On the cardioprotective effect of adenosine.

In isovolumically perfused Langendorff heart preparations of guinea pigs adenosine-depending on the experimental protocol-more or less could prevent the hypoxia-induced decrease in myocardial adenosine triphosphate [ATP], creatine phosphate [CP], glycogen and increase in lactate, i.e. showed cardioprotection.

Adenosine receptors mediate both contractile and relaxant effects of adenosine in main pulmonary artery of guinea pigs.

In guinea pig main pulmonary artery precontracted with noradrenaline, adenosine exerted an initial phasic contraction followed by a tonic contraction and a slow relaxation. After selective blockade by 1,3-dipropyl-8-cyclopentylxanthine (DPCPX: 10 nM) of A1 receptors, adenosine only elicited a rapid relaxation. This initial response was characterized by use of adenosine (AR) and its analogues N6-cyclopentyl-adenosine (CPA), R-N6-phenylisopropyladenosine (R-PIA), 2-chloroadenosine (CADO), 5'-N-ethyl-carboxamidoadenosine(NECA), N6-2-(4-aminophenyl) ethyl adenosine (APNEA) and 2-p-((carboxyethyl)-phenethylamino)-5'-carboxamidoadenosine (CGS 21 680). The order of potency of the adenosine analogues for purine-induced phasic contraction was CPA > R-PIA > NECA = APNEA > AR > CGS 21 680 suggesting the involvement of activation of A1 type adenosine receptors in the contraction phase. DPCPX antagonized the CPA-induced contraction with a pA2 = 9.27 +/- 0.26, but the Schild plot slope parameter was significantly lower than unity (0.58 +/- 0.09). In contrast, in electrically driven guinea pig atrial myocardium (a tissue reported to possess A1 receptors), the DPCPX-CPA antagonism was purely competitive (pA2 = 8.95 +/- 0.06; slope = 0.93 +/- 0.06). In the presence of 300 nM DPCPX, the rank order of potency for the purine-induced fast relaxation was NECA > CADO = AR > CGS 21 680 = R-PIA > CPA. The NECA- and adenosine-induced relaxation was influenced neither by 300 nM CP 66713 (an antagonist at A2a receptors), nor by endothelial removal and inhibition of nitric oxide synthase (100 microM NG-nitro-L-arginine: L-NOARG).(ABSTRACT TRUNCATED AT 250 WORDS)

[Cardioprotective effect of levcromakalim in isolated guinea pig heart preparations]. / A levcromakalim kardioprotektív hatásának vizsgálata izolált perfundált tengerimalac szíven.

The aim of present work was to study the cardioprotective effect of the potassium channel opener levcromakalim at a low concentration (0.3 microM) which does not depress heart rate and left ventricular developed pressure. For the determination of the protection against 10 or 20 min duration of hypoxia (95% N2 + 5% CO2) in isovolumically-perfused Langendorff heart preparations of guinea-pigs biochemical parameters (ATP, phosphocreatine, lactate and glycogen) were used. Under normoxic (95% O2 + 5% CO2) conditions 0.3 microM levcromakalim has not changed myocardial high energy phosphates, glycogen, lactate and the hypoxia induced decrease in ATP and phosphocreatine. The 10 min hypoxia induced increase in lactate and decrease in glycogen have--slightly but not significantly--been moderated and when the duration of hypoxic perfusion has lasted 20 min, this protection became significant. It is supposed that the preservation of glycogen stores induced by levcromakalim may contribute to the decrease of intracellular acidosis evoked by hypoxia. Therefore--in such experimental condition-, this mechanism may constitute the biochemical basis of cardioprotective effect of levcromokalim.

[Effect of Chinoin-103 on K(+)-activated pNPPase in the rat heart]. / Chinoin-103 hatása a patkányszív K(+)-aktivált pNPPáz aktivitására.

Effect of a new beta receptor blocking agent, an aryl oxybutanolamine derivative, the Chinoin--103 on K(+)-activated pNPP-ase activity has been studied. Propranolol and practolol were used as reference substances. It has been established that Chinoin-103 in concentration of 10(-4) M significantly hindered total pNPP-ase activity. On the other hand base pNPP-ase activity did not significantly change on the effect of the highest investigable concentration. Study of enzyme kinetic parameters has shown that decrease of base pNPP-ase activity can be attributed to decrease of reaction rate (Vmax decreases) at the same time enzyme affinity to pNPP substrate significantly increases (Km decreases). Taking into consideration the structural similarity of propranolol and Chinoin--103 it is presumable that Chinoin--103 can interact with components of membrane like propanolol and impediment of pNPP-ase activity can be attributed to the interaction.

Changes in sarcolemmal adenosine triphosphatase activity and in ouabain sensitivity of rat myocardium in isoproterenol-induced cardiac hypertrophy.

Sodium, potassium-activated adenosine triphosphatase (ATPase) activity and the sensitivity of rat myocardium to ouabain was studied in isoproterenol (IPR)-induced cardiac hypertrophy. IPR in a dose of 5 mg/kg was administered to rats intraperitoneally, once daily, for seven days. Left ventricular trabeculae originating from IPR-treated rats were significantly less sensitive than controls to ouabain-induced positive inotropy. In crude homogenate and sarcolemmal fractions the ATP hydrolysing activity both in the presence of Mg++ (basic) and Mg++, Na+, K+ (total) was significantly reduced in the heart of IPR-treated rats. The difference between the total and basic ATPase, i.e. the Na+, K(+)-stimulated portion of the activity was slightly reduced, but the sensitivity of Na+, K(+)-ATPase to ouabain remained unchanged. The results indicate that the well-known relation of sodium pump inhibition to positive inotropy in the heart of IPR-treated rats may not be valid.

[Effect of propranolol on ouabain-sensitive Na+,K+-ATPase activity of the rat heat]. / Propranolol hatása a patkányszív ouabain-szenzitív Na+,K+-ATPáz aktivitására.

Effect of propranolol on activity of basal ATP-ase measured in presence of Mg2+ activator, on that of total ATP-ase determined in presence of Mg2+, Na+, K+ activators and on that of Na+,K+-activated ATP-ase calculated from the difference of the previous two ones has been studied. Propranolol has not considerably influenced basal ATP-ase activity in 10(-7)-10(-4) M concentrations, 30-50% inhibition has been found in 1 X 10(-3)-5 X 10(-3) M concentrations. Effect of propranolol on total ATP-ase activity has been found to be dependent on doses and inhibition has been greater than inhibition experienced in case of basal ATP-ase. Enzyme kinetic investigations have shown that in case of basal ATP-ase propranolol has only decreased affinity to substrate but has not influenced rate of enzyme reaction, in case of total ATP-ase the affinity to the substrate has not changed but the reaction rate has significantly decreased. Ineffective concentrations of propranolol have potentiated inhibition effect of Na+,K+-ATP-ase. The authors have supposed that the observed changes have been results of non-specific membrane effects of propranolol and composition of membrane lipids may also play an important role.

[Effect of Chinoin-103 on Na+, K+-activated adenosine triphosphatase of the rat heart]. / Chinoin-103 hatása patkányszív Na+, K+-aktivált ATPáz aktivitására.

Effect of a new beta-blocking agent, a cardioselective aryl oxybutanolamine derivative, the Chinoin-103 on basal and total ATP-aze activities has been studied in total homogenizatum of rat heart. The effect has been compared to previous results of propranolol and to effect of practolol, respectively. It has been established that DL-Chinoin-103-similarly to DL-propranolol but in a little higher concentration--has significantly impeted both basal and total ATP-aze activities. Since in the nase of practolol similar effect was not obsreved and furthermore the effect could not be suspended by isoproterenol, the authors suppose that impediment of sarcolemmal ATP-aze activities cannot primarily be attributed to beta-blocking effect of compounds. Study of effect of racemic Chinoin-103 on enzymatic kinetic parameters of basal and total ATP-aze acticities has shown that primarily the reaction rate had decreased, affinity to substratum had not changed in the case of total ATP-aze and it has moderately decreased in the case of basal ATP-aze. The results of this publication has drawn attention to the fact that some beta-receptor blocking compounds may have other specific membrane effects besides antagonism on beta-receptors.

Study of the factors influencing cardiac growth. III. Digitoxin treatment and thyroxine-induced cardiac hypertrophy in the rat.

Thyroxine (T4) administered to rats in a dose of 1 mg/kg for 12 days induces cardiac hypertrophy. The purpose of the present study was to determine the effect of prophylactic + simultaneous digitoxin treatments on the development of T4-induced cardiac hypertrophy. Digitoxin (1 mg/kg body weight) was given per os, once daily for 6 days prior to T4 administration and continued simultaneously with T4 treatment. To determine myocardial enlargement, wet heart weight, myocardial nucleic acid and protein were measured. Digitoxin treatment induced a slight increase in wet ventricle weight and a significant elevation of myocardial RNA content (mg/ventricles) and concentration (mg/g). At the same time, the degree of T4-induced cardiac hypertrophy in digitoxin-treated and untreated animals was nearly the same. On the basis of these results it can be stated that--unlike the cardiac hypertrophy induced by pressure overload or hypoxia,--the T4-induced cardiac hypertrophy is not altered by digitoxin administration.

Alterations of contractility and sarcoplasmic reticulum function of rat heart in experimental hypo- and hyperthyroidism.

Myocardial contractility and Ca2+-pump function of sarcoplasmic reticulum (SR) were studied on hearts of untreated, thyroidectomized and thyroxine-treated rats. In hypothyroid rats the contractile force, the maximum velocity of tension development and relaxation significantly decreased (by 73.2%, 68.2%; and 67.8%, respectively), while the time to peak tension was prolonged (by 25.9%) as compared with the control group. In hyperthyroidism opposite changes were found. Since the transport of calcium opposite changes were found. Since the transport of calcium by SR plays an important role in controlling contraction and, first of all, relaxation of muscle, function of the sarcoplasmic reticulum was also investigated under the above experimental conditions. In thyroidectomized rats the rate of Ca2+-uptake and Ca2+-activated ATPase activity of SR significantly decreased (by 31.7% and 61.0%, respectively), while Ca2+-binding remained unchanged. After thyroxine treatment both the Ca2+-uptake and binding capacity of SR were even decreased (by 25.6% and 12.9%, respectively), in spite of an increase in Ca2+-activated ATPase activity (by 67.3%). These changes in Ca2+ transport function of cardiac SR may only partially be responsible for the abnormalities in contraction and relaxation observed in hearts from hypo- and hyperthyroid rats.

Study of the factors influencing cardiac growth. II. Digitoxin treatment and isoproterenol-induced cardiac hypertrophy in the rat.

Isoproterenol (IPR) administered to rats in a dose of 5 mg/kg for 4 days induces cardiac hypertrophy. The purpose of the present study was to determine the effect of prophylactic + simultaneous digitoxin treatment on the development of IPR-induced cardiac hypertrophy. Digitoxin (1 mg/kg body weight) was given per os, once daily for 6 days prior to IPR administration and continued simultaneously with IPR treatment. To determine myocardial enlargement, wet heart weight, myocardial nucleic acid and protein were measured. Digitoxin treatment induced slight but significant increase in wet ventricle weight and myocardial RNA content (mg/ventricle). At the same time the degree of IPR-induced cardiac hypertrophy in digitoxin-treated and untreated animals was nearly the same. On the basis of these results it can be stated that--unlike the cardiac hypertrophy induced by pressure overload or hypoxia,--the IPR-induced cardiac hypertrophy is not altered by digitoxin administration.

Study of the factors influencing cardiac growth. I. Comparison of cardiomegaly induced by isoproterenol in euthyroid and thyroidectomized rats.

The purpose of the present work was to study the cardiac growth-stimulating effect of IPR in hypothyroid animals, in which the in vitro sensitivity of the myocardium to beta-adrenergic agonists is significantly decreased. To determine the degree of myocardial enlargement, wet and dry ventricle weight and myocardial RNA, DNA and protein were measured. IPR administered to euthyroid rats in a dosage of 5 mg/kg/day for 4 days induced cardiomegaly. In thyroidectomized rats, a consistent depression of IPR-induced cardiomegaly was observed. This phenomenon appears to be in accordance with earlier observations showing a marked decrease in maximal beta-receptor level of ventricular membranes after thyroidectomy or PTU treatment.

Comparison of ATPase activity of cardiac myosins from different species.

Ca2+--ATPase activity of myosins prepared from hearts with different shortening speeds was measured in order to determine whether an alteration in hydrolitic activity or in the affinity of myosin for its substrate, or both, may be responsible for the species differences in Ca2+--ATPase. The KM values of ATP for cardiac myosins from rat, guinea-pig and rabbit did not differ significantly, whereas the Vmax decreased in the following order: rat greater than guinea pig greater than rabbit. These facts lead us to assume that, in spite of a great similarity of the active centres of the these myosins, the catalytic sites may not be identical.

Comparison of ATPase activity of cardiac sarcoplasmic reticulum fraction from rat and dog.

The purpose of the present study was to compare the ATPase activities of cardiac SR in two species in which the different intrinsic myocardial contractility can only partially be explained by the different properties of cardiac myosins. In cardiac SR isolated from rat heart, the total ATPase activity was 1512.5 +/- 23.3 nmol Pi/mg protein/min, nearly four times as high as in dog cardiac SR (408.8 +/- 28.9 nmol Pi/mg protein/min). The Ca2+-activated ATPase in rat cardiac SR represented only 23.8% of the total ATPase activity, while in dog cardiac SR it was approximately 50% of the total. Thus, the specific Ca2+-activated ATPase was nearly two times higher in the cardiac SR of the rat than in that of the dog. This higher rate of ATP hydrolysis in rat cardiac SR may be, at least in part, responsible for the increased intensity and shorter duration of the active state in the rat myocardium. Polyacrylamide gel electrophoresis of SR showed that the relative amount of Ca2+-pump protein was two times higher in dog heart, similar to the percentage of Ca2+-activated ATPase activity. At the same time, the specific Ca2+-activated ATPase activity and the relative amount of Ca2+ pump protein in both the rat and dog cardiac SR were inversely related.

Thyroxine-induced cardiomegaly: assessment of nucleic acid, protein content and myosin ATPase of rat heart.

1 mg/kg L-thyroxine was administered to rats for 14 days to evaluate the potential of the hyperthyroid state to induce heart hypertrophy and its effect on myosin adenosine-triphosphatase (ATPase) activity. Evidence of hyperthyroidism such as weight loss, elevation of rectal temperature, increased heart rate and oxygen consumption, was observed in all treated rats. Cardiac enlargement was determined by comparison of wet and dry ventricle weights, myocardial RNA, DNA and protein content. Wet and dry ventricle weights and the level of cardiac RNA and protein were augmented by thyroxine treatment. ATPase activity of cardiac myosin was stimulated as the Ca2+ concentration in the incubation medium increased. No difference was found in Ca2+-activation, salt sensitivity or ATPase activity of unreacted and sulphydrylmodified cardiac myosins from euthyroid or hyperthyroid groups. The results showed that in hyperthyroid rats, in contrast to some other species, the biochemical mechanism responsible for the enhancement of cardiac contractility is not an increased myosin ATPase.

ATPase activity of sulfhydryl-modified cardiac myosin from normal and isoproterenol-treated rats.

The possible role of sulfhydryl groups in the adaptation of cardiac myosin to work overload has been examined. The functional integrity of sulfhydryl groups was evaluated by measurement of Ca2+- and K+-(EDTA)-ATPase activities of myosins following sulfhydryl modification. No activation of Ca2+-ATPase of normal rat cardiac myosin was observed after pMB or NEM pretreatment. The decrease in Ca2+-ATPase of myosin from hypertrophied hearts was eliminated following sulfhydryl modification: moreover, slight stimulation of Ca2+-ATPase was observed. An increase in KCl concentration did not stimulate the Ca2+-ATPase of NEM-modified myosins obtained from either control or hypertrophied hearts. The sulfhydryl content of rat cardiac myosin expressed as moles of SH per 10(5) g of myosin was 6.99 +/- 0.30 and in IPR-induced hypertrophy did not change it significantly. In the authors' opinion an alteration in the integrity of the sulfhydryl groups may be responsible for the functional partition (decreased Ca2+-ATpase with unchanged K+-[EDTA]-ATPase activity) of myosin from hypertrophied hearts.