Increase of Gi alpha in human hearts with dilated but not ischemic cardiomyopathy

In myocardial membranes from hearts with dilated cardiomyopathy (DCM), there was a 37% increase of the Gi alpha-protein as measured by 32P-ADP-ribosylation of a approximately 40 kDa pertussis toxin substrate. Immunoblotting techniques also showed increased amounts of Gi alpha in DCM. In hearts with ischemic cardiomyopathy (ICM), Gi alpha was not altered compared with nonfailing myocardium (NF). Basal and Gpp(NH)p-stimulated adenylate cyclase activity was reduced in DCM but not in ICM. The number of beta-adrenoceptors was similarly reduced both in DCM and ICM compared with NF. Alterations of m-cholinoceptors or A1-adenosine receptors did not occur. Consistently, "indirect" negative inotropic effects of the m-cholinoceptor agonist carbachol and the A1-adenosine receptor agonist R-PIA were not different in ICM, DCM, and nonfailing myocardium. In ICM and DCM, there was a marked reduction of the positive inotropic responses to isoprenaline and milrinone. However, there was a further reduction in DCM compared with ICM. It is concluded that the increase of Gi alpha is accompanied by a reduction of basal and guanine-nucleotide-stimulated adenylate cyclase activity. Alterations of m-cholinoceptors and A1-adenosine receptors do not appear to be involved. The further decrease of the positive inotropic effects of isoprenaline and milrinone in DCM provides evidence that the increase of Gi alpha is functionally relevant in DCM but not ICM and hence might contribute to the reduced effects of endogenous catecholamines and exogenous cAMP-dependent positive inotropic agents in the former but not the latter condition.

The effects of frusemide and probenecid on the pharmacokinetics of phenprocoumon

We have studied the pharmacokinetics of phenprocoumon with and without co-administration of frusemide and probenecid in two groups of 17 healthy volunteers. Frusemide 40 mg b.i.d. for 7 days did not interact with phenprocoumon to a significant extent. Probenecid 500 mg q.i.d. for 7 days significantly accelerated the overall elimination of phenprocoumon, as indicated by a decrease in AUC from 295 to 157 micrograms.h.ml-1, and a reduction in the fraction of the dose excreted by the kidneys. The data are consistent with inhibition of the glucuronidation of phenprocoumon by probenecid. Its accelerated elimination may be a consequence of the increased formation of hydroxylated metabolites.

Different negative inotropic activity of Ca2(+)-antagonists in human myocardial tissue

To evaluate the negative inotropic effect of various Ca2(+)-antagonists in human myocardium without additional influences of preload, afterload, or frequency, we examined their effects on isometric force of contraction in isolated human papillary muscle strips and in auricular trabeculae. The 1,4-dihydropyridines isradipine, nitrendipine, and nifedipine, the phenylalkylamine verapamil, and the benzothiazepine diltiazem exerted concentration-dependent negative inotropic effects. The potency of the investigated Ca2(+)-antagonists was identical in papillary muscle strips of patients with only moderate clinical signs of heart failure undergoing mitral valve replacement-operation (NYHA II-III) and in terminally failing (heart transplantation, NYHA IV) human hearts. The IC50 values were lower in auricular trabeculae than in papillary muscle strips. The difference was significant for nifedipine, nitrendipine, and verapamil. The restorative effects of external Ca2+ after pretreatment with Ca2(+)-antagonists were significantly less strong after pretreatment with 1,4-dihydropyridine than with non-dihydropyridines in papillary muscle strips. It is concluded that 1,4-dihydropyridines and verapamil and diltiazem did differently influence Ca2(+)-mediated increase in force of contraction. Moreover, a relation between the therapeutically active free plasma concentration in vivo and the negative inotropic potency in vitro can be found. This relation follows a rank order of potency for negative inotropism (isradipine less than or equal to nitrendipine less than diltiazem less than nifedipine less than verapamil) and might have clinical relevance in the treatment of patients with compromised cardiac function.

Beta 2-adrenoceptors in human lung and peripheral mononuclear leukocytes of untreated and terbutaline-treated patients

Beta 2-adrenoceptor agonists act against bronchoconstriction by stimulating beta 2-adrenoceptors in bronchial smooth muscle. However, tachyphylaxis has been argued to occur because of beta 2-adrenoceptor down-regulation following therapy with beta 2-adrenergic agents. To investigate receptor alterations, human peripheral mononuclear leukocytes are frequently used, since human lung tissue is not easily available. In order to study whether beta 2-adrenoceptors in MNL reliably reflect the conditions in the human lung tissue, we compared MNL and human lung tissue of 18 patients who had to undergo lung resection. Ten patients were untreated, and eight had bronchodilator therapy prior to therapy with terbutaline because of bronchoconstriction. Both in human lung and MNL, the beta 2-adrenoceptor subpopulation was characterized by competition experiments with the beta 1-selective antagonist CGP 207.12 A and the beta 2-selective antagonist ICI 118.551. In MNL, a significant decrease in the density of beta 2-adrenoceptors was found in treated but not in untreated patients, while the antagonist affinity of the beta 2-adrenoceptors remained unchanged. However, in lung parenchyma, which was obtained at the very same time from the same patients, no down-regulation of the total amount of beta 2-adrenoceptors could be measured. It is concluded that MNLs are a reliable model for studying properties of beta 2-adrenoceptor regulation. However, the hereby obtained results show that MNLs do not reflect the conditions of beta 2-adrenoceptors in human lung tissue. Human lung tissue is found to be less susceptible than human MNL for beta 2-adrenoceptor down-regulation by terbutaline treatment at therapeutic doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Antagonism of novel inotropic agents at A1 adenosine receptors and m-cholinoceptors in human myocardium

The effects of the new inotropic agents saterinone, sulmazole, UD-CG 212.Cl and milrinone at A1 adenosine receptors and m-cholinoceptors were evaluated in human myocardium from patients with heart failure. At A1 adenosine receptors, all compounds inhibited 3H-DPCPX-binding to ventricular membrane preparations at micromolar concentrations. As judged from the K1-values, the rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. The new inotropic agents also displaced the binding of 3H-QNB at m-cholinoceptors. Except for saterinone, the concentration ranges of mean Ki-values were considerably higher at m-cholinoceptors than at A1 adenosine receptors. The rank order of potency was saterinone greater than sulmazole greater than UD-CG 212.Cl greater than milrinone. Competition of the A1 adenosine receptor agonist R-PIA to 3H-DPCPX-binding showed a biphasic curve with a shallow slope (Hill coefficient nH = 0.63) and revealed two affinity states of the A1 adenosine receptor. In the presence of guanine nucleotides [Gpp(NH)p], the competition curve showed one low affinity class of binding sites and was shifted to the right. In contrast, the competition curves of the new inotropic agents were characterized by a monophasic, steeper slope (mean Hill coefficient nH = 0.98). Guanine nucleotides had no effect. Similar results were obtained with saterinone and carbachol at m-cholinoceptors. Competition with carbachol revealed three affinity states of the m-cholinoceptor, the super-high affinity binding was reversed by Gpp(NH)p. Competition with saterinone revealed one class of binding sites which was not influenced by Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative inotropic properties of isradipine, nifedipine, diltiazem, and verapamil in diseased human myocardial tissue

We investigated the inotropic responses to Ca2+ antagonists using electrically driven human papillary muscle strips and human auricular trabeculae. Specimens were obtained during cardiac surgery for mitral valve replacement [New York Heart Association (NYHA) Class II-III] or heart transplantation (NYHA IV) and during aortocoronary bypass operations. The inotropic effects were studied with cumulative concentration-response curves. All Ca2+ antagonists tested significantly (p less than 0.05) depressed force of contraction at concentrations above 0.01 mumol/L, but their potencies were different. A 50% reduction of the initial force of contraction occurred at the following concentrations (NYHA II-III): nifedipine (mean IC50) 0.09 mumol/L isradipine 0.12 mumol/L, diltiazem 0.69 mumol/L, and verapamil 0.79 mumol/L. There were no significant differences in the negative inotropic effects of any tested Ca2+ antagonist between NYHA II-III and NYHA IV. When the initial force of contraction was reduced by 90%, addition of Ca2+ increased force of contraction significantly less after diltiazem (2.76 +/- 0.4 mN), isradipine (1.82 +/- 0.23 mN), and nifedipine (1.68 +/- 0.25 mN) compared to control (4.63 +/- 0.56 mN) (NYHA II-III). The negative inotropic potencies of nifedipine and verapamil were significantly greater in human auricular trabeculae compared to papillary muscle strips (p less than 0.05). However, on the relation between therapeutic vasoactive plasma concentrations and IC25 values, an entirely different rank order of potential negative inotropism could be observed: verapamil greater than nifedipine greater than diltiazem greater than isradipine.

Myocardial beta-adrenoceptors and inhibitory G-proteins in myocardial biopsies and in explanted hearts from patients with dilated cardiomyopathy

Cardiac beta-adrenoceptors and the alpha-subunits of the inhibitory guanine-nucleotide binding proteins (Gi alpha) were determined in myocardial biopsies and in larger samples of explanted hearts from patients with dilated cardiomyopathy. The myocardium of one non-failing heart was studied for comparison. There were fewer beta-adrenoceptors in right and left ventricular samples from hearts with dilated cardiomyopathy than in the non-failing myocardium. The number of beta-adrenoceptors determined by equilibrium saturation experiments closely correlated with their density as measured by a saturating concentration of the radioligand 125Iodocyanopindolol in myocardial biopsies. Using the 32P-ADP-ribosylation of a 40kDa membrane protein catalyzed by pertussis toxin, an increase of Gi alpha could also be shown in both myocardial biopsies and explanted hearts.

Effectiveness of cardiac glycosides in human myocardium with and without "downregulated" beta-adrenoceptors

We investigated the "receptor-effector-coupling" in the beta-adrenoceptor- and the Na+, K(+)-ATPase-mediated systems in nonfailing hearts and terminally failing human myocardium from patients with cardiomyopathy. The density of beta-adrenoceptors in the failing human myocardium was significantly (p less than 0.01) lower as compared with nonfailing hearts, whereas the receptor density and affinity measured by [3H]ouabain binding (cardiac glycoside receptor) was not different in either group. The maximal inotropic response to isoprenaline was significantly reduced in papillary muscle strips from failing human hearts (2.1 +/- 0.5 mN) as compared with control hearts (8.0 +/- 1.0 mN; p less than 0.05). Ouabain remained effective in both groups (6.8 +/- 1.0 vs. 5.5 +/- 0.6 mN; NS). The positive inotropic response due to extracellular Ca2+ elevation (1.8-15 mM) was studied for comparison. Maximal Ca2+ effects were reduced by 30% in failing human myocardium (7.2 +/- 0.5 mN vs. 5.1 +/- 0.8 mN, p less than 0.05). Ouabain had effectiveness (95%) similar to that of Ca2+ in nonfailing and failing human cardiac muscle. It is concluded that treatment with cardiac glycosides may still be effective in end-stage heart failure with "downregulated" beta-adrenoceptors, as judged from these in vitro studies.

Evidence against spare or uncoupled beta-adrenoceptors in the human heart

It is well established that increasing degrees of heart failure are accompanied by a reduced density of myocardial beta-adrenoceptors. It is unclear, however, whether all beta-adrenoceptors in the cardiac cell membrane are coupled to the effector system or whether "spare receptors" or "uncoupled" beta-adrenoceptors also exist. To investigate this, we measured the density of beta-adrenoceptors and the positive inotropic response to isoprenaline in preparations from the same human hearts. The myocardium from nonfailing hearts had significantly (p less than 0.01) higher numbers of beta-adrenoceptors (104 +/- 7 fmol/mg protein) compared with tissue from moderately (mitral valve disease, New York Heart Association [NYHA] class II to III, 60 +/- 2.8 fmol/mg protein) and terminally (dilated cardiomyopathy, NYHA class IV, 35 +/- 2.7 fmol/mg protein) failing human hearts. The KD values of the drug-receptor complexes did not differ within the different patient groups. There was a linear relationship (r = 0.97) between the beta-adrenoceptor density measured and the maximally obtainable positive inotropic effect elicited by isoprenaline in the three groups tested. Thus there seem to be no spare beta-adrenoceptors, that is, receptors not required for the production of the maximal inotropic response in the left ventricular human myocardium, and there are no uncoupled beta-adrenoceptors. The beta-adrenoceptors associated with the plasma membrane (marker: 3H-ouabain binding sites) remained functionally active. In addition, these results indicate that either there is no amplifier system behind the receptor level or it remains unchanged in the failing left ventricular human myocardium under the conditions tested.

The effect of xamoterol in failing human myocardium

As the dual pharmacological action of partial beta 1-adrenoceptor agonists should improve left ventricular function while also protecting the myocardium against excessive sympathetic stimulation they may be useful in the treatment of heart failure. Therefore, we studied the pharmacological effects of xamoterol (Corwin, ICI 118, 587), a compound with mixed agonist and antagonistic properties at cardiac beta-adrenoceptors in electrically driven human papillary muscle strips from failing human hearts. Specimens were obtained from patients with different grades of myocardial failure who underwent mitral valve replacement (NYHA II-III) or heart transplantation (NYHA IV). Xamoterol (0.0001-100 mumol l-1) produced only negative inotropic effects, as measured by changes in isometric force of contraction in diseased human papillary muscle strips. However, isoprenaline (0.0001-10 mumol l-1) and ouabain (0.01-0.3 mumol l-1) enhanced force of contraction in the same hearts. Prestimulation with noradrenaline (3 mumol l-1) augmented the negative inotropic effect of xamoterol. It is concluded that xamoterol exerts primarily beta-adrenoceptor antagonistic activity in the failing human myocardium.

[2 year interferon therapy of metastatic carcinoid tumor]

A 39 year old male patient was treated with daily 5 X 10(6) IU interferon alpha 2b s.c. for 22 months because of advanced carcinoid tumor. Objective response was achieved with greater than 50% reduction of 5-HIAA-excretion. Multiple metastases (liver, lung, bones, thyroid gland) were not progressing in size. An unintentional omission of treatment resulted in a rapid biochemical and morphological tumor progression. Reversibility was achieved with reinstitution of effective interferon therapy. Thus, interferon therapy of advanced carcinoid tumor is able to induce a long-term objective response. It seems to be superior to conventional chemotherapy.

Improvement of cholera toxin-catalyzed ADP-ribosylation by endogenous ADP-ribosylation factor from bovine brain provides evidence for an unchanged amount of Gs alpha in failing human myocardium

The aim of the present study was to investigate whether or not alterations of Gs alpha can be detected with cholera toxin-induced ADP-ribosylation in myocardial membranes from patients with heart failure. Therefore, Gs alpha was radiolabeled by cholera toxin-catalzyed (32P)ADP-ribosylation with (32P)NAD as substrate. In membranes from left ventricular myocardium of six patients with dilated cardiomyopathy classified as NYHA IV and three samples from two non-failing donor hearts, labeling was too weak to allow detection of possible changes in the amount of Gs alpha. Therefore, the cytosolic small molecular weight G protein ARF (ADP-ribosylation factor), a cofactor for cholera toxin-induced ADP-ribosylation of Gs alpha, was partially purified from bovine cerebral cortex. ARF activity was quantified by its ability to enhance auto-ADP-ribosylation of cholera toxin A1-subunit. Gs alpha was identified by comparing the ADP-ribosylation patterns of myocardial membranes, membranes prepared from human leukemia (HL 60) and S 49 mouse lymphoma wild type cells (45 kDa-band present) with membranes of the Gs alpha-deficient S 49 variant cyc- (45 kDa-band missing). In the presence of ARF, specific radiolabeling of the Mr 45,000 subtype of Gs alpha was markedly enhanced. The amounts of Gs alpha as measured by cholera toxin-dependent (32P)-ADP-ribosylation in the presence of ARR were similar in failing and nonfailing human hearts. It is concluded that factors other than Gs alpha are responsible for the altered regulation of the adenylate cyclase complex in heart failure. Moreover, by enhancing cholera toxin-catalyzed ADP-ribosylation, endogenous ADP-ribosylation factor from bovine brain appears to be a useful tool to study Gs alpha even in tissues in which the labeling of Gs alpha is rather weak.

Positive inotropic stimulation in the normal and insufficient human myocardium

Cardiac alpha- and beta-adrenoceptors and the positive inotropic effects of several adenylate cyclase dependent and independent agents have been measured in papillary muscle strips from patients without, as well as with moderate and severe heart failure. The number of beta-adrenoceptors was found to be decreased depending on the degree of heart failure. This does not apply to alpha-adrenoceptors, which remain unchanged. The antagonist affinity of adrenoceptors for the different ligands did not change in heart failure. Maximal increases in force of contraction were measured after raising Ca++ up to 15 mM in the muscle strips. In healthy human myocardium, isoprenaline, dobutamine, IBMX or cardiac glycosides increase force of contraction to the same maximal values as Ca++ does. However, in cardiac tissue from heart failure patients, positive inotropic agents which increase intracellular cAMP or are cAMP-dependent are less effective than Ca++. Furthermore, the results seem to indicate a homologous (agonist specific) downregulation of receptors in moderate heart failure and a heterologous downregulation in severe heart failure. Thus, many well known positive inotropic drugs lose their effectiveness just when they are needed most: in severe heart failure.

Characterization of A1 adenosine receptors in atrial and ventricular myocardium from diseased human hearts

The purpose of the present study was to characterize adenosine receptors in human atrial and ventricular myocardium. In isolated electrically driven preparations, adenosine produced "direct" negative inotropic effects in atrial myocardium (AT). In ventricular myocardium (VE), it only had negative inotropic properties when force of contraction had been stimulated with isoprenaline ("indirect" effect), but it has no inotropic effect alone. The adenosine receptor antagonist 8-phenyltheophylline antagonized the "direct" and "indirect" effects; these findings indicated that both effects were mediated by adenosine receptors. In cardiac membranes from human AT and VE, adenosine receptors were characterized with [3H]-8-cyclopentyl-1,3-dipropylxanthine (DPCPX) binding. The effects of agonists R-(-)-N6-phenylisopropyladenosine (R-PIA), S-(+)-N6-phenylisopropyladenosine (S-PIA), and 5'-(N-ethylcarboxamido) adenosine (NECA) and the effects of guanine nucleotides [Gpp(NH)p] were studied also. The antagonist affinities as judged from the apparent affinity, Kd, of [3H]DPCPX were similar in AT (2.2 nmol/l; 95% confidence limits, 1.4-3.7) and VE (1.8 nmol/l; 95% confidence limits, 1.0-3.0). The number of adenosine receptors was 1.7 times greater in AT (26.9 +/- 2.33 fmol/mg protein; n = 5) than in VE (16.2 +/- 2.3 fmol/mg protein; n = 5). High and low affinity states of adenosine receptors evaluated with the influence of Gpp(NH)p on agonist competition with R-PIA were similar in AT or VE. The rank orders of potency for agonists (R-PIA greater than S-PIA greater than NECA) and antagonists (DPCPX greater than 8-phenyltheophylline greater than theophylline) were characteristic for the A1 receptor subtype. It is concluded that A1 adenosine receptors exist in the human myocardium. Since binding properties were similar in AT and VE, the same A1 adenosine receptor probably couples to different effectors in a similar guanine nucleotide-dependent way. [3H]DPCPX is the first radiolabeled antagonist ligand that allows detection of A1 adenosine receptors and their coupling in the human myocardium.

Coupling of adenosine receptors to a pertussis toxin-sensitive G protein in the human heart

The adenosine receptors in myocardial membranes of human explanted hearts were labeled with the radiolabeled ligand [3H]DPCPX (1,3-dipropyl-8-cyclopentylxanthine). Agonist competition curves revealed high- and low-affinity states. The addition of guanylyl imidodiphosphate (Gpp (NH)p) converted all receptors to a low-affinity state. The portion of high-affinity states and the influence of guanine nucleotides were most pronounced at 22 degrees C. Only low-affinity states could be detected in pertussis toxin-treated membranes. It is concluded that adenosine receptors in the human ventricle couple via a G protein sensitive to pertussis-toxin. Alterations of the coupling of adenosine receptors might have a pathophysiological role in dilated cardiomyopathy in which pertussis toxin substrates are increased.

Reduced effects of dopexamine on force of contraction in the failing human heart despite preserved beta 2-adrenoceptor subpopulation

The results of the present study show that the reduction of the total number of beta adrenoceptors affected the beta 1-adrenoceptor subpopulation, whereas the beta 2 adrenoceptors were not detectably altered in the failing heart. Dopexamine had a 9.8-fold greater affinity to beta 2 adrenoceptors than to beta 1 adrenoceptors. In nonfailing myocardium, dopexamine increased force of contraction concentration-dependently. However, dopexamine alone had no effect in papillary muscle strips from moderately (NYHA II-III) and severely (NYHA IV) failing myocardium. However, in the presence of milrinone, it concentration-dependently increased force of contraction. Under this condition, the effectiveness was slightly less pronounced in NYHA IV than in NYHA II-III. Dopexamine concentration-dependently stimulated adenylate cyclase activity. Experiments with the beta 1-selective antagonist CGP 207.12 A and the beta 2-selective antagonist ICI 118.551 showed that both stimulation of adenylate cyclase and the increase of force of contraction are mediated by beta 2 adrenoceptors. It is concluded that although the number of beta 2 adrenoceptors is preserved in the failing myocardium, dopexamine alone does not increase force of contraction. However, the positive inotropic effect of dopexamine, which is observed in the presence of milrinone and the stimulation of adenylate cyclase activity by dopexamine are mediated by beta 2 adrenoceptors. Therefore, beta 2 adrenoceptors exist in the human myocardium, are coupled to adenylate cyclase, and are capable of increasing force of contraction. These results provide evidence for an impaired coupling of beta 2 adrenoceptors to mechanisms beyond receptor occupation in terminal heart failure.

Inotropic response to DPI 201-106 in the failing human heart

1. The present study was designed to characterize the positive inotropic response to DPI 201-106 in isolated papillary muscle strips obtained from heart failure patients undergoing surgery. 2. The positive inotropic responses to isoprenaline and milrinone and cardiac beta-adrenoceptor density were also determined. 3. DPI 201-106 increased the force of contraction in papillary muscle strips from patients with moderate (NYHA II-III) and severe (NYHA IV) heart failure, in a concentration-dependent manner. This positive inotropic effect was more pronounced in tissues from NYHA IV patients. Furthermore, these responses were greater than those produced by milrinone or isoprenaline. The positive inotropic effects of isoprenaline and milrinone were reduced in NYHA IV compared to NYHA II-III. Consistently, there was also a smaller density of beta-adrenoceptors in myocardium from NYHA IV than in NYHA II-III. The positive inotropic effect of Ca2+ was similar in tissues from both groups of patients. 4. The positive inotropic effect of DPI 201-106 was not antagonized by adenosine or carbachol, whereas both compounds reduced the positive inotropic effect of isoprenaline. 5. DPI 201-106 did not increase the Ca2+ -sensitivity of chemically skinned ventricular fibres, whereas a significant increase of the Ca2+ -sensitivity was obtained with trifluoperazine. 6. It is concluded that DPI 201-106 produces significant positive inotropic effects in tissue excised from the failing human heart. The lack of inhibition by adenosine and carbachol might contribute to its greater effectiveness in NYHA IV than NYHA II-III and indicates that its mechanism of action is cyclic AMP-independent. A sensitization of the contractile proteins to Ca2+ does not appear to be important for the positive inotropic action of DPI 201-106 in the failing human heart.

Increased number of cardiac adrenergic receptors following local heart irradiation

The effect of local X irradiation on cardiac alpha and beta receptors was studied in Wistar rats. Animals were given local heart irradiation with single doses of 15 or 20 Gy and were examined after a range of latency times of 7 to 400 days. Using the radioactive ligands [3H]CGP-12177 and [3H]prazosin, the maximal binding capacity was determined from saturation experiments. At 7 days after 20 Gy the maximal binding capacity of both alpha and beta receptors was reduced to below the level of untreated control animals. Subsequently it rose continually to a maximum of 160% of the control level for beta receptors and 130% for alpha receptors at 400 days postirradiation. The antagonist affinity as judged from the dissociation constant for [3H]CGP 12177 and [3H]prazosin did not change significantly. A similar effect was observed after 15 Gy. An increase in adrenergic receptors may represent an important pathogenetic link between early morphological and late functional changes in the pathogenesis of radiation-induced heart disease.

Myosin P-light chain isoenzymes in the human heart: evidence for diphosphorylation of the atrial P-LC form

We studied myosin light chains (LC) of human atrium and ventricle of normal and diseased individuals by a high-resolution 2-dimensional polyacrylamide gel electrophoresis (2D-PAGE) technique. Atrial LCs (ALC-1, ALC-2 (= P-LC)) revealed both higher molecular weights and lower isoelectric points (IEP) than their ventricular counterparts (VLC-1, VLC-2 (= P-LC)). Different P-LC forms with their distinct myosin isoenzymes have been designated as P-LC-polymorphism and myosin P-LC isoenzymes, respectively. In the dephosphorylated state two VLC-2 forms (VLC-2 and VLC-2*) with the same MW and different IEP, but only one ALC-2 form, were found. In the partially phosphorylated state ALC-2 appeared to be single- and double-phosphorylated (three spots in the 2D-PAGE), whereas the two VLC-2 forms appeared to be single-phosphorylated each (four spots in the 2D-PAGE). Phosphoryl-transfer from ATP to the P-LC forms was studied using skinned fibers incubated with MLCK (myosin light chain kinase) and (gamma-32P)ATP. Ventricular myosin P-LC isoenzyme pattern was usually the same in normal and diseased patients: the VLC-2 to VLC-2* ratio was approx. 70/30, but in one patient with valvular heart disease (VHD) the relation was 55/45 (shift to the VLC-2* form). In hypertrophied atria of VHD patients a shift of the myosin P-LC isoenzyme pattern to the VLC-2* form occurred, too.

Screening of positive inotropic agents in isolated cardiac preparations from different sources

The present study was designed to elucidate whether or not the positive inotropic effects of isoprenaline, milrinone, and DPI 201-106 in myocardial tissue from various sources reveal reliable results for the failing human heart. Therefore, in vitro positive inotropic responses were studied in human papillary muscle strips from patients with moderate heart failure (NNYHA II-III), human atrial trabeculae (HAT), isolated papillary muscles from Wistar-Kyoto rats (WK), and from spontaneously hypertensive rats (SHR). Results were compared with the effects of the compounds in papillary muscle strips from patients with severe heart failure (NYHA IV). In NYHA IV, positive inotropic responses were smaller for isoprenaline and milrinone than in NYHA II-III. The response to DPI 201-106 was more pronounced. In HAT, the effects of isoprenaline and milrinone were greater than in NYHA IV. The positive inotropic effect of DPI 201-106 was similar. In SHR, only the positive inotropic effect of isoprenaline was smaller than in WK. The effects of DPI 201-106 and milrinone did not differ. These data show that inotropic responses in NYHA II-III, HAT, WK, and SHR differ from the severely failing myocardium. It is concluded that new positive inotropic agents should be screened in human myocardial tissue from patients with heart failure, as experimental results from other sources may be irrelevant in this respect.

Pertussis toxin prevents adenosine receptor- and m-cholinoceptor-mediated sinus rate slowing and AV conduction block in the guinea-pig heart

The influence of pertussis toxin on the effects of adenosine, the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) and the m-cholinoceptor agonist carbachol on heart rate and atrioventricular (AV) conduction was investigated in spontaneously beating isolated perfused guinea-pig hearts. In addition, the effects of the agents on the electrocardiogram recorded from anesthetized guinea pigs were studied. Adenosine (0.1-100 mumol/l) and PIA (0.001-100 mumol/l) had concentration-dependent negative chronotropic and negative dromotropic effects. These effects were prevented by pretreatment of the animals with pertussis toxin (150 micrograms/kg; i.v.). Carbachol (0.001-100 mumol/l) had similar cardiac depressant effects. These effects were also abolished by pertussis toxin. In contrast, the negative chronotropic and negative dromotropic effects of the calcium antagonist verapamil which was investigated for comparison were not influenced by pretreatment with pertussis toxin. Since the cardiac depressant effects mediated via adenosine receptors or via m-cholinoceptors are most probably due to an activation of a K+ conductance, it is concluded that both receptors in the sinus node and in the AV node may be coupled via a common pertussis toxin-sensitive guanine nucleotide-binding protein to the K+ channel. It remains to be elucidated whether an additional inhibitory coupling to Ca2+ channels also plays a role.

A phase-II study of low-dose cyclophosphamide and recombinant human interleukin-2 in metastatic renal cell carcinoma and malignant melanoma

Recent preclinical and clinical studies that have demonstrated antitumor activity of high-dose recombinant interleukin-2 (rIL-2), and animal models that demonstrated a synergistic effect of low-dose cyclophosphamide, led us to study rIL-2 (Cetus Corp., Emeryville, Calif) in a phase II clinical trial in combination with low-dose cyclophosphamide in 32 patients, 18 with malignant melanoma and 14 with renal cell carcinoma. rIL-2 was given once daily at 3 x 10(6) U/m2, as a 30-min infusion for 14 days in cycle I and for 2 x 5 days in cycles II and III respectively; if tolerated, the dose was increased to a maximum of 6 x 10(6) U m-2 day-1; the cycles, separated by 1 week treatment-free intervals, were preceded each by a single i.v. bolus of cyclophosphamide at 350 mg/m2. The most prominent side-effects encountered in this trial consisted of a capillary leak syndrome, myalgia and fever that required dose reduction during the first cycle in one-half of the patients. Given the limit of tolerable toxicities in a standard care unit, the regimen employed achieved minor antitumor activity. No remission was achieved in patients with renal cell carcinoma, and 15% of melanoma patients showed objective responses (partial response + minor response).

Subsensitivity of the failing human heart to isoprenaline and milrinone is related to beta-adrenoceptor downregulation

The number of cardiac beta-adrenoceptors and the positive inotropic effect of isoprenaline and milrinone were measured in cardiac membranes and isolated, electrically driven muscle strips from nonfailing donor hearts and from patients with mitral valve disease (NYHA II-III), ischemic heart disease, and dilated cardiomyopathy (NYHA IV). In nonfailing hearts, the number of beta-adrenoceptors were 41.5 fmol/mg protein (mean, n = 3). In ischemic heart disease and NYHA II-III, there was a loss of cardiac beta-adrenoceptors (22.1 fmol/mg protein, mean, n = 3; 23.2 +/- 2.7 fmol/mg protein, n = 30), respectively. In NYHA IV, there was a pronounced reduction of the number of cardiac beta-adrenoceptors to 12.1 +/- 1.5 fmol/mg protein (n = 15). The Kd value did not differ in either group. Correspondingly, the positive inotropic effect of isoprenaline was more pronounced in nonfailing myocardium, reduced in NYHA II-III and ischemic heart disease and almost blunted in NYHA IV. Similar results were observed with the phosphodiesterase inhibitor milrinone. A good correlation of the beta-adrenoceptor density to the maximal positive inotropic effect of isoprenaline and milrinone was observed. Neither the number of cardiac beta-adrenoceptors nor the positive inotropic effect of isoprenaline correlated with the age of the patients. We conclude that the number of cardiac beta-adrenoceptors and the positive inotropic effect of beta-adrenoceptor agonists are reduced in the failing human heart depending on the severity of heart failure. Furthermore, the positive inotropic effect of milrinone is also reduced and related to the reduction of beta-adrenoceptors. The lack of correlation with the age of the patients provides evidence for a predominant role of heart disease rather than aging in the reduction of beta-adrenoceptors and subsensitivity to cyclic AMP-increasing positive inotropic agents in the failing human heart.

Positive inotropic effects in isolated ventricular myocardium from non-failing and terminally failing human hearts

The positive inotropic responses to isoprenaline, dobutamine, histamine, forskolin, isobutyl-methylxanthine (IBMX), dibutyryl-cyclic adenosine monophosphate (db-cAMP), ouabain and calcium were studied in isolated, electrically driven papillary muscle strips from either terminally failing human hearts or non-failing donor myocardium. The positive inotropic effect of calcium has been taken to evaluate the maximal force of contraction of each individual muscle strip ('contractile reserve'). In the non-failing heart, the maximal positive inotropic effect of isoprenaline, dobutamine, IBMX, ouabain and calcium were not significantly different, but were significantly greater than histamine. In terminally failing hearts, the positive inotropic effects of agents stimulating the adenylate cyclase by a receptor-dependent mechanism (isoprenaline, dobutamine and histamine) and the phosphodiesterase inhibitor IBMX are less than in the normal heart. Furthermore, these compounds gave a markedly reduced inotropic effect compared with forskolin, db-cAMP and ouabain, which gave maximal responses similar to calcium in the failing hearts. The data did not differ when the increase of force of contraction was related to the diameter of each preparation. These results indicate that a defect in adenylate cyclase occurs in the failing human heart, presumably located at the regulatory stimulatory subunit (Gs) of the adenylate cyclase since effects through stimulatory receptors were reduced. Responses from activation of the catalytic subunit or through cAMP-dependent protein kinases were less affected. Since the positive inotropic effect of IBMX is also impaired, it is suggested that the basal rate of cAMP production is also reduced in heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of the effectiveness of milrinone to increase force of contraction by stimulation of cardiac beta-adrenoceptors in the failing human heart

The positive inotropic effect of milrinone was investigated in isolated, electrically driven (1 Hz) human papillary muscle strips from nonfailing myocardium (control) and from patients with moderate (NYHA II-III) and severe (NYHA IV) heart failure. In the control hearts, milrinone increased force of contraction to about the same degree as Ca2+ at 15 mmol/l. In NYHA II-III, the positive inotropic effect was significantly reduced compared with the control hearts. In NYHA IV, the effectiveness (maximal increase in force of contraction) and potency (as judged from the EC50 values) were significantly less than in NYHA II-III and controls. Preexposure of the preparations to isoprenaline in NYHA II-III and NYHA IV shifted the concentration-response curve for milrinone to the left and, moreover, restored the effectiveness of the compound (i.e., similar positive inotropic effect as Ca2+). No difference in the cAMP-phosphodiesterase inhibition by milrinone could be detected between controls, NYHA II-III, and NYHA IV. It is concluded that diminished basal cAMP production is responsible for reducing the effectiveness of milrinone in the failing human heart. A diminished inhibition of cAMP-phosphodiesterase does not play a role. The fact that stimulation of cardiac beta-adrenoceptors increased the effectiveness of milrinone provides evidence that combined application of catecholamines and phosphodiesterase inhibitors may be useful in the treatment of patients with terminal heart failure.

Properties of cardiac alpha- and beta-adrenoceptors in spontaneously hypertensive rats

The effects of isoprenaline, Ca2+ and phenylephrine (in the presence of propranolol) on force of contraction were studied in isolated electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and age-matched (14-18 weeks) Wistar Kyoto control rats (WK). Cardiac alpha- and beta-adrenoceptors were characterized by radioligand binding studies. The positive inotropic effect of isoprenaline in SHR was less effective than in control rats. The EC50 values did not differ in both groups. In SHR, isoprenaline was less effective than Ca2+ to increase force of contraction whereas in WK it had the same effectiveness as Ca2+. The positive inotropic effect of phenylephrine in the presence of propranolol was similar in SHR and WK. In SHR, both the densities of cardiac alpha- and beta-adrenoceptors were reduced. In beta-adrenoceptor binding experiments, the nonhydrolysable GTP analog Gpp(NH)p caused a rightward shift of agonist competition curves of isoprenaline. Biphasic competition curves revealed a similar percentage of low and high affinity sites in SHR and WK, respectively. In alpha-adrenoceptor binding experiments, Gpp(NH)p caused no detectable shift of agonist competition curves with norepinephrine. It is suggested that cardiac beta-adrenoceptor down-regulation is involved in the reduced positive inotropic effect of isoprenaline in SHR. Functional uncoupling of beta-adrenoceptors does not appear to be involved in the reduced beta-adrenoceptor-mediated positive inotropism in SHR. Binding studies do not show evidence for a large number of alpha-adrenoceptors coupling to a guanine-nucleotide binding protein in the rat heart. Finally, in ventricular myocardium of SHR, cardiac alpha-adrenoceptors do not serve as a reserve mechanism during impaired beta-adrenergic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-adrenoceptors and alpha-adrenoceptor-mediated positive inotropic effects in failing human myocardium

Experiments were designed to characterize cardiac alpha-adrenoceptors and the alpha-adrenoceptor-mediated positive inotropic effects in human myocardial tissue from patients with moderate New York Heart Association (NYHA) class II-III and severe (NYHA class IV) heart failure. The number of cardiac alpha-adrenoceptors was low but similar in moderate and severe heart failure (NYHA class II-III: 6.7 +/- 0.8 fmol/mg protein 3H-prazosin bound, n = 12; NYHA class IV: 7.4 +/- 0.9 fmol/mg protein 3H-prazosin bound, n = 9; NS). Correspondingly, the alpha-adrenoceptor-mediated positive inotropic effect (phenylephrine in the presence of propranolol) did not significantly differ in both groups. In the same hearts, the number of beta-adrenoceptors was measured. The number of beta-adrenoceptors was significantly reduced in severe heart failure (NYHA class II-III: 22.0 +/- 1.5 fmol/mg protein 3H-CGP 12177 bound, n = 12; NYHA class IV: 11.9 +/- 0.8 fmol/mg protein 3H-CGP 12177 bound, n = 9; p less than 0.05). The positive inotropic effect of isoprenaline was significantly reduced in NYHA class IV. The positive inotropic effect of Ca2+ was similar in both groups. In conclusion, cardiac beta-adrenoceptors and the beta-adrenoceptor-mediated positive inotropic effects were reduced in severely failing myocardium. Cardiac alpha-adrenoceptors and the positive inotropic effect resulting from their stimulation is unchanged. Therefore, down regulation in response to increased sympathetic stimulation or a compensatory increase of alpha-adrenoceptors does obviously not occur in the human heart.(ABSTRACT TRUNCATED AT 250 WORDS)

Reduction of beta-adrenoceptor density and evaluation of positive inotropic responses in isolated, diseased human myocardium

Cardiac beta-adrenoceptors and the positive inotropic effects of adenylate cyclase-dependent (dobutamine, histamine, forskolin) and adenylate cyclase-independent agents (isobutylmethylxanthine (IBMX), dibutyryl-cAMP (db-cAMP), digoxin, digitoxin and calcium were measured in papillary muscle strips from severely failing (NYHA IV), moderately failing (NYHA II-III) and non-failing (NYHA I) human hearts. The density of beta-adrenoceptors in three NYHA I patients were 40.0, 42.0 and 42.9 fmol mg-1 protein. The density of cardiac beta-adrenoceptors was significantly reduced in NYHA II-III to 18.0 +/- 1.1 fmol mg-1 protein (n = 16) and further reduced in NYHA IV to 9.5 +/- 1.6 fmol mg-1 protein (n = 7). The KD values did not differ between the groups. Correspondingly, the positive inotropic effect of dobutamine was significantly reduced in NYHA II-III and almost lost in NYHA IV. The positive inotropic effect of histamine was similar in non-failing and moderately failing myocardium but reduced in preparations from severely failing hearts (NYHA IV). The positive inotropic effect of IBMX was diminished in moderately and severely failing myocardium depending on the functional class of heart failure. In contrast, the effects of forskolin, db-cAMP, digoxin and digitoxin were not impaired in NYHA IV when compared with the maximal positive inotropic effect of calcium. It is concluded that in the failing human heart (a) the number of cardiac beta-adrenoceptors is reduced proportional to the severity of heart failure; (b) the receptor coupling of H2-receptors to adenylate cyclase may be impaired, but only in severe heart failure; (c) the basal cAMP formation may be diminished; and that (d) the catalytic subunit of the adenylate cyclase and the cAMP-dependent protein kinases may be promising targets for drugs to restore force of contraction in human heart failure.

Adenosine inhibits the positive inotropic effect of 3-isobutyl-1-methylxanthine in papillary muscles without effect on cyclic AMP or cyclic GMP

1 Adenosine and the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) produced a small positive and negative inotropic effect, respectively, in isolated electrically driven papillary muscles of guinea-pigs. 2 Adenosine (100 mumol l-1) had no effect on cyclic AMP or cyclic GMP content. PIA (100 mumol l-1) slightly increased cyclic AMP. 3 In the presence of 3-isobutyl-1-methylxanthine (IBMX; 60 mumol l-1), which increased force of contraction 2 fold, adenosine and PIA exerted strong negative inotropic effects. PIA was more potent than adenosine (mean IC25 2.1 and 168 mumol -1, respectively). 4 In contrast, the nucleosides did not affect the increase in force of contraction produced by elevating extracellular Ca2+ concentration. 5 The IBMX-antagonistic effects of adenosine and PIA were not accompanied by modification of the IBMX-induced increase in cyclic AMP and cyclic GMP. 6 The effects of adenosine and PIA on force of contraction were accompanied by a partial reversal of the IBMX-induced increase in the maximal rate of depolarization of slow action potentials. 7 It is concluded that adenosine and PIA are able to attenuate the positive inotropic effect of a phosphodiesterase inhibitor. This effect is unlikely to be due to a reduction of the IBMX-induced increase in cyclic AMP content. It is conceivably due to an inhibition of the stimulant action of cyclic AMP on slow Ca2+ channels leading to the reduction of the slow inward current which in turn reduces force of contraction.

Positive inotropic effect of DPI 201-106 in spontaneously hypertensive rats: lack of inhibition by adenosine and cholinergic agents

The positive inotropic effect of DPI 201-106 has been studied in isolated, electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WK). The positive inotropic effect of isoprenaline (Iso) and Ca2+ was studied for comparison. The maximal positive inotropic effect of Iso was less in SHR than in WK. In SHR, DPI 201-106 more effectively increased force of contraction than Iso. The positive inotropic effects of DPI 201-106 and Ca2+ were not different in SHR and WK. The EC50 values of DPI 201-106, Iso, and Ca2+ did not differ in either group. Furthermore, adenosine and carbachol reduced the positive inotropic effect of Iso but failed to exert a negative inotropic action when force of contraction has been increased with DPI 201-106. The isoprenaline-antagonistic effect of adenosine and carbachol was not different in SHR and WK. We conclude that DPI 201-106 might be an effective positive inotropic agent in states in which adrenergic function is compromised. The lack of inhibition by adenosine or m-cholinoceptor agonists could contribute to the effectiveness of the compound to increase myocardial force of contraction.

Inhibition of the effects of adenosine on force of contraction and the slow calcium inward current by pertussis toxin is associated with myocardial lesions

The effects of adenosine and the adenosine receptor agonist (-)-N(6)-phenyl-isopropyladenosine (PIA) in the presence of isoprenaline on isometric force of contraction and calcium dependent slow action potentials were studied in papillary muscles from guinea pigs pretreated with pertussis toxin and control guinea pigs. Hearts from guinea pigs treated in the same way with pertussis toxin or solvent alone underwent histological examination. For comparison, hearts from isoprenaline treated guinea pigs were also studied. Pertussis toxin specifically inactivates guanine nucleotide binding proteins (N proteins) involved in transmembrane signal transduction in many receptor systems (for example, adenosine receptors, m-cholinoceptors, and and alpha 2 adrenoceptors). In papillary muscles from control guinea pigs adenosine and PIA in the presence of isoprenaline produced a negative inotropic effect and inhibited the maximal rate of depolarisation of slow calcium dependent action potentials in potassium depolarised papillary muscles. After pretreatment with pertussis toxin the inhibitory effects both on force of contraction and on the maximal rate of depolarisation of adenosine and PIA were abolished. Treatment with pertussis toxin produced disseminated myocardial necrosis and a disseminated cellular calcium overload evidenced by glyoxal-2-bis-hydroxyanil (GBHA) staining. Similar lesions (for example, myocardial necrosis and cellular calcium overload) were also observed after treatment with isoprenaline. In controls neither myocardial necrosis nor cellular calcium overload was found. It is concluded that pertussis toxin sensitive N proteins are involved in the inhibitory effects of adenosine and PIA on force of contraction and on slow calcium inward current during beta adrenergic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of beta- and alpha-adrenergic agonists, adenosine, and carbachol in heart muscle isolated from malignant hyperthermia susceptible swine

During malignant hyperthermia crisis, cardiac arrhythmias and increased myocardial oxygen consumption are observed. It was the purpose of the present study to investigate whether or not a greater sensitivity to cardiac alpha- or beta-adrenoceptor stimulation or an impaired adenosine- or m-cholinoceptor-mediated inhibition of beta-adrenergic stimulation contributes to the cardiac symptoms of malignant hyperthermia. The effects of phenylephrine, isoproterenol, adenosine, [-]-N6-phenylisopropyladenosine (PIA), and carbachol on force of contraction were studied in electrically driven trabeculae isolated from the left ventricles of malignant hyperthermia susceptible (MHS) and healthy control pigs. The positive inotropic effects of phenylephrine and of isoproterenol were similar in MHS and control pigs. The EC50 values for the inotropic effect of phenylephrine were 4.1 (2.1-8.3) mumol.l-1 (n = 9) in MHS and 6.3 (3.4-12.9) mumol.l-1 (n = 9) in control swine. The maximal positive inotropic effects at 30 mumol.l-1 phenylephrine also did not differ in both groups (176.7 +/- 14.4% of pre-phenylephrine value in MHS swine, n = 9; 170.3 +/- 15.9% of pre-phenylephrine value in control swine, n = 9). The EC50 values for isoproterenol were 0.16 (0.06-0.39) mumol.l-1 (n = 9) and 0.19 (0.05-0.29) mumol.l-1 (n = 9) in MHS and control swine, respectively. The maximal positive inotropic effects at 1 mumol.l-1 isoproterenol were also similar (213.8 +/- 12.6% of the pre-isoproterenol value in MHS swine, n = 9; 216.4 +/- 36.0% of the pre-isoproterenol value in control swine, n = 9). Also, no difference could be detected in the antiadrenergic effects of adenosine, PIA, or carbachol.(ABSTRACT TRUNCATED AT 250 WORDS)

[Cardiac effects of adenosine. Mechanism of action, pathophysiologic and clinical significance]

Adenosine has a negative inotropic effect in cardiac atrial preparations ("direct" negative inotropic effect). This effect is probably due to an activation of a potassium outward current which shortens the action potential duration and hence reduces the force of contraction. A pertussis toxin-sensitive N-protein is involved in the signal transduction from the adenosine receptor to atrial potassium channels. In ventricular cardiac preparations adenosine has no negative or even a weak positive inotropic effect, but it reduces the force of contraction in the presence of cAMP-increasing agents such as isoprenaline ("indirect" negative intropic effect). This effect is due to an inhibition of the slow Ca2+ inward current which has previously been enhanced by an increase in the cellular cAMP content. This "indirect" negative inotropic effect of adenosine is also present in the human heart. Since increased amounts of adenosine are released during cardiac stimulation via beta-adrenoceptors, the "indirect" effect might protect the heart against excessive stimulation by catecholamines. In addition, adenosine has negative chronotropic actions and prolongs AV conduction by an activation of potassium channels or an inhibition of the slow Ca2+ inward current (AV node). Cardiac bradyarrhythmias in hypoxia have been attributed to an increased formation and release of adenosine. Furthermore, adenosine has been shown to terminate supraventricular tachycardias involving the AV node. Since it has a very short duration of action it might prove safe and hence advantageous to conventional therapy in the treatment of supraventricular tachycardias.

Evidence against a role of a pertussis toxin-sensitive guanine nucleotide-binding protein in the alpha 1-adrenoceptor-mediated positive inotropic effect in the heart

Pertussis toxin, which specifically inactivates guanine nucleotide-binding proteins (N-proteins) involved in the signal transduction in various receptor systems, did not influence the positive inotropic effect of the alpha 1-adrenoceptor agonist phenylephrine in rat isolated left auricles. This indicates that the alpha 1-adrenoceptor-mediated positive inotropic effect does not involve a pertussis toxin-sensitive N-protein.

Increased responsiveness to stimulation of alpha- but not beta-adrenoceptors in the hereditary cardiomyopathy of the Syrian hamster. Intact adenosine- and cholinoceptor-mediated isoprenaline antagonistic effect

The effects of phenylephrine (in the presence of propranolol) or isoprenaline and of adenosine or carbachol (alone and in the presence of isoprenaline) were studied on the force of contraction in electrically driven papillary muscles isolated from the hearts of cardiomyopathic (strain BIO 8262) and age-matched, healthy Syrian hamsters. All experiments were performed in the so-called prenecrotic stage of the disorder within the first 30 days of life. Phenylephrine exerted a positive inotropic effect in all preparations from the cardiomyopathic hamsters. In contrast, in thirteen preparations from healthy Syrian hamsters, phenylephrine increased force of contraction in only four preparations. The positive inotropic effect of isoprenaline was similar in both cardiomyopathic and healthy Syrian hamsters. Adenosine and carbachol apparently reduced the isoprenaline-induced increase in force of contraction in both cardiomyopathic and healthy Syrian hamsters. We conclude that an increased responsiveness to alpha-adrenergic stimulation occurs in the hearts of cardiomyopathic Syrian hamsters and may be related to the myocardial injury occurring in this syndrome. An increased responsiveness to beta-adrenoceptor stimulation or an impaired adenosine-mediated or cholinoceptor-mediated feedback inhibition is unlikely to play a role in the aetiology of this syndrome.

Does an impaired adenosine mediated feedback control play a role in the development of hereditary dystrophic cardiomyopathy?

A study was carried out to investigate whether or not an impairment of the adenosine mediated negative inotropic effect in the presence of beta adrenoceptor stimulation plays a role in the pathogenesis of the hereditary cardiomyopathy of the Syrian hamster. In electrically driven papillary muscles isolated from the hearts of cardiomyopathic (strain BIO 8262) and age matched healthy control Syrian hamsters the effects of isoprenaline, adenosine, and adenosine in the presence of isoprenaline were studied within the first 30 days of life (the prenecrotic stage of the disorder). In both cardiomyopathic and control hamsters adenosine antagonised the positive inotropic effect of isoprenaline, whereas adenosine alone had no or, only a weak, inhibitory effect on the force of contraction. The effects in both groups were similar. The effect of isoprenaline on the force of contraction also did not differ in the two groups. The data show that in both cardiomyopathic and control hamsters adenosine reduces the force of contraction during beta adrenergic stimulation. The potency or efficacy of adenosine did not differ in the two groups. An impaired adenosine mediated feedback control of the heart does not therefore seem to play a role in the pathogenesis of the hereditary dystrophic cardiomyopathy of the Syrian hamster.

Increased sensitivity to alpha-adrenoceptor stimulation but intact purinergic and muscarinergic effects in prehypertensive cardiac hypertrophy of spontaneously hypertensive rats

The effects of phenylephrine, isoprenaline and adenosine, (-)-N6-phenylisopropyladenosine (PIA) or carbachol alone and in the presence of isoprenaline on force of contraction were studied in isolated electrically driven papillary muscles of spontaneously hypertensive rats (SHR) and age-matched Wistar control rats. In SHR an increased heart to body weight ratio was observed when blood pressure was not yet elevated. During this stage of the syndrome (i.e. between the 27th and 35th day of life) phenylephrine was about 3.4 times more potent to increase force of contraction in SHR (mean EC50: 2.8 mumol l-1) than in control rats (mean EC50: 9.4 mumol l-1). The positive inotropic effect of isoprenaline was similar in SHR and control rats. Also no difference could be detected in the isoprenaline-antagonistic effect of adenosine, the adenosine receptor agonist PIA or carbachol. We conclude that an increased sensitivity to cardiac alpha-adrenoceptor stimulation might be related to prehypertensive cardiac hypertrophy in SHR.