Regional distribution of beta 1- and beta 2-adrenoceptors in the failing and nonfailing human heart

Total beta-adrenoceptor density and beta 1- and beta 2-subtype distribution in right and left atria and in different ventricular regions from 14 failing and seven nonfailing human hearts have been compared. End-stage heart failure was due to idiopathic dilated cardiomyopathy (n = 8) or ischaemic cardiomyopathy (n = 6). In nonfailing hearts the total beta-adrenoceptor density was similar in the right and left atria and in all the ventricular regions studied (about 70 to 80 fmol/mg protein). The beta 1:beta 2-adrenoceptor ratio in both nonfailing atria was similar (about 70:30%) and was significantly smaller than in the different regions of both ventricles (about 80:20%). The beta 1-subtype density was similar in nonfailing atria and ventricles (about 55 fmol/mg protein). The beta 2-subtype density was significantly higher in the right and left atrium (about 25 fmol/mg protein) than in both ventricles (about 15 fmol/mg protein). In patients with end-stage heart failure due to idiopathic dilated cardiomyopathy or ischaemic cardiomyopathy the total beta-adrenoceptor density was reduced by 50-60% in all regions. On the other hand, the beta 1- and beta 2-subtype distribution differed with the cause of heart failure. In patients with idiopathic dilated cardiomyopathy, the beta 1-adrenoceptor density was not significantly reduced. In patients with ischaemic cardiomyopathy both beta 1- and beta 2-adrenoceptors were reduced in all regions. It is concluded that downregulation of beta-adrenoceptors in patients with end-stage idiopathic dilated cardiomyopathy or ischaemic cardiomyopathy occurs uniformly throughout the heart. The results support the hypothesis that changes in beta-adrenoceptor subtypes may be related to the cause of heart failure.

Reduced alpha 1- and beta 2-adrenoceptor-mediated positive inotropic effects in human end-stage heart failure

1. alpha 1-Adrenoceptor (phenylephrine in the presence of propranolol) and beta 2-adrenoceptor (fenoterol)-mediated positive inotropic effects were investigated in human ventricular preparations isolated from five non-failing (prospective organ donors) and from eight explanted failing hearts with end-stage idiopathic dilative cardiomyopathy (NYHA IV). 2. For comparison, the nonselective beta-adrenoceptor agonist isoprenaline, the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), the cardiac glycoside dihydroouabain, and calcium were studied. 3. Furthermore, the influence of IBMX on adenosine 3':5'-cyclic monophosphate (cyclic AMP) PDE activity as well as total beta-adrenoceptor density, beta 1- and beta 2-adrenoceptor subtype distribution, and alpha 1-adrenoceptor density were compared in nonfailing and failing human heart preparations. The radioligands (-)-[125I]-iodocyanopindolol for beta-adrenoceptor binding and [3H]-prazosin for alpha 1-adrenoceptor binding were used. 4. The inotropic responses to calcium and dihydroouabain in failing human hearts were unchanged, whereas the maximal alpha 1- and beta 2-adrenoceptor-mediated positive inotropic effects were greatly reduced. The inotropic effects of the other cyclic AMP increasing compounds, i.e. isoprenaline and IBMX, were also reduced to about 60% of the effects observed in nonfailing controls. The potency of these compounds was decreased by factors 4-10. 5. The basal PDE activity and the PDE inhibition by IBMX were similar in nonfailing and failing preparations. 6. The total beta-adrenoceptor density in nonfailing hearts was about 70 fmol mg-1 protein. In failing hearts the total number of beta-adrenoceptors was markedly reduced by about 60%. The betal/beta2-adrenoceptor ratio was shifted from about 80/20% in nonfailing to approximately 60/40% in failing hearts which was due to a selective reduction of beta1-adrenoceptors. The beta2-adrenoceptor population remaining unchanged. alpha-Adrenoceptor density was increased from about 4 fmol mg-' protein in nonfailing to 10 fmol mgprotein in failing hearts.7. Changes in PDE activity and adrenoceptor downregulation cannot completely explain the reduced positive inotropic effects of alpha 1- and beta 2-adrenoceptor agonists in failing human hearts. This supports the hypothesis that impairment of other processes such as the coupling between receptor and effector system, i.e. the respective G-proteins, are equally important in end-stage heart failure.

[The effect of seasonal undernutrition on the protein fractions in the blood of Moroccan cattle]

In a trial including 84 cattle from three farms near Rabat (Morocco) the influence of undernutrition (dry season September to February) on the concentration of total protein and its fractions in the blood of cattle was investigated. The results out of the field were confirmed by comparing these concentrations during an experimental cycle of sufficient--insufficient--sufficient feed. It was obvious that malnutrition decreases the concentration of total protein and some of its fractions (albumins, gamma-globulins) in the blood by about 10%, while alpha- and beta-globulins remained nearly uneffected.

[The composition of rumen fluid of cattle collected from fistulas using ultrasound]

Concentrations of rumen fluid characteristics were compared between rumen fluid which was taken by standardized conditions via stomach tube and via rumen fistula respectively depending on time after feeding (2.5 or 9 hours ppr.). The results were as follows: Differences (%) of the characteristics of rumen fluid taken via stomach tube to that by rumen fistula: [table: see text] Other investigations showed that the admixture of saliva to rumen fluid resulted in a degradation of the concentrations especially of VFA. The pH was not appreciable changed. The consequences for veterinary practitioners are discussed.

Increased messenger RNA level of the inhibitory G protein alpha subunit Gi alpha-2 in human end-stage heart failure

In human heart failure the positive inotropic and cAMP-elevating effects of both beta-adrenoceptor agonists and phosphodiesterase inhibitors are diminished. This has been explained at least in part by an increase in the inhibitory signal-transducing G protein (Gi) and unchanged stimulatory G protein (Gs). In the present study we determined the mRNA expression pattern of the alpha subunits of Gi-1, Gi-2, Gi-3, and Gs in myocardial tissue samples of patients undergoing heart transplantation. Northern blot analysis of total RNA extracted from left ventricles with 32P-labeled cDNAs demonstrated expression of Gi alpha-2, Gi alpha-3, and Gs alpha mRNA. In contrast, Gi alpha-1 mRNA was not detectable. To investigate whether the increased ratio of Gi/Gs might be due to altered gene expression, we compared mRNA levels of Gi alpha-2, Gi alpha-3, and Gs alpha in left ventricular myocardium from failing hearts with idiopathic dilated cardiomyopathy (n = 8) and ischemic cardiomyopathy (n = 6) and from nonfailing hearts from transplant donors (n = 8). Compared with nonfailing control hearts, the Gi alpha-2 mRNA was increased by 75 +/- 26% (p less than 0.05) in idiopathic dilated cardiomyopathy hearts and 90 +/- 26% (p less than 0.05) in ischemic cardiomyopathy hearts. Gi alpha-3 and Gs alpha mRNA levels were similar in the three groups. The results suggest that as in other mammalian species, Gi alpha-2 and Gi alpha-3 mRNA are the predominant Gi alpha mRNA subtypes in human ventricular myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation and possible functional implications of G-protein mRNA expression in nonfailing and failing ventricular myocardium

In human end-stage heart failure an increased amount of inhibitory G-protein alpha-subunits (Gi alpha) is assumed to play a role in desensitization of the adenylyl cyclase signaling pathway. In the present study, northern blot experiments with 32P-labeled cDNA probes in ventricular tissue samples from explanted human hearts revealed that Gi alpha-2- and Gi alpha-3- mRNA are the predominant Gi alpha-mRNA subtypes in human ventricles, whereas Gi alpha-1-mRNA was not detectable. The mRNA for the stimulatory G-protein alpha-subunit (GS alpha) consisted of two mRNA sizes. Quantification of mRNA levels revealed a 103 +/- 38% increase in Gi alpha-2-mRNA levels in hearts with idiopathic dilative cardiomyopathy (IDC; n = 8), and a 77 +/- 25% increase in hearts with ischemic cardiomyopathy (ICM; n = 6) as compared to nonfailing controls (NF, n = 8). In contrast, Gi alpha-3- and GS alpha-mRNA levels were similar in failing and nonfailing hearts. To investigate whether or not the increased expression of Gi alpha-2-mRNA might be due to chronically elevated catecholamine levels, we determined the influence of a 4-day infusion of isoprenaline (Iso; 2.4 mg/kg.d), propranolol (Prop; 9.9 mg/kg.d), Iso + Prop or 0.9% NaCl as control (Ctr) on myocardial Gi alpha-mRNA and Gi alpha-protein levels in rats. In Iso-treated rats, hybridization experiments revealed a 49 +/- 18% (n = 7) and 27 +/- 7% (n = 8) increase in Gi alpha-2 and Gi alpha-3-mRNA, respectively. Pertussis toxin-catalyzed ADP-ribosylation revealed a 22 +/- 7% (n = 8) increase in Gi-protein as compared to Ctr (n = 8). These alterations were accompanied by an increased potency for the negative inotropic effect (NIE) of carbachol (mean EC50: 0.04 microM vs. 0.28 microM) in the presence of Iso in isolated electrically driven (1 Hz) papillary muscles. Prop itself had no effect, but it antagonized all Iso-induced effects. We conclude that, in human heart failure due to IDC or ICM, increased Gi alpha-2-, but not Gi alpha-3- mRNA levels accompany the increased amount of Gi alpha-protein, suggesting that this increase is at least in part due to increased de novo synthesis. The experiments in rats demonstrated that chronic beta-adrenergic stimulation leads to an increased expression of Gi alpha-mRNA and -protein, and to an enhanced potency of the negative inotropic effect of muscarinic agonists.(ABSTRACT TRUNCATED AT 400 WORDS)

Phosphodiesterase inhibition and positive inotropy in failing human myocardium

Positive inotropic effects of phosphodiesterase inhibitors like 3-isobutyl-1-methylxanthine (IBMX), pimobendan, adibendan, milrinone, saterinone, and enoximone are greatly diminished in isolated heart muscle preparations from human failing myocardium as compared to nonfailing myocardium. This is accompanied by a reduced increase in cAMP content in intact isometrically contracting human trabeculae. With anion exchange chromatography four peaks of phosphodiesterase activities (PDE I-IV) could be separated from both nonfailing and failing human myocardium. Substrate specificity, Km, and Vmax were similar in nonfailing and failing myocardium. Furthermore, the PDE inhibitors investigated exhibited similar IC50-values in both tissues, indicating that the sensitivity of the enzymes from nonfailing and failing tissue was unchanged. Thus, changes in PDE are probably not responsible for the reduced positive inotropic and cAMP-increasing effects of PDE inhibitors in human failing heart muscle preparations. Instead, an increase in signal transducing inhibitory G-proteins may keep the adenylyl cyclase at reduced activity, resulting in an attenuated formation of cAMP, even in the presence of PDE inhibitors.

Isoprenaline-induced increase in the 40/41 kDa pertussis toxin substrates and functional consequences on contractile response in rat heart

Chronic beta-adrenoceptor stimulation leads to desensitization of the myocardial adenylyl cyclase signalling pathway which includes beta-adrenoceptor downregulation and upregulation of Gi-protein alpha-subunits. However, these investigations have mainly been done in cellular preparations. In this study we report that isoprenaline infusion in vivo leads to an increase in myocardial Gi alpha and present evidence for functional consequences of this increase. Rats were treated by a 4-day subcutaneous infusion with isoprenaline (2.4 mg/kg.d), propranolol (9.9 mg/kg.d) and triiodothyronine (T3, 0.5 mg/kg.d) for comparison. Isoprenaline treatment increased the pertussis toxin-sensitive amount of Gi alpha by 22 +/- 6% and decreased beta 1- and beta 2-adrenoceptor density from 35 +/- 4 to 23 +/- 6 fmol/mg protein and 24 +/- 4 to 8 +/- 6 fmol/mg protein, respectively. Contraction experiments on electrically driven papillary muscles revealed that the negative inotropic potency of the M-cholinoceptor agonist carbachol in the presence of isoprenaline was increased as compared to control (mean EC50-values: 0.04 mumol/l vs. 0.28 mumol/l). All isoprenaline-induced effects were antagonized by simultaneously administered propranolol. T3 treatment had no influence on the parameters investigated. The results suggest that chronic beta-adrenoceptor stimulation desensitizes myocardial adenylyl cyclase by at least two mechanisms: beta-adrenoceptor downregulation leading to diminished signal transduction in the stimulatory pathway and Gi alpha upregulation leading to sensitization of the inhibitory pathway. Such adaptation might protect the heart from chronic exposure to catecholamines in heart diseases with elevated plasma catecholamine levels.

The role of alpha 1-adrenergic and muscarinic receptors in cardiac function

The positive inotropic effect of an alpha 1-adrenoceptor agonist, such as phenylephrine, is accompanied by an increase in the presumed second messenger, inositol 1,4,5-trisphosphate (1,4,5-IP3) and inositol 1,3,4,5-tetrakisphosphate (1,3,4,5-IP4), which may release calcium from the sarcoplasmic reticulum (SR) and/or facilitate calcium entry from the extracellular space. In addition, phenylephrine sensitizes the contractile proteins for calcium. Alpha 1-adrenergic positive inotropic effects are enhanced in heart muscle preparations from cardiomyopathic hamsters and are reduced in heart muscle preparations from human failing myocardium. How the negative inotropic effects of M-cholinoceptor agonists work in the presence of cAMP-increasing agents in ventricular heart muscle preparations is discussed. It involves cAMP-reduction, an increase in cGMP and activation of phosphatase activity. In a rat model, chronic beta-adrenergic stimulation leads to increased sensitivity of rat ventricular tissue for the negative inotropic effect of the M-cholinoceptor agonist, carbachol. This might be due to facilitated signal transduction via increased Gi proteins. In human ventricular tissue from hearts with end-stage heart failure, due to idiopathic dilated cardiomyopathy (IDC), an increased Gi protein has also been found. However, the negative inotropic effects of carbachol were unchanged. The data indicate that changes in alpha-adrenergic and M-cholinergic responses in the heart may depend on underlying causes that induce changes.

Beta-adrenoceptor stimulation-induced increase in cardiac Gi-protein expression and in carbachol sensitivity

Increased plasma concentrations of catecholamines are assumed to be responsible for the decreased sensitivity to catecholamines of the failing heart. We investigated in rat heart the influence of a 4-day infusion of isoprenaline (Iso; 2.4 mg.kg-1.d-1), propranolol (Prop; 9.9 mg.kg-1.d-1), Iso + Prop or 0.9% NaCl as control (Ctr) on myocardial Gi-mRNA and Gi-protein levels and on the negative inotropic effect of carbachol in papillary muscles. In Iso-treated rats, hybridization experiments with 32P-cDNAs revealed a 49 +/- 18% (n = 7-8) and 27 +/- 7% (n = 8) increase in Gi alpha-2- and Gi alpha-3-mRNA respectively, and pertussis toxin-catalyzed ADP-ribosylation revealed a 22 +/- 7% (n = 8) increase in Gi-protein as compared to Ctr. These alterations were accompanied by an increased potency of carbachol (mean EC50: 0.04 microM vs. 0.28 microM) in the presence of Iso in isolated electrically driven (1 Hz) papillary muscles. Prop had no effect on Gi-protein expression but antagonized all Iso-induced effects. In conclusion, beta-adrenergic stimulation leads to an increased expression of Gi and to an enhanced negative inotropic potency of muscarinic agonists. An enhanced muscarinic receptor coupling via Gi might play a pathophysiological role in heart diseases with increased plasma catecholamine levels.

Evidence for isoproterenol-induced phosphorylation of phosphatase inhibitor-1 in the intact heart

The positive inotropic effect of the beta-adrenoceptor agonist isoproterenol is accompanied by inhibition of phosphatase type 1 activity in myocardium. Indirect assays suggest that this effect is due to activation of protein phosphatase inhibitor-1, which inhibits phosphatase activity only when phosphorylated. To test this hypothesis directly, electrically stimulated (3 Hz) guinea pig ventricular preparations were perfused according to the Langendorff method with physiological buffers with or without 5 mCi 32P/heart, and then various concentrations of isoproterenol were applied. Contractility was recorded. Hearts were freeze-clamped and cAMP and inhibitor-1 activities were measured. In 32P-labeled hearts a protein at about 26 kd on autoradiograms of 12% sodium dodecyl sulfate gels was detected. Isoproterenol (1 microM) increased rate of tension development to 238% of the predrug value, cAMP concentrations 1.5-fold, and inhibitor-1 activity threefold. Concomitantly, there was an increase in a 32P-labeled band at about 26 kd from 380 to 540 pmol 32P/mg protein. This protein at about 26 kd, after transfer to nitrocellulose, was recognized by an antiserum prepared against rabbit skeletal muscle inhibitor-1. More radioactive protein of about 26 kd could be immunoprecipitated by the antiserum from isoproterenol-treated than from untreated hearts. It is concluded that a protein, probably identical to phosphatase inhibitor-1, is phosphorylated in vivo in the heart in the presence of isoproterenol. Phosphorylation of inhibitor-1 with consequent modification of type 1 phosphatase activity may contribute to the effects of isoproterenol in the heart.

[Malignant hyperthermia and inositol phosphate metabolism in the heart and skeletal musculature]

There are recent reports that inositol phosphate metabolism is involved in the development of malignant hyperthermia (MH). Consequently, we investigated the basal concentration of inositol phosphate products in skeletal and heart muscles of malignant hyperthermia-susceptible (MHS) and healthy control (MHN) swine. Different inositol phosphates were measured by high pressure liquid chromatography, including inositol trisphosphate, tetrakisphosphate, pentakisphosphate and hexakisphosphate. All inositol phosphate products measured had a higher concentration in MHS than MHN in skeletal (304-1330%) as well as heart muscles (134-440%). An activation of the inositol phosphate metabolism has been shown to mobilise intracellular calcium from the sarcoplasmic reticulum. It is therefore concluded that, firstly, besides involvement of the skeletal muscles a primary myocardial abnormality in MHS is possible; and secondly, the idea that the inositol phosphate metabolism could be involved in the development of MH is additionally supported.

Antiviral therapy of varicella-zoster virus infection in immunocompromised children--a prospective randomized study of aciclovir versus brivudin

Both aciclovir and brivudin are effective in the treatment of immunocompromised children with varicella-zoster virus infection. To determine which drug is preferable, a prospective randomized trial aciclovir vs. brivudin was conducted. Forty-three immunocompromised children were randomly assigned to receive aciclovir intravenously at a dose of 1,500 mg/m2/d and brivudin orally at a dose of 15 mg/kg/d, respectively. Twenty-two patients were treated with aciclovir and 21 with brivudin. In all children the general status improved within two days. The eruption of new lesions stopped within one to five days, fever stopped within one to nine days, complete remission occurred within five to six days after introduction of the virustatic therapy. There was no difference in therapeutic efficacy between aciclovir and brivudin. Two children in each group did not respond to the medication. No myelo-, hepato- and nephrotoxic side effects due to aciclovir or brivudin were observed. All obviously immunocompromised children with varicella or zoster may be treated with aciclovir or brivudin.

Effects of isomazole on force of contraction and phosphodiesterase isoenzymes I-IV in nonfailing and failing human hearts

The phosphodiesterase (PDE) inhibitor isomazole increased the force of contraction to 278.3 +/- 89.1% (n = 7) of the predrug value in ventricular trabeculae carneae isolated from nonfailing human hearts. This effect can be attributed mainly to a PDE III or a combined PDE III/IV inhibition since at the concentration of the maximal positive inotropic effect of isomazole, PDE III and PDE IV were completely inhibited. In explanted failing human hearts (end-stage myocardial failure, NYHA IV), isomazole increased the force of contraction only marginally to 110.1 +/- 10.7% of the predrug value. The lack of a distinct positive inotropic efficacy of isomazole in failing human hearts could not be explained by an impairment of PDE inhibition since the properties of the PDE I-IV isoenzymes separated by DEAE-Sepharose chromatography and the inhibitory effects of isomazole did not differ in both preparations. The positive inotropic effect of the beta-adrenoceptor agonist isoprenaline was also reduced in failing hearts. However, in the presence of isomazole, the diminished positive inotropic effect of isoprenaline was restored to values obtained with isoprenaline alone in nonfailing hearts. Thus, the decreased effect of inotropic drugs like isoprenaline or isomazole in preparations from failing human heart might be explained mainly by a diminished cAMP formation due to a defect in receptor-adenylate cyclase coupling.

Differential response of renin secretion to vasoconstrictors in the isolated perfused rat kidney

1. We have examined whether an increase of renal vascular resistance is generally accompanied by an inhibition of renin secretion. The effects of vasoconstriction produced by angiotensin II (Ang II), arginine-vasopressin (AVP), and potassium (KCl) depolarization on vascular resistance and on renin release from isolated rat kidneys perfused at constant pressure of 100 mmHg were investigated. 2. Histological examination performed on some representative kidneys revealed that the tubular lumina of all segments within the cortex were patent and the brush borders of the proximal tubules were well preserved. The renal vasculature and the juxtaglomerular region appeared to be morphologically intact. By immunocytochemistry, renin-positive cells were found exclusively in the wall of the afferent arterioles. 3. Basal flow rate through isolated kidneys was 14.5 +/- 2.0 ml min-1 (g kidney weight (gkw))-1 (mean +/- S.E.M., n = 10). Under control conditions renin secretory rates were in the range of 30-40 (ng Ang I h-1) min-1 gkw-1. 4. Ang II (100 pM) caused a decrease of renal flow rate to 42 +/- 2% of control which was accompanied by a reduction of renin secretion rates by a factor of 4. 5. AVP (10 pM to 1 nM) reduced renal perfusate flow in a dose-dependent fashion to a minimum of 25 +/- 3% of control. The vasoconstrictor effect of AVP was paralleled by a concentration-dependent increase of renin secretory rates reaching a factor of maximally 5 when AVP was used at a concentration of 1 nM. The stimulatory effect of AVP on renin release could be mimicked by [deamino-Cys1, D-Arg8]-vasopressin (dDAVP), a vasopressin analogue with prevalent V2 receptor agonistic properties. In the presence of dDAVP (100 nM, 1 microM) renal flow rate reversibly increased by 8 and 12% of control values, respectively. 6. Depolarizing concentrations of KCl (30 mM) decreased perfusate flow to 20 +/- 4% of control. The vasoconstrictor effect of KCl was paralleled by an increase of the arterio-venous difference of perfusate renin activity to such an extent that the rate of renin release remained unaltered. 7. Our findings suggest that there exists no general inverse relationship between renal arteriolar resistance and renin secretion. Our study, moreover, does not support a functional role of potential operated calcium channels in the control of renin secretion. Finally, we conclude that V2 receptors are present on juxtaglomerular epithelioid cell membranes and mediate the stimulatory effect of AVP on renin release from isolated rat kidneys.

Amiloride enhances the secretion but not the synthesis of renin in renal juxtaglomerular cells

In this study we have examined a potential role of the sodium/proton exchange system in the regulation of renin secretion. We found that the inhibitors of the Na+/H+ antiport, amiloride (1 mM) and ethylisopropylamiloride (EIPA, 50 microM), led to a 125% increase of renin secretion from cultured mouse juxtaglomerular cells. The stimulatory effect of EIPA on renin secretion was dependent on the extracellular concentrations of sodium and hydrogen ions. While lowering the extracellular pH from 7.3 to 7.0, and lowering [Na+]e from 130 mM to 5 mM had no effect on basal renin release, it markedly attenuated or even blunted the effect of EIPA on renin secretion. The stimulatory effect of forskolin on renin secretion, however, was not altered by decreases of extracellular pH and of sodium. Inhibition of basal renin release was achieved with angiotensin II (1 microM). In the presence of EIPA the inhibitory effect angiotensin II was markedly attenuated. Although effective on renin secretion, neither amiloride nor EIPA exerted a significant effect on the denovo synthesis of renin in cultured mouse JG cells. These findings are compatible with the idea that an amiloride-sensitive transport process, presumably the Na+/H+ exchanger, acts indirectly as an inhibitory signal transduction system for renin secretion from renal juxtaglomerular cells.

Mechanism underlying the reduced positive inotropic effects of the phosphodiesterase III inhibitors pimobendan, adibendan and saterinone in failing as compared to nonfailing human cardiac muscle preparations

The present study was performed to compare the effects of the new positive inotropic phosphodiesterase III inhibitors pimobendan, adibendan, and saterinone on the isometric force of contraction in electrically driven ventricular trabeculae carneae isolated from explanted failing (end-stage myocardial failure) with those from nonfailing (prospective organ donors) human hearts. In preparations from nonfailing hearts the phosphodiesterase inhibitors, as well as the beta-adrenoceptor agonist isoprenaline, the cardiac glycoside dihydro-ouabain, and calcium, which were studied for comparison, revealed pronounced positive inotropic effects. The maximal effects of pimobendan, adibendan, and saterinone amounted to 56%, 36% and 45%, respectively, of the maximal effect of calcium. In contrast, in preparations from failing hearts the phosphodiesterase III inhibitors failed to significantly increase the force of contraction and the effect of isoprenaline was markedly reduced. The effects of dihydroouabain and calcium were almost unaltered. The diminished effects of isoprenaline were restored by the concomitant application of phosphodiesterase inhibitors. To elucidate the underlying mechanism of the lack of effect of the phosphodiesterase III inhibitors in the failing heart we also investigated the inhibitory effects of these compounds on the activities of the phosphodiesterase isoenzymes I-III separated by DEAE-cellulose chromatography from both kinds of myocardial tissue. Furthermore, the effects of pimobendan and isoprenaline on the content of cyclic adenosine monophosphate (determined by radioimmunoassays) of intact contracting trabeculae were studied. The lack of effect of the phosphodiesterase inhibitors in failing human hearts could not be explained by an altered phosphodiesterase inhibition, since the properties of the phosphodiesterase isoenzymes I-III and also the inhibitory effects of the phosphodiesterase inhibitors on these isoenzymes did not differ between failing and nonfailing human myocardial tissue. Instead, it may be due to a diminished formation of cyclic adenosine monophosphate in failing hearts, presumably caused mainly by a defect in receptor-adenylate cyclase coupling at least in idiopathic dilated cardiomyopathy. Both the basal and the pimobendan-stimulated or isoprenaline-stimulated contents of cyclic adenosine monophosphate of intact contracting trabeculae from failing hearts were decreased compared with the levels in nonfailing hearts. However, under the combined action of isoprenaline and pimobendan the cyclic adenosine monophosphate level reached values as high as with each compound alone in nonfailing preparations, and in addition the positive inotropic effect of isoprenaline was restored. These findings may have important clinical implications. Along with the elevated levels of circulating catecholamines the positive inotropic effects of the phosphodiesterase inhibitors may be maintained in patients with heart failure.(ABSTRACT TRUNCATED AT 400 WORDS)

Possible involvement of inositol-lipid metabolism in malignant hyperthermia

Alpha-adrenoceptor stimulation may induce malignant hyperthermia (MH) in vivo. Consequently, we have investigated the effects of the alpha-adrenoceptor agonist phenylephrine and, for comparison, the effects of the beta-adrenoceptor agonist isoproterenol on inositol-lipid metabolism of malignant hyperthermia susceptible (MHS) and healthy control (MHN) swine. The experiments were performed on electrically stimulated (frequency 0.2 Hz) trabeculae isolated from the right ventricles of the hearts of MHS and MHN animals. After labelling with [3H]inositol for 6 h, different inositol phosphates were measured by high pressure liquid chromatography, including inositol 1-phosphate, inositol 1,4-bisphosphate, inositol 1,3,4-trisphosphate, inositol 1,4,5-trisphosphate (1,4,5-IP3) and inositol 1,3,4,5-tetrakisphosphate. After stimulation with isoproterenol, the inositol phosphate content did not increase or vary between muscle from MHS and MHN animals. In contrast, all inositol phosphates increased after stimulation with phenylephrine in both muscle types, the effects being greater in MHS than in MHN, especially as regards 1,4,5-IP3 content. As 1,4,5-IP3, a presumed second messenger, has been shown to mobilize intracellular calcium, it is concluded that an enhanced alpha-adrenergic response is involved in the development of MH.

Isoprenaline-induced increase in mRNA levels of inhibitory G-protein alpha-subunits in rat heart

Long-term beta-adrenergic stimulation has been shown to desensitize the beta-adrenoceptor/adenylyl cyclase signalling pathway at both the receptor and the G-protein level. To further elucidate the cellular mechanism of G-protein regulation we investigated the influence of prolonged infusion of isoprenaline (2.4 mg/kg.d) on myocardial mRNA levels of different G-protein alpha-subunits in rats. For comparison rats were treated with triiodothyronine (T3; 0.5 mg/kg.d) which induces cardiac hypertrophy like isoprenaline but has different effects on the adenylyl cyclase system. Isoprenaline- and T3-treated animals developed an increase in heart/body weight ratio of 41 +/- 3% and 27 +/- 4%, respectively (P less than 0.05). Isoprenaline increased myocardial total RNA concentration by 39 +/- 6% (P less than 0.05). Hybridization with 32P-labeled rat cDNAs demonstrated an expression rank order of Gs alpha-mRNA greater than Gi alpha-2-mRNA greater than Gi alpha-3-mRNA and no detectable expression of Gi alpha-1-mRNA in rat myocardium. mRNA levels of Gs alpha, Gi alpha-2 and Gi alpha-3 were 36.9 +/- 1.28, 10.7 +/- 1.07 and 3.7 +/- 0.19 pg/micrograms total RNA, respectively. Isoprenaline increased Gi alpha-2- and Gi alpha-3-mRNA concentrations per microgram total RNA by 49 +/- 18% and 27 +/- 7%, respectively (P less than 0.05). This effect was abolished by simultaneously administered propranolol (9.9 mg/kg.d), indicating a beta-adrenoceptor-mediated mechanism. In contrast, T3-induced cardiac hypertrophy was not accompanied by changes in Gi alpha-mRNA expression. Gs alpha-mRNA levels were unaffected by either treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of caffeine, halothane, succinylcholine, phenylephrine and isoproterenol on myocardial force of contraction of malignant hyperthermia susceptible swine

The effects of caffeine, halothane, succinylcholine, phenylephrine and isoproterenol on force of contraction were studied in electrically driven (0.2 Hz) trabeculae isolated from the right ventricles of the hearts of malignant hyperthermia susceptible (MHS) and healthy control (nMHS) swine. Caffeine (0.1-10 mmol/l) had positive inotropic effects, amounting to 275 +/- 35% of control in nMHS and 268 +/- 34% in MHS (n = 16). Halothane (0.25-4 vol%) decreased the force of contraction maximally to 52 +/- 4% in nMHS and 51 +/- 5% of control in MHS (n = 16). Propranolol did not change these effects. Succinylcholine (0.1-10000 mumol/l) had a small positive inotropic effect in both groups, which was blocked by propranolol. Phenylephrine (0.1-300 mumol/l) increased the force of contraction maximally to 188 +/- 24% of control in nMHS and to 193 +/- 23% in MHS (n = 16). The inotropic effect was blocked by prazosin but not by succinylcholine (1 mmol/l). Isoproterenol (0.01-10 mumol/l) had a positive inotropic effect of maximally 275 +/- 21% of control in nMHS and 396 +/- 31% in MHS (n = 17) (P less than 0.05). Succinylcholine potentiated this effect, and propranolol shifted the concentration-response curves to the right. We conclude that caffeine, halothane, succinylcholine and phenylephrine have similar inotropic effects in the hearts of nMHS and MHS, whereas isoproterenol has a significantly greater effect in MHS than in nMHS.

Effects of the triazolopyrimidine trapidil on force of contraction, beating frequency and phosphodiesterase I--IV activity in guinea-pig hearts

The effects of the triazolopyrimidine trapidil (5-methyl-7-diethylamino-s-triazolo [1,5-alpha]pyrimidine, CAS 15421-84-8) on force of contraction, beating frequency and phosphodiesterase (PDE) activity were investigated in isolated preparations from guinea-pig hearts. The effects of 3-isobutyl-1-methylxanthine (IBMX), theophylline and milrinone were studied for comparison. Trapidil exerted a concentration-dependent (1000-3000 mumol(s)/l) positive inotropic effect (EC50 562.4 mumol(s)/l) in guinea-pig papillary muscles. The positive inotropic effect was accompanied by a shortening of the duration of contraction as described for IBMX, or isoprenaline. The efficacy of trapidil was lower than that of IBMX or milrinone. Both agents maximally enhanced force of contraction to a 3fold (milrinone) or even 6fold greater amount (IBMX). The potency of trapidil was almost in the same order of magnitude as that of milrinone. The positive inotropic effect of trapidil is at least partially due to a cyclic adenosine monophosphate (cAMP)-dependent mechanism because carbachol antagonized the increase in force of contraction. Trapidil concentration-dependently but nonselectively inhibited the activities of cAMP PDE isoenzymes I-IV as did theophylline or IBMX. Based on IC50 values (275 mumol(s)/l on the average) trapidil had a potency similar to that of theophylline while IBMX was about one order of magnitude more potent. Regarding the inhibition of PDE III, IBMX was 49fold and milrinone 114fold more potent than trapidil. Trapidil revealed only a marginal positive chronotropic effect. The frequency of spontaneously beating right auricles was increased by 13% at most. Trapidil did not produce any tachyarrhythmias or contractures. It is concluded that the positive inotropic effect of trapidil is mainly due to PDE inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxygen-dependent erythropoietin production by the isolated perfused rat kidney

In this study we have investigated the role of oxygen delivery and of classic second messengers on erythropoietin production by the isolated perfused rat kidney. We found that the rat kidney was capable of de novo synthesis of erythropoietin. The erythropoietin production rate was inversely related to the oxygen pressure in the perfusate and increased from 0.17 to 1.85 U erythropoietin h-1 g kidney-1 when arterial PO2 was lowered from 500 mmHg to 30 mmHg. Addition of forskolin (10 microM) and 8-bromo-cGMP (100 microM) to the perfusate elicited significant effects on the renal vascular resistance, but had no significant effect on erythropoietin production. Hypoxia-induced erythropoietin formation, however, was blocked by calmidazolium (1 microM) and W-7 (10 microM), two structurally different putative calmodulin antagonists. Calmidazolium and W-7 had no effect on other functional parameters of the isolated perfused rat kidney such as flow rate, glomerular filtration rate or sodium reabsorption. Our findings suggest that the oxygen-sensing mechanism that controls renal erythropoietin production is primarily located in the kidney itself. A calcium/calmodulin-dependent cellular reaction could be involved in the signal transduction process.