Intermedin (adrenomedullin-2): a novel counter-regulatory peptide in the cardiovascular and renal systems.

Intermedin (IMD) is a novel peptide related to calcitonin gene-related peptide (CGRP) and adrenomedullin (AM). Proteolytic processing of a larger precursor yields a series of biologically active C-terminal fragments, IMD(1-53), IMD(1-47) and IMD(8-47). IMD shares a family of receptors with AM and CGRP composed of a calcitonin-receptor like receptor (CALCRL) associated with one of three receptor activity modifying proteins (RAMP). Compared to CGRP, IMD is less potent at CGRP(1) receptors but more potent at AM(1) receptors and AM(2) receptors; compared to AM, IMD is more potent at CGRP(1) receptors but less potent at AM(1) and AM(2) receptors. The cellular and tissue distribution of IMD overlaps in some aspects with that of CGRP and AM but is distinct from both. IMD is present in neonatal but absent or expressed sparsely, in adult heart and vasculature and present at low levels in plasma. The prominent localization of IMD in hypothalamus and pituitary and in kidney is consistent with a physiological role in the central and peripheral regulation of the circulation and water-electrolyte homeostasis. IMD is a potent systemic and pulmonary vasodilator, influences regional blood flow and augments cardiac contractility. IMD protects myocardium from the deleterious effects of oxidative stress associated with ischaemia-reperfusion injury and exerts an anti-growth effect directly on cardiomyocytes to oppose the influence of hypertrophic stimuli. The robust increase in expression of the peptide in hypertrophied and ischaemic myocardium indicates an important protective role for IMD as an endogenous counter-regulatory peptide in the heart.

NPY and cardiac diseases.

Hypertension-induced left ventricular hypertrophy (LVH), along with ischemic heart disease, result in LV remodeling as part of a continuum that often leads to congestive heart failure. The neurohormonal model has been used to underpin many treatment strategies, but optimal outcomes have not been achieved. Neuropeptide Y (NPY) has emerged as an additional therapeutic target, ever since it was recognised as an important mediator released from sympathetic nerves in the heart, affecting coronary artery constriction and myocardial contraction. More recent interest has focused on the mitogenic and hypertrophic effects that are observed in endothelial and vascular smooth muscle cells, and cardiac myocytes. Of the six identified NPY receptor subtypes, Y(1), Y(2) and Y(5) appear to mediate the main functional responses in the heart. Plasma levels of NPY become elevated due to the increased sympathetic activation present in stress-related cardiac conditions. Also, NPY and Y receptor polymorphisms have been identified that may predispose individuals to increased risk of hypertension and cardiac complications. This review examines what understanding exists regarding the likely contribution of NPY to cardiac pathology. It appears that NPY may play a part in compensatory or detrimental remodeling of myocardial tissue subsequent to hemodynamic overload or myocardial infarction, and in angiogenic processes to regenerate myocardium after ischemic injury. However, greater mechanistic information is required in order to truly assess the potential for treatment of cardiac diseases using NPY-based drugs.

SB-271258, a major metabolite in the dog of the thiazolidinedione, rosiglitazone, stimulates protein synthesis in re-differentiated rat cardiomyocytes in vitro.

In dogs, chronic administration of thiazolidinediones causes cardiac hypertrophy in vivo at high doses. The hypertrophic action of rosiglitazone in dogs might be attributed to production of SB-271258 (a major metabolite in dogs but minor in rats and man), rather than a direct effect on myocardium. The hypothesis that SB-271258 had potential to initiate cardiomyocyte hypertrophy or to modify responses elicited by other hypertrophic stimuli was tested in an in vitro bioassay utilising adult rat ventricular cardiomyocytes. SB-271258 increased protein and incorporation of 14C-phenylalanine, a marker of protein synthesis, in rat cardiomyocytes maintained in serum-free culture (24 h), maximally at 1 microM by 15.0% and 9.1%, respectively. In the presence of serum (10% v/v), SB-271258 elicited a moderate trophic effect: cellular protein and incorporation of 14C-phenylalanine were increased, maximally at 100 nM by 31.7% and 36.3%, respectively, above basal values (18.6% and 13.3% increases above serum response). In the presence of IGF-1 (10 nM) plus SB-271258, protein synthesis was increased, maximally by 45.5% above basal value (increase of 6.9% above IGF-1 alone). In contrast, SB-271258 attenuated the increase (12.0%) in cellular protein elicited by IGF-1. In re-differentiated cardiomyocytes, a model of relevance to established hypertrophy, SB-271258 (1 nM-1 microM) elicited a marked trophic effect per se, as evidenced by the maximum increase (at 100 nM), in protein synthesis of 24.5%. In conclusion, these data imply that cardiac hypertrophy associated with chronic administration of rosiglitazone in dogs in previous in vivo studies might be partly attributable to production of the metabolite SB-271258 since this metabolite was shown to elicit trophic effects directly on rat cardiomyocytes.

Myocardial expression of the endothelin system in endotoxin-treated rats.

Although circulating plasma levels of endothelin (ET)-1 are elevated in endotoxemia, little is known about the myocardial expression of the ET system in endotoxic shock. We assessed the temporal mRNA expression pattern of key components of the ET system (pre-pro ET (ppET) -1, -2, ET-converting enzyme-1, ET(A) and ET(B) receptors) by reverse transcription polymerase chain reaction in a rat model of early endotoxic shock. Lipopolysaccharide (5 mg/kg, i.p.) caused a transient increase (p < 0.05) in inducible nitric oxide synthase mRNA expression. ppET-1 mRNA expression was increased at 2 h (approximately 12-fold increase; p < 0.05) in the lipopolysaccharide compared with the saline group and ppET-2 mRNA expression was unaltered. ET-converting enzyme-1, ET(A), and ET(B) receptor mRNA expression was unaltered in the lipopolysaccharide compared with the saline group. While ppET-1 mRNA expression is selectively upregulated in ventricular myocardium of lipopolysaccharide-treated rats, an absence of alteration in ET-converting enzyme-1 mRNA expression suggests an excess capacity of ET-converting enzyme-1 to cope with the increased expression of ET-1. At the level of the receptor, endotoxic shock did not affect the expression of either ET(A) or ET(B) receptor mRNA. These data are consistent with the increased expression of myocardial ET-1 as an acute-phase response due to hemodynamic instability associated with the early stages of endotoxic shock.

Use of A-192621 to provide evidence for involvement of endothelin ET(B)-receptors in endothelin-1-mediated cardiomyocyte hypertrophy.

Increased plasma levels of endothelin-1 correlate with the severity of left ventricular hypertrophy in vivo. The aim of the study was to determine the relative contribution of stimulation of endothelin ET(A) and endothelin ET(B) receptors, and the associated activation of protein kinase C, to the hypertrophic response initiated by endothelin-1 in adult rat ventricular cardiomyocytes maintained in culture (24 h). Endothelin-1 (10(-7) M) increased the total mass of protein and the incorporation of [14C] phenylalanine into protein to 26% and 25% greater (P<0.05) than respective basal values. The total content of RNA and the incorporation of 2-[14C] uridine into RNA were increased by 23% and 21%, respectively, by endothelin-1 (10(-8) M). Actinomycin D (5x10(-6) M), an inhibitor of transcription, abolished the incorporation of [14C] phenylalanine and the increased protein mass elicited by endothelin-1 (10(-8) M). The selective agonists at the endothelin ET(B) receptor, sarafotoxin 6c (10(-7) M) and endothelin-3 (10(-7) M), increased the incorporation of [14C] phenylalanine to 13% and 13% greater than respective basal values. The incorporation of [14C]phenylalanine in response to endothelin-1 (10(-7) M) was reduced by 50% (P<0.05) by the selective antagonist at endothelin ET(A) receptors, ABT-627 (10(-9) M), while the response to sarafotoxin 6c was not attenuated. The selective antagonist at endothelin ET(B) receptors, A192621 (10(-10) M), abolished the response to sarafotoxin 6c (10(-7) M) and attenuated the response to endothelin-1 (10(-7) M) by 43% (P<0.05). The selective inhibitor of protein kinase C, bisindolylmaleimide (5x10(-6) M) attenuated the response to sarafotoxin 6c (10(-7) M) by 78% and that to endothelin-1 (10(-7) M), elicited in the presence of A192621 (10(-10) M), by 52%. In conclusion, these data implicate endothelin ET(B) receptors, in addition to endothelin ET(A) receptors, in endothelin-1-mediated cardiomyocyte hypertrophy and provide evidence for the involvement of protein kinase C, at least in part, in the hypertrophic signalling pathways associated with activation of each receptor subpopulation.

Positive and negative contractile effects of somatostatin-14 on rat ventricular cardiomyocytes.

Somatostatin-14 elicits negative inotropic and chronotropic actions in atrial myocardium. Less is known about the effects of somatostatin-14 in ventricular myocardium. The direct contractile effects of somatostatin-14 were assessed using ventricular cardiomyocytes isolated from the hearts of adult rats. Cells were stimulated at 0.5 Hz with CaCl2 (2 mM) under basal conditions and in the presence of the beta-adrenoceptor agonist, isoprenaline (1 nM), or the selective inhibitor of the transient outward current (Ito), 4-aminopyridine (500 microM). Somatostatin-14 did not alter basal contractile response but it did inhibit (IC50 = 13 nM) the response to isoprenaline (1 nM). In the presence of 4-aminopyridine (500 microM), somatostatin-14 stimulated a positive contractile response (EC50 = 118 fM) that was attenuated markedly by diltiazem (100 nM). These data indicate that somatostatin-14 exerts dual effects directly in rat ventricular cardiomyocytes: (1) a negative contractile effect, observed in the presence of isoprenaline (1 nM), coupled to activation of Ito; and (2) a previously unreported and very potent positive contractile effect, unmasked by 4-aminopyridine (500 microM), coupled to the influx of calcium ions via L-type calcium channels. The greater potency of somatostatin-14 for producing the positive contractile effect indicates that the peptide may exert a predominantly stimulatory influence on the resting contractility of ventricular myocardium in vivo, whereas the negative contractile effect, observed at much higher concentrations, could indicate that localized elevations in the concentration of the peptide may serve as a negative regulatory influence to limit the detrimental effects of excessive stimulation of cardiomyocyte contractility.

Endothelin(A) receptor subtype mediates endothelin-induced contractility in left ventricular cardiomyocytes isolated from rabbit myocardium.

Endothelin (ET)-1 is a potent positive inotropic agent, the effects of which are mediated by increases in cytosolic Ca(2+) in the myocardium. The object of this study was to examine 1) the influence of ET(A) and ET(B) receptor subtypes, and 2) the role of the phospholipase C (PLC) pathway in mediating ET-1-induced contraction. Left ventricular cardiomyocytes were isolated from the hearts of New Zealand White rabbits (2-2.5 kg) by the use of Langendorff perfusion with collagenase. Cardiomyocyte function was examined during unloaded, electrically stimulated (0.5 Hz) contractions with a video-edge detection system. ET-1 increased cell shortening with greater potency than ET-3: mean EC(50) values were 1.1 x 10(-11) and 2.6 x 10(-10) M, respectively. With the same order of potency, ET-1 and ET-3 increased (P <.05) velocity of cell shortening. The ET(A) receptor-selective antagonist ABT-627 shifted the ET-1-induced cell shortening response curve to the right with a pA(2) value of 10.3. The ET(B) receptor-selective antagonist A-192621 (10(-8)-10(-7) M) did not alter the concentration-response of ET-1. Moreover, the ET(B) receptor-selective agonist sarafotoxin 6c did not have any effect on cell shortening over the concentration range of 10(-11) to 10(-7) M. ET-1 in the presence of the PLC inhibitor U-73122 did not alter the contractile amplitude. However, ET-1 in the presence of the protein kinase C inhibitor bisindolylmalemide increased cell shortening. These findings indicate that 1) the ET(A) receptor subtype, and not the ET(B) receptor subtype, mediates the positive inotropic effect of ET-1, and 2) the response of ET-1 is mediated by a PLC pathway, but not through protein kinase C, in ventricular cardiomyocytes isolated from rabbit myocardium.

Inotropic response to endothelin-1, isoprenaline and calcium in cardiomyocytes isolated from endotoxin treated rats: effects of ethyl-isothiourea and dexamethasone.

1. The contractile effects of endothelin-1, isoprenaline and extracellular calcium were assessed on ventricular cardiomyocytes isolated from lipopolysaccharide-treated rats. The involvement of nitric oxide was investigated using dexamethasone (in vivo) and ethyl isothiourea (in vitro). 2. Male Wistar rats (n=70) were injected with either saline (1 ml kg(-1)) or lipopolysaccharide (LPS; 5 mg kg(-1)) alone, or following pre-treatment with dexamethasone (DEX+LPS; 5 mg kg(-1)). Ventricular cell shortening was recorded using a video edge detection system, and concentration-response relationships were established for endothelin-1, isoprenaline and calcium, in the absence or presence of ethyl isothiourea (ETU; 10 microM). iNOS expression was assessed using reverse transcription-polymerase chain reaction. 3. iNOS mRNA expression was greater (P<0.001) in the LPS (iNOS/GAPDH ratio: 0.90+/-0.09) treated group compared to saline (iNOS/GAPDH ratio: 0.36+/-0.02). Baseline contractile amplitude was reduced (P<0.05) in the LPS (7.3+/-0.2 microm) and DEX+LPS groups (6.7+/-0.3 microm) compared to saline (8. 0+/-0.2 microm). 4. The concentration-dependent contractile response to endothelin-1 was attenuated (P<0.05) in the LPS group compared to saline (maximum change: 0.45+/-0.2 vs 1.8+/-0.2 microm). Neither ETU nor dexamethasone improved contractile function in the LPS-treated animals. 5. The concentration-dependent increase in the contractile response to isoprenaline was attenuated in the LPS-treated group compared to saline (P<0.05; maximum change: 1.7+/-0.4 vs 3.1+/-0.4 microm). This effect was reversed by ETU (maximum change: 3.7+/-0.6 microm). Pre-treatment with dexamethasone prevented a significant fall in contraction amplitude (maximum change: 2.4+/-0.4 microm). 6. The contractile response to calcium was reduced (P<0.05) in the LPS group compared to saline (maximum change: 8.7+/-0.6 vs 10.7+/-0.8 microm). Neither ETU nor dexamethasone restored contractile function in the LPS-treated group. 7. In conclusion, a nitric oxide-mediated inhibitory pathway is not responsible for the diminished contractile response to either endothelin-1 or extracellular calcium, but contributes to the hyporesponsiveness to isoprenaline in lipopolysaccharide treated rats.

Contribution of de novo protein synthesis to the hypertrophic effect of IGF-1 but not of thyroid hormones in adult ventricular cardiomyocytes.

BACKGROUND: Enhanced expression of IGF-1 occurs in left ventricular hypertrophy (LVH) associated with systemic hypertension. Cardiac dysfunction accompanied by LVH is also observed in hyperthyroidism. OBJECTIVE: to assess the relative contributions of de novo protein synthesis and attenuated protein degradation to increased protein mass associated with cardiomyocyte hypertrophy elicited by IGF-1 and thyroid hormones (tri-iodo thyronine T3, and l-thyroxine T4), respectively. METHODS: total mass of protein, and both the incorporation, and removal of previously incorporated l-U-14C-phenylalanine, indices of protein synthesis and degradation, respectively, were assessed in quiescent adult rat ventricular cardiomyocytes maintained in short-term culture, and corrected for DNA content, as a index of cell number. RESULTS: IGF-1 (1 pM-100 nM) increased cell protein significantly, maximally at 1 nM and by 38% above basal value after 24 h. T3 (10 pM-2 microM) and T4 (10 pM-2 microM) increased cell protein significantly maximally at 1 microM and by 33.2 and 30.5%, respectively, above basal value. IGF-1 (< or = 10 pM), T3 (10 pM-2 microM) and T4 (10 pM-2 microM) did not increase incorporation of l-U-14C-phenylalanine above basal values. IGF-1 (100 pM-100 nM) increased incorporation of radiolabel significantly maximally at 100 nM and by 56%. T4 (100 pM) and IGF-1 (10 pM), concentrations that did not stimulate de novo protein synthesis, attenuated the degradation of radiolabelled protein by 13.6 and 11.8%, respectively, compared to control values after 48 h. CONCLUSION: These data indicate that the acute hypertrophic response to (i) thyroid hormones cannot be attributed to initiation of de novo protein synthesis; (ii) IGF- 1 comprises two components; the response elicited by IGF-1 (< 10 pM) is independent of, while the response elicited by IGF-1 (> 100 pM) is due to de novo protein synthesis.

Troglitazone does not initiate hypertrophy but can sensitise cardiomyocytes to growth effects of serum.

Chronic administration of troglitazone might predispose to cardiac hypertrophy. The aims of the study were to determine if troglitazone could (i) initiate a trophic response directly in ventricular cardiomyocytes and (ii) modify responses to other trophic stimuli. After 24 h, troglitazone (10 nM-10 microM) (i) did not increase cellular protein mass and decreased incorporation of [14C]phenylalanine, a marker of protein synthesis, (ii) interacted with serum (10% v/v) and insulin-like growth factor-1 (10 nM) to produce small trophic responses, (iii) increased cellular protein mass but not protein synthesis with insulin (1 unit/ml). Troglitazone (1 microM) attenuated responses to phorbol-12-myristate-13-acetate (PMA) (100 nM), and noradrenaline (5 microM) and endothelin-1 (100 nM), which also activate protein kinase C. In summary, troglitazone does not initiate cardiomyocyte growth directly in vitro, and can inhibit protein kinase C-mediated growth mechanisms. However, the interaction of troglitazone with serum growth factors may contribute modestly to the development of hypertrophy. As troglitazone produced a moderate hypertrophic effect per se in re-differentiated cardiomyocytes, it may directly increase the severity of established hypertrophy.

A comparison of enalapril 20 mg once daily versus 10 mg twice daily in terms of blood pressure lowering and patient compliance.

OBJECTIVE: To compare enalapril 20 mg once daily with 10 mg twice daily in terms of blood pressure reduction and patient compliance. DESIGN: Cross-over study of patients randomly assigned to a sequence of enalapril 20 mg once daily or 10 mg twice daily in three 4-week periods following a 4-week placebo run-in. SETTING: General practices in the greater Belfast and Lisburn area in Northern Ireland. PATIENTS: Twenty-five hypertensive patients who had a mean diastolic blood pressure of between 90 and 110 mm Hg after receiving placebo for 4 weeks. MAIN OUTCOME MEASURES: Reduction in blood pressure and estimation of patient compliance. RESULTS: Patient compliance was superior on the once daily regimen. However, the twice daily regimen was associated with a greater blood pressure reduction which almost reached statistical significance at the 5% level. CONCLUSIONS: Enalapril 20 mg should be prescribed as 10 mg twice daily and measures taken to improve patient compliance.

D-myo inositol 1,2,6, triphosphate (alpha-trinositol, pp56): selective antagonist at neuropeptide Y (NPY) Y-receptors or selective inhibitor of phosphatidylinositol cell signaling?

1. D-myo inositol 1,2,6 trisphosphate (alpha-trinositol, pp56), an isomer of the second messenger substance, D-myo inositol 1,4,5 trisphosphate, has an interesting pharmacological profile that includes anti-inflammatory and analgesic effects and antagonism of neuropeptide Y (NPY)-mediated cellular responses. 2. However, not all responses elicited by this neuropeptide are sensitive to antagonism by pp56. Evidence is emerging, at least in certain tissues, that other receptor populations, in addition to those for NPY, are also sensitive to inhibition by pp56. 3. A direct or allosteric interaction of pp56 at receptors for NPY is now considered unlikely and it is more probable that pp56 might interfere at some point in the phosphatidylinositol signaling pathway, possibly at the level of the plasmalemmal inositol 1,3,4,5, tetrakisphosphate receptor. 4. Full realization of the therapeutic potential of this novel compound, however, must await a thorough characterization of the cellular mechanism(s) associated with the various pharmacological effects of pp56.

Receptor-mediated effects of endothelin on the L-type Ca++ current in ventricular cardiomyocytes.

The purpose of this study was to establish whether specific receptor subtypes are responsible for mediating the effects of endothelin-1 (ET-1) and endothelin-3 (ET-3) on the L-type calcium current (ICa) using a number of receptor-selective antagonists, including PD155080 (ETA), BQ-788, RES-701 and IRL-1038 (ETB) and the ETA/ETB receptor-non-selective antagonist PD145065. Ventricular cardiomyocytes were isolated from adult New Zealand White rabbits using Langendorff perfusion with collagenase. ICa was recorded using a whole-cell patch-clamp technique. ET-1 decreased, whereas ET-3 increased, ICa at equimolar concentrations of 10 nM. The decrease in ICa produced by ET-1 was completely blocked by PD155080 and PD145065 (1 and 10 microM); however, ICa was increased upon washout of PD155080. Although the decrease in ICa produced by ET-1 was partially blocked by BQ-788 (1 and 10 microM), ET-1 in combination with either RES-701 (1 and 10 microM) or IRL-1038 (1 microM) produced a decrease in ICa similar to that produced by ET-1 alone. The increase in ICa by ET-3 was completely abolished by either BQ-788 or IRL-1038 (1 microM). These data indicate that the decrease in ICa produced by ET-1 in rabbit ventricular cardiomyocytes is mediated by the ETA receptor subtype, because PD155080 completely inhibited this response. The ETB receptor-selective antagonists RES-701 and IRL-1038 did not alter the decrease in current produced by ET-1, although the response was partially sensitive to BQ-788, which may lack receptor-subtype selectivity in these cells. In contrast, the increase in ICa produced by ET-3 was mediated by the ETB receptor subtype, because BQ-788 and IRL-1038 abolished this response.

Enhancement of latent inhibition in the rat by the CCK antagonist proglumide.

The behavioral paradigm of latent inhibition (LI) involves the retardation of conditioning to a stimulus when paired with reinforcement, if preexposure to that stimulus with no significant consequence has occurred. This phenomenon is believed to reflect a process of learning to ignore stimuli as irrelevant. Disruption in LI can be considered to be an attentional deficit observed in schizophrenia. The neuropeptide cholecystokinin (CCK), which coexists with dopamine (DA) in some brain regions, has been implicated in the pathophysiology of schizophrenia. The present study examined the effects of the nonselective CCK antagonist proglumide on LI (0.25, 0.5, and 1.0 mg/kg) using a conditioned suppression of drinking procedure in rats. For purposes of comparison the effects of haloperidol (0.1 mg/kg) were also investigated. Administration of 1.0 and 0.5 mg/kg, but not 0.25 mg/kg, proglumide was found to reduce suppression of drinking behavior in animals preexposed (PE) to a flashing light stimulus. These animals developed LI under conditions where preexposed control animals exhibited suppression of drinking behavior similar to that of nonpreexposed (NPE) control animals. These findings for proglumide were comparable to the effects on drinking behavior of 0.1 mg/kg haloperidol. The enhancement of LI by proglumide may be interpreted in terms of CCK dopamine interactions. Because CCK may modulate dopamine, the results reported here for proglumide strengthen the argument for the investigation of CCK-based drugs as potential antipsychotic agents.

Stimulation of L-type Ca2+ current by the endothelin receptor A-selective antagonist, BQ-123 in ventricular cardiomyocytes isolated from the rabbit myocardium.

BQ-123 is extensively used as an antagonist at endothelin (ET) receptors, having selectivity at the ET(A) receptor subtype. In this study, the effects of BQ-123 per se on action potentials, L-type calcium currents, and potassium currents, were examined in ventricular cardiomyocytes isolated from adult, male, New Zealand White rabbits, using the patch-clamp technique. BQ-123 (1 microM) increased (P < 0.02) the duration of the action potential to 267 +/- 36 ms from a control duration of 228 +/- 30 ms. BQ-123 did not have any effect on the inward rectifier or transient outward potassium currents, but increased (P < 0.02) the L-type Ca2+ current to -2.76 +/- 0.3 nA from a control value of -2.45 +/- 0.28 nA. The increases in both duration of the action potential and L-type Ca2+ current were reversed upon washout (233 +/- 28 ms and -2.32 +/- 0.31 nA, respectively) and were not different from the control values in the absence of BQ-123. In contrast, the endothelin receptor antagonists, BQ-788, PD155080 and PD145065 (1-10 microM) did not affect the L-type Ca2+ current. These results indicate that, unlike PD155080, BQ-788 and PD145065, the conventional ET(A) receptor-selective antagonist, BQ-123, exerts a unique positive effect on the L-type Ca2+ current in ventricular cardiomyocytes isolated from rabbit myocardium. The mechanism of action of BQ-123, therefore, is not confined to ET receptor antagonism.

Calcitonin gene-related peptide (CGRP) increases cell surface area and induces expression of skeletal alpha-actin ANP mRNA in hypertrophying neonatal cardiomyocytes.

We have reported previously that calcitonin gene-related peptide (CGRP) exerts hypertrophic effects, defined in the broadest sense as increased mass of protein per cell, in adult rat ventricular in vitro. The aim of the present investigation was to determine whether the peptide also increases the cell surface area of, and induces expression of ANP and skeletal alpha-actin mRNA in hypertrophying neonatal rat ventricular cardiomyocytes. Cells cultured in the presence of CGRP were invisibly hypertrophied after 48 h compared to cells cultured in serum-free MEM for the same period. CGRP, 100 pM and 1 nM, increased cell surface area significantly and to values 1.82- and 2.15-fold greater, respectively, than in the absence of peptide (659.64 +/-23.48 microns 2, n = 10). The selective antagonist at CGRP1, receptors, CGRP8-37(200nM), significantly attenuated the effects of CGRP (100 pM and 1 nM). CGRP caused a marked up-regulation of the expression of mRNA encoding skeletal alpha-actin and ANP, respectively, maximally after 12 h and at a concentration of 100 pM, to values approximately 3.6- and 2.5-fold greater than in the absence of peptide. These effects of the peptide were completely abolished in the presence of CGRP8-37(100 nM). In conclusion, CGRP increases cell surface area and induces expression of ANP and skeletal alpha-actin mRNA in hypertrophying cardiomyocytes via the CGRP1, receptor subtype.

Involvement of endothelin (ET)A and ETB receptors in the hypertrophic effects of ET-1 in rabbit ventricular cardiomyocytes.

The question was addressed whether endothelin-1 (ET-1) exerts hypertrophic effects in cardiomyocytes isolated from ventricles of adult rabbits and maintained in short-term (24 h) serum-free primary culture providing mechanical quiescence. ET-1 (> or =100 pM) increased significantly total mass of cellular protein and incorporation of L-U-[(14)C]phenylalanine and 2-[(14)C]uridine into cellular protein and RNA, respectively. Cycloheximide (35 microM), an inhibitor of protein synthesis, significantly reduced the incorporation of L-U-[(14)C]phenylalanine and 2-[(14)C]uridine into cellular protein and RNA, respectively, under control conditions and in response to ET-1. Actinomycin D (5 microM), a selective inhibitor of transcription, abolished the incorporation of 2-[(14)C]uridine into cellular RNA and significantly reduced the incorporation of L-U-[(14)C]phenylalanine into cellular protein under control conditions and in response to ET-1. The selective antagonists at the ET(A) receptor [BQ123 (100 nM) and PD155080 (100 nM)] and the selective antagonist at the ET(B) receptor [BQ788 (100 nM)] significantly reduced the incorporation of L-U-[(14)C]phenylalanine into cellular protein in response to ET-1 (10 nM). The selective inhibitor of protein kinase C (PKC), bisindolylmaleimide (BIM) (5 microM), reduced markedly the incorporation of 2-[(14)C]uridine into cellular RNA and, to a lesser degree, the incorporation of L-U-[(14)C]phenylalanine into cellular protein in response to ET-1 (100 pM to 10 nM). ET-1 exerts hypertrophic effects directly in vitro in ventricular cardiomyocytes isolated from the hearts of adult rabbits. These effects are (a) due to de novo synthesis since total mass of cellular protein and incorporation of L-U-[(14)C]phenylalanine and 2-[(14)C]uridine into cellular protein and RNA, respectively, were increased; (b) mediated by both the ET(A) and ET(B) receptor subtypes; and (c) may be associated, at least partly, with the activation of PKC.

Enhanced endothelium-dependent vasodilator responses in patients with systemic vasculitis.

The aim of the study was to investigate endothelium-dependent vasodilator responses in the forearm vasculature of patients with systemic vasculitis. We studied 10 patients with systemic vasculitis and 16 healthy control subjects using forearm venous occlusion plethysmography to assess changes in forearm blood flow in response to acetylcholine (ACh). NG-monomethyl-L-arginine (L-NMMA), was also used to assess the contribution of endothelium derived relaxing factor/nitric oxide (EDRF/NO) to the vasodilator responses to acetylcholine. A significantly greater vasodilator response to ACh was seen in the patient group at all doses infused (p < 0.01). After pre-infusion of L-NMMA, the differences in ACh dilator responses were no longer significant. There was a greater magnitude of inhibition of ACh responses by L-NMMA in the patient group. The enhanced vasodilator response to ACh observed was in part abolished by L-NMMA, suggesting that EDRF/NO is produced in excess in systemic vasculitis. The precise role played by EDRF/NO in the pathogenesis of systemic vasculitis requires further study.

Evidence for Y1 and Y2 subtypes of neuropeptide Y receptors linked to opposing postjunctional effects observed in rat cardiac myocytes.

The aim of this study was to confirm the existence of and identify the receptor subtypes for neuropeptide Y that are present post-junctionally in myocardium. The effects of the selective agonists, [Leu31, Pro34] neuropeptide Y (neuropeptide Y Y1 receptors), neuropeptide Y-(13-36) and peptide YY-(3-36) (neuropeptide Y Y2 receptors), and neuropeptide Y and the related peptide YY, which have differential action at neuropeptide Y Y3 receptors, on amplitudes of contraction of adult rat ventricular cardiomyocytes were studied. Also, the effect of the neuropeptide Y Y1-selective antagonist, bis(31/31')[[Cys31, Trp32, Nva34] neuropeptide Y-(31-36)] on neuropeptide Y-mediated changes in myocyte contraction was investigated. Neuropeptide Y, peptide YY, neuropeptide Y-(13-36) and peptide YY-(3-36) attenuated the isoprenaline (10(-7) M)-stimulated contractile response, and the EC50 values were 9.0 x 10(-9), 4.3 x 10(-10), 3.1 x 10(-11) and 8.5 x 10(-11) M, respectively. [Leu31, Pro34] neuropeptide Y increased the contractile response of cardiomyocytes, and the EC50 values were 8.1 x 10(-9) and 1.5 x 10(-9) M, in the absence and presence of isoprenaline, respectively. Since [Leu31, Pro34] neuropeptide Y caused a positive effect on ventricular myocyte contraction and neuropeptide Y-(13-36) and peptide YY (3-36) produced the most potent negative effects, it is proposed that both neuropeptide Y Y1 and neuropeptide Y Y2 receptors, linked respectively to the positive and negative responses, are expressed in cardiomyocytes. The finding of receptors with neuropeptide Y Y2 characteristics on cardiomyocytes represents a further example of a postjunctional location for this subtype. As there was no significant discrepancy between the potencies of peptide YY and neuropeptide Y to attenuate the contractile response, it appears that neuropeptide Y Y3-like receptors are not linked principally to contractile function in rat cardiomyocytes. Bis(31/31')[[Cys31, Trp32, Nva34] neuropeptide Y-(31-36)] antagonised the neuropeptide Y-mediated stimulation of contractile activity through neuropeptide Y Y1 receptors, but the compound also inhibited the attenuation of isoprenaline-stimulated contraction, apparently by acting as a partial agonist at the neuropeptide Y Y2 receptors.

Use of D-myo inositol 1,2,6 trisphosphate to inhibit contractile activity in rat ventricular cardiomyocytes induced by neuropeptide Y and other cardioactive peptides through phospholipase C.

1. D-Myo inositol 1,2,6 trisphosphate (alpha-trinositol, pp56), an isomer of the second messenger substance, inositol 1,4,5 trisphosphate, has an interesting pharmacological profile that includes antagonism of a number of neuropeptide Y (NPY)-mediated cellular processes. The ability of pp56 to inhibit selectively the myocardial contraction mediated by NPY in relation to the responses to other cardioactive peptides, including endothelin-1, calcitonin gene-related peptide (CGRP), secretin and vasoactive intestinal peptide (VIP), was assessed. In order to investigate the possible interaction of pp56 with mechanisms of inositol phosphate signalling generated in heart muscle cells by activation of the beta-isoenzyme of phospholipase C (PLC beta), noradrenaline was used as a positive control, and isoprenaline and forskolin were included as negative controls. 2. Ventricular cardiomyocytes, isolated from the hearts of adult rats, were stimulated to contract at 0.5 Hz in the presence of calcium ion (2 mM). The concentrations of agonists used were in the region of their maximally effective concentrations for myocyte contraction and the concentration of pp56 was in the range of 1-100 microM. Contractile activity was monitored by video microscopy and maximum shortening determined by image analysis. 3. In the absence of agonist, contractile amplitudes following 20 min preincubation with pp56 were not different from that observed in the absence of pp56. Pp56 (1-100 microM) inhibited significantly the positive contractile response to noradrenaline (5 microM) in the presence of propranolol (500 nM), such that the response was almost completely attenuated at the highest concentration of the inhibitor. Pp56 did not inhibit the positive contractile responses to forskolin (40 microM) or isoprenaline (100 nM). 4. NPY alone does not influence the basal level of contraction of cardiomyocytes, but can attenuate isoprenaline-stimulated contraction and can increase contractile amplitude from basal when the transient outward current is blocked with 4-aminopyridine. In the presence of isoprenaline (100 nM), the negative response to NPY (100 nM) was attenuated significantly by pp56 (1-100 microM). With 4-aminopyridine, the positive contractile response to NPY (200 nM) was decreased by pp56, although this was not statistically significant. 5. Pp56 inhibited the positive contractile responses to CGRP (1 nM) and endothelin-1 (20 nM) completely, but did not affect the responses to secretin (20 nM) or VIP (20 nM). 6. In conclusion, these data challenge the previously obtained selectivity of pp56 as an antagonist of NPY-mediated cellular processes, since responses to CGRP and endothelin-1 were at least equally sensitive. Furthermore, as pp56 discriminated clearly in its inhibition of responses to alpha-adrenoceptor by comparison with beta-adrenoceptor/adenylate cyclase stimulation, it appears that pp56 may be a useful pharmacological agent with which to distinguish between PLC beta-dependent and PLC beta-independent coupling mechanisms. On this basis, further evidence has been obtained that, in rat cardiomyocytes, the contractile responses to NPY, CGRP and endothelin-1 are attributable to the activation of PLC beta-dependent pathways, whereas the responses to secretin and VIP are mediated by PLC beta-independent pathways.