Hypertrophic cardiomyopathy: a disease of impaired adenosine-mediated autoregulation of the heart

Adenosine and ventricular automaticity

Adenosine is considered a cardiodepressant agent due to its negative chronotropic and inotropic effects. However, the effect of adenosine on ventricular automaticity is less well established since both an increase and a decrease in ventricular automaticity have been reported. The experimental and clinical evidence dealing with the effects of adenosine on ventricular automaticity as well as the possible mechanisms involved, is presented in this review.

Effects of adenosine on human coronary arterial circulation

Adenosine is a potent vasodilator used extensively to study the coronary circulation of animals. Its use in humans, however, has been hampered by lack of knowledge about its effects on the human coronary circulation and by concern about its safety. We investigated in humans the effects of adenosine, administered by intracoronary bolus (2-16 micrograms), intracoronary infusion (10-240 micrograms/min), or intravenous infusion (35-140 micrograms/kg/min) on coronary and systemic hemodynamics and the electrocardiogram. Coronary blood flow velocity (CBFV) was measured with a 3F coronary Doppler catheter. The maximal CBFV was determined with intracoronary papaverine (4.5 +/- 0.2.resting CBFV). In normal left coronary arteries (n = 20), 16-micrograms boluses of adenosine caused coronary hyperemia similar to that caused by papaverine (4.6 +/- 0.7.resting CBFV). In the right coronary artery (n = 5), 12-micrograms boluses caused maximal hyperemia (4.4 +/- 1.0.resting CBFV). Intracoronary boluses caused a small, brief decrease in arterial pressure (similar to that caused by papaverine) and no changes in heart rate or in the electrocardiogram. The duration of hyperemia was much shorter after adenosine than after papaverine administration. Intracoronary infusions of 80 micrograms/min or more into the left coronary artery (n = 6) also caused maximal hyperemia (4.4 +/- 0.1.resting CBFV), and doses up to 240 micrograms/min caused a minimal decrease in arterial pressure (-6 +/- 2 mm Hg) and no significant change in heart rate or in electrocardiographic variables. Intravenous infusions in normal patients (n = 25) at 140 micrograms/kg/min caused coronary vasodilation similar to that caused by papaverine in 84% of patients (4.4 +/- 0.9.resting CBFV). At submaximal infusion rates, however, CBFV often fluctuated widely. During the 140-micrograms/kg/min infusion, arterial pressure decreased 6 +/- 7 mm Hg, and heart rate increased 24 +/- 14 beats/min. One patient developed 1 cycle of 2:1 atrioventricular block, but otherwise, the electrocardiogram did not change. In eight patients with microvascular vasodilator dysfunction (delta CBFV, less than 3.5 peak/resting velocity after a maximally vasodilating dose of intracoronary papaverine), the dose-response characteristics to intracoronary boluses and intravenous infusions of adenosine were similar to those found in normal patients.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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.

ATP and other adenine compounds increase mechanical activity and inositol trisphosphate production in rat heart

1. The effects of adenosine 5'-triphosphate (ATP) and other adenine compounds were examined on rat papillary and right ventricular muscles in the presence of 10 microM-propranolol, 10 microM-atropine and 0.1 microM-prazosin or 10 microM-phentolamine. 2. Adenosine, adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), ATP and alpha,beta-methylene ATP (APCPP) produced small positive inotropic effects, sometimes preceded by transient negative effects. 3. 8-Phenyltheophylline (8-PT), a P1-purinoceptor antagonist antagonized the negative effects and increased the positive inotropy induced by ATP and adenosine. 4. In the presence of APCPP, a P2-purinergic agonist, ATP had only negative inotropic effects. 5. Adenosine and ATP increased inositol 1, 4, 5- and inositol 1, 3, 4-trisphosphate as well as inositol mono- and bisphosphate formation. Maximal effects were obtained at concentrations of 0.5 mM. 6. APCPP increased inositol phosphate formation while 8-PT did not prevent the effects of adenosine and ATP. 7. It is suggested that P2-purinoceptor activation induces both a positive inotropy and an increase in inositol-lipid metabolism in rat ventricular muscles.

[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.

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.

Human lymphocyte antigens in hypertrophic cardiomyopathy

An increasing number of genetic studies in hypertrophic cardiomyopathy challenge conventional views on inheritance and suggest genetic heterogeneity or non-genetic disease. We have found changes in relative risk for some antigens with significantly increased frequency of HLA antigen DR4 in this condition. These findings are consistent with there being a genetic component in susceptibility to hypertrophic cardiomyopathy. No evidence was found for HLA linkage using either sib pair analysis or lod scores. This suggests that hypertrophic cardiomyopathy does not have a disease susceptibility gene related to the HLA region on the short arm of chromosome number six. Population HLA associations with hypertrophic cardiomyopathy must thus be explained by other influences of the genetic background on disease susceptibility.

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.

Functional evidence for the existence of adenosine receptors in the human heart

Adenosine added to isolated electrically driven preparations of human ventricular heart muscle antagonized the positive inotropic effect of isoprenaline (mean EC50 19 mumol 1(-1), n = 9). Similar effects were observed with the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (mean EC50 0.5 mumol 1(-1), n = 7). These data provide functional evidence for the existence of adenosine receptors in the human myocardium which may modulate the force of contraction during beta-adrenergic stimulation and thus could be involved in the autoregulation of myocardial contractility.

Sick sinus syndrome: an adenosine-mediated disease

It is suggested that adenosine is a mediator of the paroxysmal bradycardias and tachycardias characterising the sick sinus syndrome. Increased sensitivity to adenosine could account for the arrhythmias, which might therefore be suppressed by an adenosine antagonist such as theophylline.