On the mechanism of positive inotropic effects of alpha-adrenoceptor agonists

Angiotensin II and cardiac excitation-contraction coupling: questions and controversies

Angiotensin II (AngII) is a circulating peptide that produces a positive inotropic effect in the heart in several species, including humans. The subcellular mechanisms involved in producing this effect have been the focus of numerous studies; however, the results of these studies have generated considerable controversy. Although part of the controversy might arise from species and developmental differences, conflicting results have also been reported in the same species. To further complicate the understanding of the cardiac actions of AngII, the binding of the peptide to its transmembrane G-protein-coupled receptors has been shown to activate signalling cascades that involve numerous second messengers. Among these, inositol 1,4,5-triphosphate (IP3) and protein kinase C (PKC) have been shown to have the potential to modulate either one or both of the two basic mechanisms known to increase contractility: (i) an increase in the intracellular Ca2+ concentration ([Ca2+]i); or (ii) an increase in myofilament responsiveness to Ca2+. The aim of this review is to examine the effect of AngII on the fundamental components of cardiac excitation-contraction coupling: calcium currents, Na+/Ca2+ exchange, sarcoplasmic reticulum (SR)-CaZ+ release, calcium transients and contractile proteins. An answer to the following question is sought: Is the positive inotropic effect of AngII due to an increase in [Ca2+]i, to an increase in myofilament responsiveness to Ca2+, or to both?

Adrenergic-mediated effects of cocaine on the myocardial force-frequency relationship

We studied the role of sarcolemmal alpha- and beta-adrenoceptors activation in the effects of cocaine on the positive force staircase in isolated guinea pig atria. The preparations were superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed (PTD), maximum velocity of development of tension (Vmax T) and time to peak tension (TPT). The positive force staircase was not affected by propranolol or phentolamine, but it was abolished by nifedipine. Cocaine 1 mg/l (2.9 microM) enhanced PTD and Vmax T, while TPT remained unchanged. On the other hand, cocaine did not modify the increase in PTD induced by the increase in frequency of stimulation, but significantly reduced the magnitude of the increase in Vmax T. The cocaine-induced attenuation of the increase in Vmax T in response to changes in the frequency of stimulation was abolished by both propranolol and phentolamine. It is concluded that the effect of cocaine on the force-frequency relationship required background activation of alpha- and beta-adrenergic receptors.

Mechanism of the positive inotropic action of cocaine in the guinea pig atrium

We studied the mechanism of the positive inotropic action of cocaine in isolated guinea pig atria superfused with Tyrode's solution at 31 degrees C while attached to a force transducer to measure peak tension developed, maximum velocity of development of tension, and time to peak tension. Cocaine 2.9 microM enhanced peak tension developed and velocity of development of tension, and prolonged time to peak tension. The increase in peak tension developed produced by cocaine was not affected by propranolol. On the other hand, the cocaine-induced increase in velocity of development of tension was reduced, but not abolished. In the presence of propranolol and phentolamine combined, the cocaine-induced prolongation of time to peak tension was abolished and the increases of both peak tension developed and velocity of development of tension were significantly smaller than those observed in the absence of the two adrenergic blockers. For all practical purposes, nifedipine completely abolished the increase in peak tension developed induced by cocaine. It is concluded that the positive inotropic effect of cocaine in the guinea pig atrial muscle is predominantly the result of adrenergic-dependent, both alpha- and beta- receptor mediated, as well as adrenergic-independent increases in calcium influx through the L-type calcium channels in the sarcolemma.

Myocardial contractile response and IP3, cAMP and cGMP interrelationships

An experimental study in the perfused working normal and pressure overloaded rat heart. A mini review based on a doctoral thesis.

Phenylephrine plus propranolol improves the balance between myocardial oxygen supply and demand during experimental cardiopulmonary resuscitation

Epinephrine increases coronary blood flow but may not improve the balance between myocardial oxygen supply and demand during cardiopulmonary resuscitation (CPR). The objective of this study was to determine whether this balance can be improved by administering a relatively pure alpha-adrenergic vasoconstrictor, alone or in combination with a beta-adrenergic blocker. We measured coronary perfusion pressures during CPR and myocardial adenosine 5'-triphosphate (ATP) and lactate concentrations in biopsies obtained immediately after 10 minutes of CPR in six control dogs and in three groups of six dogs each given large doses of epinephrine, phenylephrine, or phenylephrine plus propranolol during CPR. Coronary perfusion pressure during CPR was higher in the three treated groups than in the control group, although differences were limited to the early portion of CPR in dogs given epinephrine or phenylephrine alone. Postresuscitation myocardial ATP concentration was significantly higher (29.5 +/- 3.0 vs 22.6 +/- 1.8 nmol/mg of protein, p < 0.05) and myocardial lactate concentration tended to be lower (52.8 +/- 13.6 vs 78.5 +/- 15.2 nmol/mg of protein) than in the control group in dogs given both phenylephrine and propranolol. In contrast, myocardial ATP concentration tended to be lower than in the control group in epinephrine-treated dogs, and myocardial lactate concentrations were higher than in the control group in dogs treated with either epinephrine (p < 0.05) or phenylephrine alone (p = 0.052). We conclude that the balance between myocardial oxygen supply and demand during CPR can be improved by administering a combination of phenylephrine and propranolol, but not by administering large doses of epinephrine or phenylephrine alone.

Relationship between intracellular calcium and oxygen consumption: effects of perfusion pressure, extracellular calcium, dobutamine, and nifedipine

All of the mechanisms that connect the cardiac mechanical work load with energy production have not been clearly defined. The purpose of this study was to evaluate the relationship between intracellular calcium and oxygen consumption in intact hearts, to further understand this relationship. Intracellular calcium was measured in isolated nonworking perfused rat hearts loaded with Indo-1 by means of a surface fluorometry technique. Glucose was used as a substrate. Myocardial contraction and oxygen consumption were modulated by perfusion pressure (80, 110, and 140 cm of water), extracellular calcium (1, 2, 3, and 4 mmol/L), dobutamine (10(-6) mol/L), and nifedipine (10(-6) mol/L). With all of these interventions there was a close correlation between intracellular calcium (systolic, diastolic, and amplitude) and oxygen consumption or left ventricular developed pressure. Observations in this study support the hypothesis that intracellular calcium plays a regulatory role in the link between cardiac mechanics and energy production.

Myocardial [3H]polyinositol phosphates and their response to burn trauma

Polyinositol phosphates comprise a portion of the phosphatidyl signal transducing system. The most well known is IP3 which stimulates Ca2+ release from Ca2+ sequestering organelles within cells. In this study, polyinositol phosphate changes in the heart subjected to the systemic effects of burn trauma were examined. The hypothesis was that systemic trauma induced by large body surface area (% BSA) burn may perturb the phosphatidylinositol signal transducing system. At postburn day 21 left ventricular tissues were harvested from mice with varying burn sizes (i.e. 0, 20 and 50 per cent). Levels of the polyinositol phosphates were measured by incorporation of myo-[2-3H]inositol with separation of the phosphates by anion-exchange chromatography. Analysis of variance was used for statistical evaluation. Multivariate relationships between the independent polyinositol forms (inositol, Il,4P2 and I1P) existed for control (r2 = 0.71 and 50 per cent burn groups (r2 = 0.78). Numerous interdependent relationships existed within each of the multivariate tests. These analyses confirm that several independent polyinositol phosphates contribute to changes in the second messenger IP3 in ventricular tissue subjected to the systemic effects of burn trauma. Disruption of the polyinositol phosphates may underlie either the cause, or exacerbation, of heart dysfunction and cellular damage in burn patients.

Different effects of alpha- and beta-adrenergic stimulation on cytosolic pH and myofilament responsiveness to Ca2+ in cardiac myocytes

alpha-Adrenergic stimulation (alpha-AS) and beta-adrenergic stimulation (beta-AS) of the myocardium are associated respectively with an increase and a decrease in myofilament responsiveness to Ca2+. We hypothesized that changes in cytosolic pH (pH(i)) may modulate these opposite actions of alpha-AS and beta-AS. The effects of alpha-AS (50 microM phenylephrine and 1 microM nadolol) and beta-AS (0.05 microM isoproterenol) on contraction and either cytosolic Ca2+ (Cai) or pH(i) were assessed in adult rat ventricular myocytes bathed in bicarbonate buffer (pH 7.36 +/- 0.05). In cells loaded with the ester derivative (AM form) of indo-1, the 410/490-nm ratio of emitted fluorescence indexed Cai. Myofilament responsiveness to Ca2+ was assessed by the relaxation phase of the length-indo-1 fluorescence relation during a twitch. alpha-AS and beta-AS shifted this relation in opposite directions, indicating that alpha-AS increased and beta-AS decreased myofilament responsiveness to Ca2+. In addition, the positive inotropic action of alpha-AS was associated with an increased Cai transient amplitude in 50% of the myocytes (n = 12), whereas beta-AS always increased Cai (n = 5). In cells loaded with the fluorescent pH(i) probe SNARF-1 AM, the emitted 590/640-nm fluorescence is a measure of pH(i). The effect of alpha-AS on the extent of cell shortening during the twitch (ES) was expressed as the percentage of resting cell length. Both ES and pH(i) were assessed in myocytes bathed in 1.5 mM [Ca2+] and stimulated at 0.5 Hz (control ES, 7.4 +/- 1.5%; control pH(i), 7.11 +/- 0.05; n = 10). alpha-AS enhanced both ES (delta ES, 1.8 +/- 0.6%; p less than 0.05) and pH(i) (delta pH(i), 0.06 +/- 0.01; p less than 0.005), and there was a significant correlation between delta ES and delta pH(i) (r = 0.76, p less than 0.05). A similar effect of alpha-AS on pH(i) was observed in the absence of electrical stimulation (n = 8). The alpha-AS-induced enhancement of ES and pH(i) was abolished by 10 microM ethylisopropylamiloride, a Na(+)-H+ exchange inhibitor (n = 7). In additional experiments, myocytes were preincubated either with 0.2 microM 4 beta-phorbol 12-myristate 13-acetate (n = 8) or with 5 nM staurosporine (n = 8), which have been shown to downregulate and inhibit Ca(2+)-activated phospholipid-dependent protein kinase C, respectively. In either group, alpha-AS had no effect on pH(i) and decreased ES to approximately 60% of control.(ABSTRACT TRUNCATED AT 400 WORDS)

Beta- and alpha-adrenoceptor-agonists and -antagonists in chronic heart failure

In patients with chronic heart failure, cardiac beta-adrenoceptor function is decreased, and this decrease is related to the degree of heart failure. Under these conditions, treatment with beta-adrenoceptor agonists seems to be of limited value as it might further down-regulate cardiac beta-adrenoceptors, resulting, finally, in a loss of therapeutic efficacy. However, beta-adrenoceptor antagonists might have beneficial effects, because they can protect the myocardium from the deleterious effects of elevated endogenous catecholamines and can, simultaneously, restore the previously down-regulated beta-adrenoceptor function. Stimulation of cardiac alpha-adrenoceptors, however, seems not to be of any therapeutic value in patients with chronic heart failure, because a) the number of alpha-adrenoceptors in the human heart is very low and its function is not completely understood, and b) no alpha-adrenoceptor agonist is presently available that selectively stimulates cardiac alpha-adrenoceptors without concomitantly activating vascular alpha-adrenoceptors. In acute myocardial ischemia, cardiac beta-adrenoceptors increase; this increase is--at least in early acute myocardial ischemia--accompanied by an increased beta-adrenoceptor functional responsiveness; thus, under these conditions, beta-adrenoceptor agonists again might not be of clinical value, while beta-adrenoceptor antagonists may exert beneficial effects, because they can block (over)activation of the sensitized beta-adrenoceptors by elevated endogenous catecholamines.

Physiology and pharmacology of cardiovascular catecholamine receptors: implications for treatment of chronic heart failure

In the sympathetic nervous system the physiologic effects of the endogenous catecholamines noradrenaline (NA) and adrenaline (A) are mediated by alpha- and beta-adrenoreceptors (ARs). Both AR-types can be subdivided into two major subtypes: alpha-ARs into alpha-1 (predominant effect: vasoconstriction) and alpha-2 (presynaptic: inhibition of NA-release; postsynaptic: vasoconstriction), beta-ARs into beta-1 (cardiac effects, renal renin release, and lipolysis) and beta-2 (presynaptic: facilitation of NA-release; postsynaptic: vascular, bronchial, and uterine smooth muscle relaxation, glycogenolysis and possibly part of the A-mediated cardiac effects). During the last 30 years growing evidence has accumulated that dopamine (DA), the third endogenous catecholamine and the immediate precursor of NA, may also cause peripheral effects through stimulation of specific DA-receptors, in addition to its known action at alpha- and beta-ARs. It is now well accepted that at least two different DA-receptors are present in many peripheral tissues (DA1 and DA2), including those of the cardiovascular and autonomic nervous system. They seem to be involved in dilation of certain vascular beds, inhibition of NA-release during nerve stimulation, natriuresis, and aldosterone release. In chronic heart failure cardiac beta-AR function decreases (presumably due to endogenous "down-regulation" by the elevated catecholamines), and this decrease is related to the severity of heart failure (judged clinically by New York Heart Association functional class). The human heart contains both functional beta-1 and beta-2 ARs; cardiac beta-1 and beta-2 ARs seem to be differentially affected by different kinds of heart failure; in end-stage dilated cardiomyopathy beta-1 ARs are selectively reduced, whereas beta-2 ARs are nearly normal.(ABSTRACT TRUNCATED AT 250 WORDS)