as evidenced by the n-6:n-3 fatty acid ratio and the unsaturation index (U.I.) in cellular lipids. The final result of ageing in culture and of fatty acid supplementations was in all the groups of cells but one (n-6:n-3, 2:1) an unbalance in the n-6:n-3 fatty acid ratio. All the supplementations were able to counteract the decrease in the U.I. observed with senescence, but only the n-6:n-3 (2:1) was able to do so by increasing the cellular content of the fatty acids which are precursors of anti-aggregation eicosanoids without altering the n-6:n-3 fatty acid ratio.

Manipulation of lipid composition of rat heart myocytes aged in culture and its effect on alpha1-adrenoceptor stimulation

The fatty acid composition of the phosphoinositides was evaluated in cultured neonatal rat cardiomyocytes during the aging-like process in vitro, comparing data obtained from control and gamma-linolenic acid supplemented cardiomyocytes. The response to alpha1 stimulation was evaluated in both control and supplemented cells to verify the relationship between the alterations of the phosphoinositide fatty acid composition concomitant to culture aging and the cell response to exogenous stimuli. Arachidonate level decreased as a function of age in all the phosphoinositides, which appeared to be more saturated as cells aged in culture. Inositol phosphate production in response to alpha1 stimulation decreased as cells aged in culture. Supplementation of culture medium with gamma-linolenic acid caused significant modifications in the fatty acid pattern of the phosphoinositides, which appeared less saturated than the corresponding fractions isolated from unsupplemented cells during the aging-like process. The modifications induced by the supplementation in the phosphoinositide fatty acid composition prevented the age-related reduction of inositol phosphate production upon stimulation. These results clearly indicate a major role for the lipid composition in determining the response to alpha1 stimulation, suggesting a nutritional approach to overcome some of the impairments of molecular events related to the process of aging.

Age-related changes in essential fatty acid metabolism in cultured rat heart myocytes

We previously demonstrated that cultured neonatal rat myocytes have the capacity to desaturate/elongate essential fatty acids, alpha-linolenic acid conversion being higher than linoleic acid conversion. The whole process of highly unsaturated fatty acid formation from linoleic and alpha-linolenic acids slows with aging. In this study we grew heart myocytes in culture for different periods of time, and we observed a decrease in the desaturating/elongating activities for both substrates as the cells aged in culture. Alpha-linolenic acid conversion into highly unsaturated fatty acids was less impaired by aging than linoleic acid conversion. These modifications are correlated to the age-dependent alterations observed in the total lipid fatty acid composition, which caused a decrease in the unsaturation index. Changes in the lipid composition that occur in aging cultures parallel those reported for several tissues upon aging in the whole animal. The data herein reported may suggest the possibility of counteracting the effects of aging on lipid metabolism by supplementing cultures with appropriate amounts of highly unsaturated fatty acids.

Endothelin-1 mediated inhibition of the acetylcholine-activated potassium current from rabbit isolated atrial cardiomyocytes

1. Endothelin-1 is a 21 amino acid peptide with potent inotropic and chronotropic actions in the heart. Relatively little is known about the underlying electrophysiological effects of the peptide. In this study, the effects of endothelin-1 (ET-1) on the acetylcholine-activated potassium current (IK(ACh) were investigated in the absence and presence of the receptor-selective antagonists, PD155080 (ETA receptor-selective) and RES-701 (ETB receptor-selective) in rabbit atrial cardiomyocytes. 2. Cells were obtained from New Zealand White rabbits (2.5-3 kg) by enzymatic dissociation with collagenase. Potassium currents were recorded, in the presence of nifedipine (5 microM), by use of the whole cell ruptured patch-clamp technique. Following stabilization, control recordings were made with standard pulse protocols, and drugs were applied by a gravity fed microperfusion system. 3. Endothelin-1 (10 nM) alone did not affect the "steady state' potassium current. Acetylcholine (1 microM) increased (P < 0.05) the potassium current to-1321 +/- 290 pA, from a control value of -955 +/- 191 pA, at a step potential of -100 mV. Acetylcholine also increased the holding current at -40 mV from +80 +/- 9 pA to +242 +/- 38 pA, and this effect was abolished (P < 0.05) in the presence of endothelin-1 (+44 +/- 13 pA). The responses to acetylcholine were attributed to activation of the atrial muscarinic-activated potassium current (IK(ACh)) as they were blocked by atropine (10 microM). Endothelin-1 (10 nM) in the presence of acetylcholine did not affect the "steady state' potassium current (-882 +/- 88 pA compared to a control value of -870 +/- 98 pA, at -100 mV). 4. The ETA receptor-selective antagonist, PD155080 (1 microM), prevented (P < 0.05) the ET-1 induced inhibition of IK(ACh) at all potentials. PD155080, in the presence of endothelin-1 and acetylcholine, increased the inward component of the "steady state' potassium current to -1030 +/- 210 pA from a control value of -804 +/- 224 pA at a step potential of -100 mV. Also the outward component was increased at a potential of -20 mV from +90 +/- 17 pA to +241 +/- 47 pA. 5. Unlike PD155080, the ETB receptor-selective antagonist, RES-701 (1 microM), only prevented (P < 0.05) the inhibitory effect of endothelin-1 on the inward component of the IK(ACh); at -100 mV, RES-701, in the presence of endothelin-1 and acetylcholine, increased the "steady state' potassium current to -913 +/- 137 pA from -733 +/- 116 pA. Furthermore, RES-701, in contrast to PD155080, failed to sustain this inhibitory effect as, in the presence of endothelin-1 and acetylcholine, the "steady state' potassium current returned to a value of -768 +/- 96 pA, at a step potential of -100 mV. 6. In conclusion, endothelin-1 clearly inhibits the effects of acetylcholine on IK(ACh) in rabbit atrial cardiomyocytes. This effect is primarily mediated by an ETA receptor-subtype, but is transiently and partially mediated by a RES-701-sensitive ETB receptor subtype. Inhibition of the IK(ACh) may account for the positive chronotropic properties of endothelin-1.

Molecular pharmacology and pathophysiological significance of endothelin

Since the discovery of the most potent vasoconstrictor peptide, endothelin, in 1988, explosive investigations have rapidly clarified much of the basic pharmacological, biochemical and molecular biological features of endothelin, including the presence and structure of isopeptides and their genes (endothelin-1, -2 and -3), regulation of gene expression, intracellular processing, specific endothelin converting enzyme (ECE), receptor subtypes (ETA and ETB), intracellular signal transduction following receptor activation, etc. ECE was recently cloned, and its structure was shown to be a single transmembrane protein with a short intracellular N-terminal and a long extracellular C-terminal that contains the catalytic domain and numerous N-glycosylation sites. In addition to acute contractile or secretory actions, endothelin has been shown to exert long-term proliferative actions on many cell types. In this case, intracellular signal transduction appears to converge to activation of mitogen-activated protein kinase. As a recent dramatic advance, a number of non-peptide and orally active receptor antagonists have been developed. They, as well as current peptide antagonists, markedly accelerated the pace of investigations into the true pathophysiological roles of endogenous endothelin-1 in mature animals; e.g., hypertension, pulmonary hypertension, acute renal failure, cerebral vasospasm, vascular thickening, cardiac hypertrophy, chronic heart failure, etc. Thus, the interference with the endothelin pathway by either ECE-inhibition or receptor blockade may provide an exciting prospect for the development of novel therapeutic drugs.

cGMP formation in rat atrial slices is ligand and endothelin receptor subtype specific

Involvement of a cGMP pathway in signal transduction stimulated by endothelins(ETs) and sarafotoxins (SRTXs) was examined in rat atrial slices. These peptides activated different receptor-binding sites (ET-1 and SRTX-b reacted with picomolar binding sites of the ET(A) receptor, and ET-3 and SRTX-c reacted with the nanomolar binding sites of the ET(B) receptor) to produce cGMP. ET-1 and SRTX-b stimulated an increase in cGMP levels via a Ca2+-dependent NO pathway involving a pertussis toxin-insensitive G protein, whereas ET-3 and SRTX-c elevated cGMP levels via a Ca2+-independent CO pathway involving a pertussis toxin-sensitive G protein. These results can best be explained in terms of formation of different ligand-receptor-G-protein complexes. The ligands had no effect on ventricular slices, indicating that these signal transduction mechanisms are unique to the atria.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.

The structure and specificity of endothelin receptors: their importance in physiology and medicine

In addition to involvement in vascular endothelium-smooth muscle communication, the secretion of and receptors for, endothelins are widely distributed. Two cloned receptor subtypes are G-protein-coupled to several intracellular messengers, predominantly inositol phosphates. From a knowledge of structure-activity relationships and peptide conformations, details of receptor architecture and selective agents, including nonpeptides and antagonists, have been discovered. From the nature of the actions of endothelins, receptor distributions (including CNS) and plasma levels, it is concluded that they are paracrine factors normally involved in long-term cellular regulation, but which may be important in several pathologies, many of which are stress-related.

Solution conformation of endothelin and point mutants by nuclear magnetic resonance spectroscopy

Two-dimensional NMR techniques were utilized to determine the secondary structural elements of endothelin-1 (ET-1), a potent vasoconstrictor peptide, and two of its point mutants, Met-7 to Ala-7 (ETM7A), and Asp-8 to Ala-8 (ETD8A) in acetic acid-d3/water solution. Sequence specific NMR assignments were determined for all three peptides, as well as chemical shifts and NOE connectivity patterns. The chemical shifts of ET-1 and ETM7A are identical (+/- 0.05 ppm) except for the site of substitution, whereas marked shift changes were detected between ET-1 and ETD8A. These chemical shift differences imply that the Asp-8 to Ala-8 mutation has induced a conformational change relative to the parent conformation. All three molecules show the same basic nuclear Overhauser effect (NOE) pattern, which suggests that the gross conformation of all three molecules is the same. Small changes in sequential NOE intensities and changes in medium-range NOE patterns indicate that there are subtle conformational differences between ET-1 and ETD8A.

Structure-function relationships of endothelins, sarafotoxins, and their receptor subtypes

The endothelins (ETs) and sarafotoxins (SRTXs) are two structurally related families of potent vasoactive peptides. Although their physiological functions have yet to be precisely elucidated, it seems likely that the ETs are involved in pathophysiological conditions such as hypertension and heart failure. In this minireview, recent advances in the biochemical characterization of the ET/SRTX system, with special reference to structure-function relationships and ET/SRTX receptor subtypes, are described, as well as the recent cloning and expression of ET receptors.

Endothelins and sarafotoxins: physiological regulation, receptor subtypes and transmembrane signaling

The endothelins and sarafotoxins are two structurally related families of potent vasoactive peptides. Although the physiological functions of these peptides are not entirely clear, the endothelins are probably involved in pathophysiological conditions such as hypertension and heart failure. This review summarizes the state of the art in some areas of this intensively studied subject, including: (1) structure-function relationships of ET/SRTX, (2) ET concentrations in plasma, (3) ET/SRTX receptor subtypes and (4) signaling events mediated by the activation of ET/SRTX receptors.

Influence of aging on the contractile response to endothelin of rat thoracic aorta

Age-related changes in the contractile response to endothelin-1 and ACh were assessed in thoracic aortas isolated from 2-, 6- and 24-month-old male Fischer 344 rats. In aortic strips with an intact endothelium, the maximal contractile response to endothelin-1 decreased with development to maturity. Removal of the endothelium did not affect the contractile response to endothelin-1. Endothelin-1 did not elicit a relaxant response in phenylephrine-precontracted strips. The ACh-induced relaxation decreased in senescent rats. These results indicate that the contractile response of aortic smooth muscle to endothelin-1 decreases with age, and that the endothelial vasorelaxant factors do not contribute to this age-induced modulation.

Determination of the structure of [Nle7]-endothelin by 1H NMR

[Nle7]-endothelin was synthesized and studied by 1H NMR and distance geometry calculations. The NMR study was performed first in DMSO-d6 and then in 50% acetonitrile/water since this peptide aggregates in pure water. In both cases, all spin systems were identified and assigned with the aid of two-dimensional spectroscopy (2D): COSY (for scalar couplings) and NOESY (for dipolar couplings). On the basis of the acetonitrile/water NMR parameters, and using the DISGEO program, a three-dimensional structure of [Nle7]-endothelin is proposed and discussed.

Endothelin/sarafotoxin receptor induced phosphoinositide turnover: effects of pertussis and cholera toxins and of phorbol ester

The induction of phosphoinositide hydrolysis (PI) by endothelin/sarafotoxin (ET/SRTX) receptors in rat heart myocytes was investigated by the use of bacterial toxins as well as a phorbol ester. Both pertussis- and choleratoxin enhanced the stimulation of PI hydrolysis. Phorbol ester treatment of the myocytes for short periods distinguished between two types of PI-hydrolysis, the one induced by endothelins and the other by sarafotoxins. The possible mediation of G-protein (s) in the induction by ET/SRTX receptors of PI-hydrolysis is discussed.