Evidence for alpha 1-adrenoceptor-mediated increase of inositol trisphosphate in the human heart

Lithium interacts with cardiac remodeling: the fundamental value in the pharmacotherapy of bipolar disorder

Patients with bipolar disorder (BD) have an increased risk of cardiovascular morbidity and mortality during the course of their illness. For over half a century, lithium has been the gold-standard medication used to treat the mood burdens of BD. In addition, lithium possesses several biological effects that may modulate cardiovascular risk in patients with BD. In this review, we update the current knowledge of cellular and molecular mechanisms underlying the possible cardiac actions of lithium. The mechanistic insights suggest that lithium at therapeutic levels potentially exerts cardioprotective effects on ischemic hearts by modulating structural and electrical remodeling. The possible cardioprotective actions of lithium may involve an extensive range of signaling pathways, including the Wnt/glycogen synthase kinase-3β, phosphatidylinositol-3-kinase/protein kinase B, phosphoinositide/protein kinase C, and mitogen-activated protein kinase/extracellular signal-regulated kinase cascades. Accordingly, understanding the cardioprotective effects of lithium may lead to the development of a potential strategy for reducing cardiovascular morbidity in patients with BD.

Chiral recognition of doxazosin enantiomers in 3 targets for therapy as well as adverse drug reactions in animal experiments

Doxazosin used in benign prostatic hyperplasia has the side effects of causing hypotension and the risk of heart failure. The 3 targets of α1A-adrenoceptors (in the prostate), α1D-adrenoceptors (in the aorta), and an unknown mechanism (in the heart) are involved, respectively. We hypothesized that there is a chiral recognition of doxazosin enantiomers in the 3 targets. Using isolated rat aorta (α1D-adrenoceptors) and rabbit prostate (α1A-adrenoceptors), we examined pA2 and pKB values of doxazosin enantiomers. We observed chronotropic and inotropic effects of doxazosin enantiomers in isolated rat and rabbit heart tissues. (–)Doxazosin and (+)doxazosin produced a shift to the right of concentration–contraction curves for noradrenalin (aorta) and phenylephrine (prostate smooth muscle). The pA2 value of (–)doxazosin (8.625 ± 0.053) was smaller than (+)doxazosin (9.503 ± 0.051) in rat aorta, but their pKB values in rabbit prostate were the same. In rat and rabbit heart tissues, (+)doxazosin (3–30 µmol·L−1) significantly decreased atrial rate, and produced negative inotropic effects; however, (–)doxazosin did not affect the atrial rate, and produced positive inotropic effects in the atria. Thus, the chiral carbon atom of doxazosin does not affect its activity at the therapeutic target of α1A-adrenoceptors in the prostate, but significantly changes its blocking activity against α1D-adrenoceptors in the aorta, and produces opposite inotropic effects in the atria via an α1-adrenoceptor-independent mechanism.

Cardiovascular α1-adrenoceptor subtypes: functions and signaling

α1-Adrenoceptors (α1AR) are G protein-coupled receptors and include α1A, α1B, and α1D subtypes corresponding to cloned α1a, α1b, and α1d, respectively. α1AR mediate several cardiovascular actions of sympathomimetic amines such as vasoconstriction and cardiac inotropy, hypertrophy, metabolism, and remodeling. α1AR subtypes are products of separate genes and differ in structure, G protein-coupling, tissue distribution, signaling, regulation, and functions. Both α1AAR and α1BAR mediate positive inotropic responses. On the other hand, cardiac hypertrophy is primarily mediated by α1AAR. The only demonstrated major function of α1DAR is vasoconstriction. α1AR are coupled to phospholipase C, phospholipase D, and phospholipase A2; they increase intracellular Ca2+ and myofibrillar sensitivity to Ca2+ and cause translocation of specific phosphokinase C isoforms to the particulate fraction. Cardiac hypertrophic responses to α1AR agonists might involve activation of phosphokinase C and mitogen-activated protein kinase via Gq. α1AR subtypes might interact with each other and with other receptors and signaling mechanisms.Key words: cardiac hypertrophy, inotropic responses, central α1-adrenoreceptors, arrythmias.

Methoxamine inhibits transient outward potassium current through alpha1A-adrenoceptors in rat ventricular myocytes

alpha1-Adrenoceptor agonists are known to reduce transient outward potassium current (I(to)) in the heart. The aim of this study was to analyze the effect of methoxamine (mtx) on I(to) and to elucidate which adrenoceptor subtype was involved in this effect. We used the whole-cell configuration of the patch-clamp technique to record I(to). Our experiments confirm that mtx induces a dose-dependent decrease of I(to) that is characterized by an acceleration of time to peak (3.5 +/- 0.2 and 2.3 +/- 0.3 ms for control and mtx, respectively), and a decrease in both inactivation time constants (T(fast) was reduced from 20.8 +/-2.6 to 14.9 +/- 1.1 ms, and tau(slow) was reduced from 138 +/- 32.1 to 114 +/- 28.7 ms; n = 7). All these effects were antagonized by prazosin and the alpha1A-antagonist 5-methylurapidil but not by the irreversible alpha1B-antagonist chloroethylclonidine. These data indicate that stimulation of alpha1A-adrenoceptor subtype is involved in the methoxamine-induced reduction of I(to) in rat ventricular myocytes.

Myocardial alpha1-adrenoceptor: inotropic effect and physiologic and pathologic implications

Alpha1-adrenergic receptors have been found in myocardium of all mammalian species. Although the exact underlying mechanisms have not been conclusively determined, it would appear that the myocardial effects of alpha1-adrenoceptors may vary in importance according to the pathophysiologic process involved. In physiological conditions, this receptor system plays a role in cardiac growth, cardiac contraction, and has both an antiarrhythmic function as well as a role in cardiac adaptation to various situations. This system is also involved in some pathological processes such as ischemia/reperfusion, ischemic preconditioning, and cardiac hypertrophy. The role of alpha1-adrenoceptors in heart failure is somewhat controversial. Experimental evidence suggests that myocardial alpha1-adrenoceptors can have either beneficial or deleterious effects on the heart. It thus seems possible that the development of agents specific to certain subtypes of alpha1-adrenoceptor and a better understanding of their role in pathophysiologic states could be clinically relevant.

Inositol-1,4,5-trisphosphate mass content in isolated perfused rat heart during alpha-1-adrenoceptor stimulation

Inositol-1,4,5-trisphosphate (IP3) has been proposed to be a second messenger in response to alpha-1-adrenoceptor stimulation also in myocardial cells. We studied the effect of alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine or 5 x 10(-5) mol/l noradrenaline both in the presence of 10(-6) mol/l timolol) on IP3 mass content in isolated perfused rat hearts. IP3 content was determined by a specific receptor-binding assay-kit (TRK 1000, Amersham) after validating the method. For comparison also the effect of muscarinic stimulation (10(-4) mol/l carbachol in the presence of 10(-6) mol/l timolol) on IP3 content was measured in corresponding preparations. A basal IP3 level of about 75 pmol/mg protein was found. There were no prominent effects of alpha-1-adrenoceptor stimulation on total IP3 content in isolated perfused rat hearts. Phenylephrine gave a statistically significant increase of about 40% at 1/4 min and a statistically significant decrease of about 25% at 4 min after start of exposure. Noradrenaline, however, gave no statistically significant change of IP3 at the time-points studied. Muscarinic stimulation caused a slight, statistically insignificant, increase of IP3 at 1/4 min. The results are compatible with an assumption that agonist stimulation evokes a localized increase of IP3 which may be masked by a relatively high total IP3 mass content. The IP3 peak after phenylephrine coincided with the early positive inotropic phase of the response reported earlier in perfused rat hearts for alpha-1-adrenoceptor stimulation by phenylephrine. Although this might be compatible with a role for IP3 in this early and transient phase, a mediator function of IP3 in the inotropic response is not established.

The norepinephrine-stimulated inositol phosphate response in human atria

Inositol phosphate release and metabolism were studied in right atrial appendages obtained from 18 patients undergoing coronary artery bypass surgery and/or mitral valve replacement. [3H]Inositol-labeled human atria contained inositol(1,4. 5)trisphosphate, inositol(1,4)bisphosphate and the 1- (or 3) and 4-isomers of inositol monophosphate. Addition of norepinephrine (100 mumol/l) activated the release of inositol phosphates, as indicated by increased [3H]inositol label in all of these inositol phosphates. However, the phosphorylation product of inositol (1.4.5)trisphosphate, inositol-(1,3,4,5)tetrakisphosphate, and its metabolic products were not detected, either in control or stimulated atria. Similar inositol phosphate profiles were observed in rat right atria. Furthermore, both human and rat atria contained high concentrations of inositol(1,4,5)trisphosphate, which were not observed to increase with norepinephrine stimulation. The inositol phosphate responses to norepinephrine in rat and human cardiac tissue appear to be similar, except for the generally lower activity observed in human tissue. Thus, the rat provides a suitable model for the study of cardiac phosphatidylinositol turnover.

Prazosin reduces myocardial ischemia/reperfusion-induced Ca2+ overloading in rat heart by inhibiting phosphoinositide signaling

The aim of this study was to establish whether or not alpha 1-adrenergic receptors are implicated in triggering phosphoinositide hydrolysis and intracellular Ca2+ accumulation during myocardial ischemia and reperfusion. In isolated perfused rat hearts, the selective alpha 1-receptor antagonist prazosin abolished the increase in radioactivity incorporation into cellular inositol phosphates induced by 30 min ischemia followed by 30 min reperfusion, and selectively blocked the degradation of phosphoinositides; only minor changes in the ischemia/reperfusion-induced loss of other classes of phospholipids were seen. In addition, a prazosin-induced decrease of ischemia/reperfusion Ca2+ overloading was documented in real-time recordings of epicardial cytosolic free Ca2+ in fura 2-loaded hearts. An inhibition of early ischemic Ca2+ rise was observed, as well as a lower peak of cytosolic free Ca2+ and a more rapid reversal to normal values during reperfusion. Moreover, alpha 1-adrenergic blockade resulted in a significant improvement in the recovery of myocardial function during reperfusion: an increased left ventricular developed pressure and maximum rate of rise of systolic pressure paralleled the decrease in time-averaged cytosolic Ca2+ and the increase in amplitude of Ca2+ transients, respectively. It is concluded that myocardial Ca2+ overloading during ischemia and reperfusion may be triggered by alpha 1-adrenergic receptor-induced polyphosphoinositide hydrolysis.

Response of the rat heart to catecholamines and thyroid hormones

Catecholamines and thyroid hormones have a similar influence on heart function and metabolism, but this may occur in a differential manner and to a different extent. In this study, the effects of norepinephrine (NE) and of triiodothyronine (T3) were studied in regard to the function of the left (LV) and right ventricle (RV) and to the oxidative pentose phosphate pathway (PPP). NE was applied in rats as continuous i.v. infusion (0.2 mg/kg/h) for three days. T3 was given as daily s.c. injections (0.2 mg/kg) for the same period of time. LV and RV function was measured in the closed-chest trapanal-anesthetized animals using special Millar ultraminature catheter pressure transducers. NE induced an increase in heart rate, in mean arterial pressure, and in total peripheral resistance (TPR). The cardiac RNA/DNA and the left ventricular weight/body weight ratios were increased by about 40%. These effects were prevented by simultaneous alpha- and beta-receptor blockade with prazosin and metoprolol, respectively, but not by verapamil which abolished the hemodynamic effects. RVSP was significantly elevated by NE in a dose-dependent manner. The functional effects of T3 on the LV were not as pronounced as those induced by NE. Heart rate and LV dp/dtmax were increased by T3, and this increase was prevented by concomitant beta-receptor blockade with metoprolol. In contrast to NE, T3 induced an increase in cardiac output and a concomitant decrease in TPR. The RNA/DNA ratio was elevated and cardiac hypertrophy had developed after treatment for three days with T3. These changes were not affected by beta-receptor blockade with metoprolol. RVSP was increased by T3 to a lesser extent than with NE. In metabolic terms it turned out that only NE, but not T3 had a stimulating effect on the cardiac PPP. NE increased the mRNA and activity of glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of this pathway. However, there was no effect of T3 on G-6-PD activity nor on 6-phosphogluconate dehydrogenase activity, one of the following enzymes in the pathway within the first 5 days of T3 treatment. These results demonstrate that the functional effects of T3 were not as pronounced as or even different from those of NE, and that T3 lacked a stimulating effect on the cardiac PPP.

Effects of isoprenaline, phenylephrine on heart and influence of nifedipine on these effects

In a perfused isovolumetrically contracting rat heart model, the effects of isoprenaline (IPN) and phenylephrine (PE) on myocardial contraction and relaxation were investigated, and the influence of nifedipine on these effects was studied. Both IPN and PE increased the myocardial contraction and improved its relaxation, but some differences existed. Nifedipine (10 nmol/L) substantially inhibited the PE-mediated inotropic effect, but in case of IPN-mediated inotropic ones, it did not. It was assumed that there may be various types of slow channels, one was activated by IPN, and the other, by PE.

Spontaneous and renal hypertensive rats: responsiveness of isolated right and left atria to noradrenaline, isoprenaline and methoxamine

The adrenergic responsiveness of right and left atria isolated from spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR) was studied. Right atria isolated from SHR showed subsensitivity to the chronotropic effect of noradrenaline (21.3-fold at the EC50 level, P < 0.05) and isoprenaline (12.0-fold, P < 0.05). However, atria isolated from RHR did not exhibit any significant alteration (P > 0.05) in sensitivity to the chronotropic effect of noradrenaline or isoprenaline. Chronotropic responsiveness to theophylline was not altered in right atria isolated from SHR or RHR. alpha 1-Mediated inotropic responses to noradrenaline and methoxamine were blunted in left atria isolated from SHR. Left atria isolated from RHR showed supersensitivity to the inotropic effect of noradrenaline (5.4-fold at the EC50 level, P < 0.05) and subsensitivity to the inotropic effect of methoxamine (6.0-fold, P < 0.05). It is concluded that the present results could, at least partially, explain the reduced cardiac output observed during established hypertension in SHR and the increased cardiac output observed in the initial phase of renal hypertension in RHR.

Preconditioning against myocardial dysfunction after ischemia and reperfusion by an alpha 1-adrenergic mechanism

Preconditioning may find ready applicability in humans facing scheduled global cardiac ischemia-reperfusion (IR) during bypass or transplantation, where such a maneuver is feasible before arrest. Our objective was to delineate and exploit the endogenous preconditioning mechanism triggered by transient ischemia (TI) and thereby attenuate myocardial postischemic mechanical dysfunction by clinically acceptable means. Preconditioning by 2 minutes of TI followed by 10 minutes of normal perfusion protected isolated rat left ventricle function assessed after 20 minutes of global, 37 degrees C ischemia and 40 minutes of reperfusion. Final recovery of developed pressure (DP) was improved (91.5 +/- 1.9% of equilibration DP versus unconditioned IR control, 57.4 +/- 2.4%, P < .01) and was accompanied by increased contractility (+/- dP/dt). Norepinephrine release increased after TI, and reserpine pretreatment abolished TI preconditioning. This suggests that endogenous norepinephrine mediates functional preconditioning in rat. Brief pretreatment (2 minutes) with exogenous norepinephrine reproduced the protection (89.1 +/- 1.4%) of postischemic function. Functional protection persisted after the hemodynamic effects had resolved. Norepinephrine-induced preconditioning was simulated by phenylephrine and blocked by alpha 1-adrenergic receptor antagonist. TI preconditioning was similarly lost after selective alpha 1-adrenergic receptor blockade. We conclude that transient ischemic preconditioning is mediated by the sympathetic neurotransmitter release and alpha 1-adrenergic receptor stimulation. Although the postreceptor mechanism remains unclear, functional protection after IR does not seem related to the magnitude of ATP depletion and elevation of resting pressure during ischemia. Rather, the endogenous mechanisms facilitate both recovery of mechanical function and ATP repletion during reperfusion.

Phosphatidylinositol metabolism in hypertrophic rat heart

The accumulation of inositol 1,4,5-trisphosphate (IP3) after hormonal stimulation has a physiological role, possibly by alteration of Ca2+ levels in cardiac myocyte. However, this accumulation has not been studied under pathophysiological conditions. In this report, we examine phosphatidylinositol metabolism during cellular response to norepinephrine in pressure-overloaded hypertrophic rat heart. After stimulation with norepinephrine, the accumulations of IP3 and diacylglyceride significantly increased in isolated myocytes from stroke-prone spontaneously hypertensive rat (SHRSP) heart, indicating phosphatidylinositol-specific phospholipase C activity increased in SHRSP heart cells. Protein kinase C activity was also enhanced in SHRSP, with a marked increase in particulate activity. We determined the intracellular calcium concentration and found it to be higher in SHRSP than in Wistar-Kyoto (WKY) rats at 30-40 weeks of age. Ca2+ influx was also elevated in SHRSP stimulated by norepinephrine. In SHRSP heart, cytosolic Ca2+ concentration may rise quickly in response to some stimuli, such as alpha 1-adrenergic stimulation, which is shown to be one of the pathways that increases cytosolic Ca2+ levels in hypertrophied rat heart. These data suggest that a part of the phosphatidylinositol-turnover pathway, such as the phosphatidylinositol 4,5-bisphosphate-IP3-Ca2+ pathway or the diacylglyceride-protein kinase C pathway, may play an important role in the development of hypertrophy in SHRSP heart.

Comparison of the positive inotropic effects of serotonin, histamine, angiotensin II, endothelin and isoprenaline in the isolated human right atrium

The receptor systems through which serotonin (5-HT), histamine, angiotensin II and endothelin increase the force of contraction were studied in isolated right atria from patients without apparent heart failure. All agonists increased the atrial force of contraction in a concentration-dependent manner; maximal effects, however, were significantly less than those evoked by isoprenaline or Ca2+. 5-HT and histamine, but not angiotensin II and endothelin, activated adenylate cyclase, whereas endothelin and angiotensin II stimulated inositol phosphate generation. Experiments with subtype-selective antagonists revealed that histamine effects were mediated by H2-receptors (sensitive to ranitidine), 5-HT-effects by 5-HT4-receptors (sensitive to SDZ 205-557) and angiotensin II effects by AT1-receptors (sensitive to losartan). We conclude that in human right atria the force of contraction can be increased by cyclic AMP-dependent (histamine, 5-HT) and -independent (angiotensin II, endothelin) pathways. Compared to beta-adrenoceptors, however, all other receptor systems increase the force of contraction only submaximally indicating that the beta-adrenoceptor pathway is the most important physiological mechanism to regulate force of contraction and/or heart rate in the human heart.

Alteration of left ventricular endocardial function by intracavitary high-power ultrasound interacts with volume, inotropic state, and alpha 1-adrenergic stimulation

BACKGROUND

High-power intracavitary ultrasound abbreviates left ventricular (LV) ejection duration, thereby decreasing mechanical LV performance, presumably by selective impairment of endocardial endothelial function.

METHODS AND RESULTS

Effects of ultrasound were evaluated in the ejecting LV of anesthetized, open-chest dogs under different conditions of LV volume and contractile state and after mild selective alpha 1-adrenergic stimulation. LV pressures, left atrial pressures, and regional segment lengths were measured in anterior and posterior midwall. A cylindrical ultrasound probe (0.9 MHz, 25 W) mounted on a catheter was inserted into the LV cavity through the apex and was activated for 4 minutes in each condition. In protocol A (n = 7), LV volume was altered with caval vein occlusion and intravenous dextran infusion. The ultrasound probe was activated at low (4.1 +/- 0.9 mm Hg), mid (10.6 +/- 1.5 mm Hg), and high (17.9 +/- 1.8 mm Hg) LV end-diastolic pressure (EDP). Effects of ultrasound were less pronounced at higher EDP. For example, the time interval from end-diastole to peak (-)dP/dt decreased by 7.5 +/- 2.3% at low, 4.4 +/- 2.2% at mid, and 1.9 +/- 1.6% at high LVEDP (p < 0.001). In protocol B (n = 7), LV inotropic state was altered by slow intravenous infusion of low-dose calcium. The ultrasound probe was activated before and after calcium. Effects of ultrasound were less pronounced after calcium. Time from end-diastole to peak (-)dP/dt decreased by 8.4 +/- 3.1% at baseline and by 3.5 +/- 2.1% after calcium (p < 0.001). In protocol C (n = 7), activation of the ultrasound probe was performed at baseline and after mild selective alpha 1-adrenergic stimulation (propranolol plus phenylephrine). Effects of ultrasound were similar at baseline and after propranolol but increased after phenylephrine. Time from end-diastole to peak (-)dP/dt decreased by 5.2 +/- 2.4% at baseline, by 5.3 +/- 1.9% after propranolol, and by 8.9 +/- 3.2% after phenylephrine (p < 0.05).

CONCLUSIONS

Effects of intracavitary ultrasound, which are presumably mediated through modulation of endocardial endothelial function, were more important at low volume, lower calcium, and under mild selective alpha 1-adrenergic stimulation.

Increased calcium release from sarcoplasmic reticulum stimulated by inositol trisphosphate in spontaneously hypertensive rat heart cells

It is known that inositol (1, 4, 5)-trisphosphate (IP3) stimulates Ca2+ release from sarcoplasmic reticulum (SR) in several tissues, but in cardiac myocytes this phenomenon has not been confirmed. The purpose of the present study was to confirm the effect of (1, 4, 5)-IP3 on Ca2+ release from SR in cardiac myocytes. The effect of IP3 on Ca2+ release from SR in hypertrophic cardiac cells was also determined. We examined the effects of IP3 on Ca2+ release from cardiac myocyte SR by the digital-image method in a single cell. We also determined the effect of IP3 on calcium release from isolated SR. SR was prepared from spontaneous hypertensive rat hearts and Wistar kyoto rat hearts. The SR was prelabeled with 45Ca2+, and then incubated with the indicated concentrations of IP3 for 1 min at 37 degrees C. In cardiac myocytes treated with saponin, Ca2+ release stimulated by 10 microM (1, 4, 5)-IP3 was detected by fura-2. In 45Ca2+ prelabeled SR, the maximal Ca2+ release was achieved at 10 microM IP3 incubated for 1 min. The release of Ca2+ was higher in SR of SHR than in the SR of WKY. IP3 stimulates Ca2-release from cardiac SR, and this release is greater in SHR than in WKY. However, it is uncertain whether this phenomenon plays a role in cardiac hypertrophy.

Effects of norepinephrine on the oxidative pentose phosphate pathway in the rat heart

To examine whether stimulation of alpha-adrenergic receptors may affect the oxidative pentose phosphate pathway (PPP) in the rat heart, norepinephrine (NE) and the alpha-adrenergic agonist norfenephrine were used. NE was administered as a continuous intravenous infusion in awake rats for 3 days. It stimulated the activity of cardiac glucose-6-phosphate dehydrogenase (G-6-PD), the first and regulating enzyme of the oxidative PPP, in a dose-dependent manner. With the highest dose (0.2 mg.kg-1.hr-1), there was also a time-dependent enhancement. The increase observed after 48 hours was attenuated partially by the beta-receptor blocker metoprolol and the alpha-receptor blocker prazosin. It was entirely abolished when both drugs were administered. Carvedilol, a beta-adrenergic blocker and vasodilator with alpha 1-blocking activity (0.5 mg.kg-1.hr-1), prevented the NE-induced increase in cardiac G-6-PD activity, in functional parameters (heart rate, left ventricular systolic pressure, and left ventricular dP/dtmax), and in the heart weight/body weight ratio. The alpha-adrenergic stimulator norfenephrine increased myocardial G-6-PD activity; prazosin prevented this stimulation. NE and norfenephrine also elevated the available pool of cardiac 5-phosphoribosyl-1-pyrophosphate. G-6-PD activity was enhanced in cardiac myocytes freshly isolated from the left ventricle of rats that had received NE infusion for 3 days (12.3 +/- 1.4 units/g protein) compared with control rats (1.5 +/- 0.4 units/g protein). The activity of 6-phosphogluconate dehydrogenase, one of the enzymes in the oxidative PPP, was elevated only moderately from 12.7 +/- 0.7 to 19.1 +/- 1.4 units/g protein. Combined alpha- and beta-receptor blockade with carvedilol attenuated these effects.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Receptor systems in the non-failing human heart

Catecholamines acting through beta 1- and beta 2-adrenoceptors cause positive inotropic and chronotropic effects in the human heart. In recent years, however, evidence has accumulated that in the human heart also other receptor systems can affect heart rate and/or contractility. Positive inotropic effects can be mediated by receptor systems acting through accumulation of intracellular cAMP (Gs-protein coupled receptors such as 5-HT4-like, histamine H2, and vasoactive intestinal peptide) or by receptor systems acting independent of cAMP possibly through the phospholipase C/diacylglycerol/inositol-1,4,5-trisphosphate pathway (such as alpha 1-adrenergic, angiotensin II, and endothelin). In the non-failing human heart, however, activation of all these receptor systems induces only submaximal positive inotropic effects when compared with those caused by beta-adrenoceptor stimulation, indicating that in humans the cardiac beta-adrenoceptor-Gs-protein-adenylate cyclase pathway is the most powerful mechanism to increase heart rate and contractility. On the other hand, at least three receptor systems acting through inhibition of cAMP formation (Gi-protein coupled receptors) exist in the human heart: muscarinic M2-, adenosine A1-, and somatostatin-receptors. Activation of M2- and A1-receptors causes negative inotropic effects in the non-failing human heart: in atria activation of both receptors causes decreases in basal as well as in isoprenaline-stimulated force of contraction, but in ventricles only isoprenaline-stimulated force of contraction is depressed.

Phospholipid metabolism in cardiomyopathic hamster heart cells

We demonstrated that the activities of phosphatidylinositide-specific phospholipase C, inositol 1,4,5-trisphosphate (IP3) kinase, and IP3 phosphatase were enhanced in cardiomyopathic hamster hearts (BIO 14.6 and BIO 53.58) in comparison to control hamsters (F1b). Release of both arachidonic acid and prostacyclin was markedly enhanced by norepinephrine in the cardiomyopathic hamsters. Phospholipase C in heart has high substrate specificity to phosphatidylinositol. IP3 production was markedly enhanced in the cardiomyopathic hamsters. We also determined the intracellular calcium concentration, which was higher in BIO 53.58 hamsters than in BIO 14.6 hamsters at 5-20 weeks of age. There was no significant difference in the intracellular calcium level between F1b and BIO 14.6 hamsters at 5 weeks of age. These results suggest that phosphatidylinositol turnover stimulated by norepinephrine may produce high intracellular calcium levels in both BIO 14.6 and BIO 53.58 myocytes. In addition, in BIO 53.58 hamsters, some mechanism such as the sarcoplasmic reticulum, which controls the intracellular calcium level, may deteriorate in function. We concluded from these results that a prolonged high intracellular calcium level may lead to the death of BIO 53.58 myocytes.

Phospholipid metabolism and prostacyclin synthesis in hypoxic myocytes

We observed that in hypoxic myocardial cells prostacyclin and arachidonic acid release increased and that during hypoxia phospholipid degradation also occurred. In order to clarify the mechanism of phospholipid degradation, we determined the activity of phospholipases A2 and C. We found that phosphatidylcholine (PC) and phosphatidylethanolamine (PE) were markedly decreased and that lysophosphatidylcholine and lysophosphatidylethanolamine were increased. In contrast, there was only slight phosphatidylinositol degradation and no lysophosphatidylinositol elevation was observed. These results show that phospholipase A2 was activated in hypoxic myocytes and had substrate specificity towards PC and PE. To study phospholipase C activity, membrane phospholipids were labeled with [3H]choline, [3H]inositol or [3H]ethanolamine. The release of inositol was observed, but neither choline nor ethanolamine was released. In hypoxia, myocardial-cell phospholipase C has high substrate specificity towards phosphatidylinositol. The activation of phospholipases is closely related to the intracellular Ca2+ concentration; it is though that inositol polyphosphatides may regulate intracellular Ca2+. We determined how Ca2+ influx occurs in hypoxia. beta-Adrenergic blockade and Ca2+ antagonists markedly suppressed Ca2+ influx, phospholipase A2 activity, phospholipase C activity and cell death. However, the alpha 1-adrenergic blockade was less effective in suppressing these phenomena. These results suggest that in hypoxic myocardial cells Ca2+ influx mediated by beta-adrenergic stimulation activates phospholipases A2 and C, and that phospholipid degradation and prostacyclin release then occur.

Myocardial alpha 1-adrenoceptors mediate positive inotropic effect and changes in phosphatidylinositol metabolism. Species differences in receptor distribution and the intracellular coupling process in mammalian ventricular myocardium

Species-dependent variations of myocardial alpha 1-adrenoceptor-mediated positive inotropic effects of epinephrine were assessed in relation to characteristics of alpha 1-receptor bindings and acceleration of phosphatidylinositol metabolism in the isolated rat, rabbit, and dog ventricular myocardium. Epinephrine in the presence of the beta-adrenoceptor antagonist bupranolol (10(-6) M) elicited a positive inotropic effect through activation of alpha 1-adrenoceptors in rat and rabbit, whereas in dog ventricular myocardium, bupranolol abolished the positive inotropic effect of epinephrine. [3H]Prazosin bound to membrane fractions derived from rat, rabbit, and dog ventricular muscle with high affinities in a saturable and reversible manner. In dog, Bmax and Kd values of alpha 1-adrenoceptor binding sites were identical to those in rabbit ventricular muscle. The Bmax value of alpha 1-adrenoceptors in rat ventricle was the highest, amounting to two to four times those in rabbit and dog. Epinephrine displacement curves for the specific binding of [3H]prazosin in the membrane fraction of these species showed high and low affinity sites with slope factors significantly less than unity, which were shifted to single low affinity sites with slope factors close to unity by addition of 5'-guanylylimidodiphosphate. Accumulation of [3H]inositol 1-phosphate [( 3H]IP1) in ventricular slices prelabeled with [3H]myo-inositol was increased by epinephrine in a time- and concentration-dependent manner in rat ventricular slices. [3H]IP1 accumulation likewise was facilitated by alpha 1-adrenoceptor stimulation in rabbit ventricular slices, whereas the extent of [3H]IP1 accumulation was much less than that in rat. In dog ventricular slices, [3H]IP1 was not accumulated by epinephrine. In rabbit papillary muscle, the time course of increase in contractile force induced by alpha-adrenoceptors coincided with the prolongation of the action potential duration with a similar time course, which is in strong contrast to previous findings in rat that the contractile response was dissociated from the electrophysiological response to alpha-adrenoceptor stimulation. The present results indicate that a wide range of variation of alpha 1-adrenoceptor-mediated regulation of myocardial contractility may be ascribed to different contributions of facilitatory as well as inhibitory regulatory processes that lead to intracellular Ca2+ mobilization subsequent to myocardial alpha 1-adrenoceptor activation among mammalian species.

Effects of benzyltetrahydropalmatine on delayed after depolarization and triggered activity and on His-bundle electrogram and ECG in animal experiment

Standard microelectrode techniques were used to study the effects of benzyltetrahydropalmatine (BTHP) on ouabain-induced delayed after depolarization (DAD) and triggered activity in isolated guinea papillary muscles. The results indicate that ouabain-induced DAD and triggered activity were abolished by BTHP 100 mumol/L. In anesthetized rabbit ECG heart rate was reduced in a dose-dependent manner from control value of 288 +/- 14 to 261 +/- 14 (BTHP 5 mg/kg) and 226 +/- 36 bpm (BTHP 10 mg/kg). P-R interval was prolonged. In His-bundle electrogram, H-V interval and V duration were not affected, but A-H interval was prolonged from 41 +/- 3 to 45 +/- 5 ms.

Ca(2+)-oscillations and Ca(2+)-waves in mammalian cardiac and vascular smooth muscle cells

In this article, we review briefly the available theories and data on [Ca2+]i-waves and [Ca2+]i-oscillations in mammalian cardiac and vascular smooth muscles. In addition to our review, we also report: (i) the existence and characterization of rapid agonist-induced [Ca2+]i-waves in cultured vascular smooth muscle cells (A7r5 cells); and (ii a new method for studying rapid [Ca2+]i-waves in mammalian cardiac ventricular cells. In mammalian cardiac muscle several types of Ca(2+)-release from sarcoplasmic reticulum (SR) are known to occur and might be involved in Ca(2+)-waves and Ca(2+)-oscillations: (a) Ca(2+)-induced release of Ca2+, of the type thought to be important in normal excitation-contraction coupling; (b) spontaneous, cyclic release of Ca2+ related to a Ca(2+)-overload of the SR; and (c) Ins(1,4,5)P3-induced Ca(2+)-release. The available data support the idea that [Ca2+]i-waves in heart propagate by a mechanism somewhat different than that involved in normal excitation-contraction coupling (a, above), perhaps involving spontaneous release of Ca2+ from an overloaded SR (b, above). In mammalian vascular smooth muscle, our data support the idea that agonist-receptor interaction (vasopressin, in this case) initiates [Ca2+]i-waves that then propagate via some form of Ca(2+)-induced release of Ca2+, perhaps in a manner similar to that proposed by Berridge and Irvine [1].

Modulation of alpha-adrenergic receptors and their intracellular coupling in the ischemic heart

The alpha 1-adrenergic receptor exists as at least two distinct subtypes, alpha 1a and alpha 1b. Based on hydrophobic exclusion studies and limited proteolysis of the cloned receptor, it appears to possess characteristics analogous to other membrane-bound receptors including seven membrane spanning domains, three extracellular, and three intracellular loops, with extensive glycosylation near the extracellular amino terminus. Although the receptor is coupled to phospholipase C in cardiac myocytes, with activation resulting in the production of inositol trisphosphate (IP3) and diacylglycerol, recent findings suggest that the receptor may also be linked to phospholipase A2, phospholipase D, and cyclic nucleotide phosphodiesterase. The alpha 1-adrenergic receptor has been shown to increase in response to myocardial ischemia in a number of different species and to mediate not only positive inotropic effects, but also to contribute substantially to arrhythmogenesis. The increase in alpha 1-adrenergic receptors can also occur in isolated adult ventricular myocytes in response to hypoxia, a mechanism which appears to be secondary to the sarcolemmal accumulation of long-chain acylcarnitines. This increase in alpha 1-adrenergic receptors in hypoxic myocytes is also linked to an enhanced increase in IP3 in response to receptor stimulation. These and other findings obtained in vivo during ischemia suggest that alpha 1-adrenergic mechanisms can become prominent in myocardium under pathophysiologic conditions in which a depressed contractile state exists and may therefore serve as a secondary inotropic system. However, the arrhythmogenic effects of stimulation of the alpha 1-adrenergic receptor in the ischemic heart in man may contribute substantially to arrhythmogenesis and, thereby, to the incidence of sudden cardiac death.

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.

Effects of carbachol and (-)-N6-phenylisopropyladenosine on myocardial inositol phosphate content and force of contraction

1. The effects of carbachol and the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) on force of contraction and inositol lipid metabolism were studied in electrically driven left auricles and papillary muscles isolated from guinea-pig hearts. Both carbachol and PIA (0.01-10 microM) had concentration-dependent negative inotropic effects in auricles. In papillary muscles PIA had no inotropic effect. Carbachol also had no inotropic effect at low concentrations (0.01-1 microM) but at 10-100 microM it exerted a slight positive inotropic effect. 2. In auricles and papillary muscles both carbachol and PIA concentration-dependently increased inositol trisphosphate (IP3; significant at 1 microM). Accordingly phosphatidylinositol bisphosphate (PIP2), the precursor of IP3, was reduced. All effects of carbachol and PIA were antagonized by atropine (10 microM) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 20 microM) respectively, indicating receptor-mediated effects. 3. In auricles the negative inotropic effects of carbachol and PIA preceded the increase in IP3. 4. In papillary muscles the increase in IP3 preceded the slight positive inotropic effect of carbachol, indicating that the M-cholinoceptor-mediated increase in IP3 and force of contraction may be related. However, PIA showed a comparable increase in IP3 but no inotropic effect, indicating a dissociation between those parameters. 5. In conclusion, in previous studies a close relation between increases in IP3 and force of contraction has been shown after alpha 1-adrenoceptor stimulation. The present study with carbachol supports this view. However, the present data for PIA could not show such a close relationship, questioning the role of IP3 as an endogenous regulator of force of contraction.

Endothelin stimulates phosphatidylinositol turnover in rat right and left atria

The effect of endothelin on phosphatidylinositol (PI) turnover has been investigated in isolated rat atria by measuring the generation of inositol phosphates (IPs) following [3H]inositol phospholipid labelling. In the presence of 10 mM LiCl, endothelin caused dose-dependent increases in the accumulations of inositol mono-, bis- and tris-phosphate (IP1, IP2 and IP3) which were maximal at 10(-6) M endothelin. The dose-response relationship was similar in right and left atria, but right atria showed a higher maximal IP response. Endothelin produced a rapid and transient stimulation of IP3 accumulation, which was maximal at 30 s followed by a slower increase which continued linearly past 20 min. During both the initial phase and the sustained phase the only isomer of IP3 present at detectable levels was the 1,4,5-isomer. As with endothelin, responses to noradrenaline also were higher in right atria compared with left atria and showed a biphasic pattern of release of IP3. These data demonstrate that endothelin receptors in rat atria are coupled to stimulation of the PI turnover pathway in an apparently similar manner to alpha 1-adrenoceptors. The PI pathway may be important in mediating the reported cardiac actions of endothelin.

1,2-diacylglycerol content in myocardium from spontaneously hypertensive rats during the development of hypertension

1,2-Diacylglycerol (DAG) has been considered to play an important role as an activator of protein kinase C in the signal transduction of inositol phospholipid metabolism. To examine the relation of 1,2-DAG in heart tissues to cardiac hypertrophy associated with hypertension, we measured the amount of 1,2-DAG in spontaneously hypertensive rat (SHR) hearts at 4, 10 and 20 weeks of age, and in age-matched normotensive Wistar-Kyoto (WKY) rat hearts using thin-layer chromatography with flame ionization detection (TLC-FID). Significant cardiac hypertrophy was found in 4-week-old SHR, while SHR did not yet have significant hypertension. Major phospholipids such as phosphatidylcholine and phosphatidylethanolamine increased from 4 to 20 weeks in the myocardium, but there was no difference between the two strains. The cholesterol levels of 4- and 20-week-old SHR were significantly higher than WKY rats. The 1,2-DAG contents of SHR hearts were significantly higher than WKY rats at 4 weeks. An increase in the RNA content was also observed in 4-week-old SHR hearts. However, analysis of the fatty acid composition of 1,2-DAG revealed no difference between the two strains. However, there was no significant difference in the 1,2-DAG content or in its fatty acid composition between SHR and WKY rat hearts at 10 and 20 weeks of age. It is suggested that an increase in the 1,2-DAG content of SHR hearts during the early stages appears related to the initiation of cardiac hypertrophy in SHR hearts before developed hypertension.

Functional and autoradiographic evidence for endothelin 1 receptors on human and rat cardiac myocytes. Comparison with single smooth muscle cells

This study aimed to determine whether receptors for endothelin were present on the cardiac myocyte as well as on vascular smooth muscle cells. Low- and high-resolution autoradiography was performed using 125I-endothelin 1 on intact rat myocardium and samples of human ventricle obtained from explanted hearts at the time of transplant. In addition to specific binding to the smooth muscle of the blood vessel lumen, there was considerable binding associated with cardiac myocytes. To discover whether there was any functional correlate for this binding, muscle cells were isolated enzymatically from human and rat ventricle and from rat femoral artery, and their contractile characteristics were studied. Single cardiac cells were superfused with physiological saline at 32 degrees C, and their length change was displayed continuously on a chart recorder. Endothelin 1 had a pronounced effect on shortening in both rat and human myocytes. The contraction amplitude was approximately doubled in both cases, from 4.1 +/- 0.8% cell length to 8.1 +/- 1.3% for rat (mean +/- SEM, n = 9, p less than 0.001), and from 2.1 +/- 0.5% to 4.0 +/- 0.5% in human (n = 10, p less than 0.001). In rat, the magnitude of the effect was comparable to that of the alpha-adrenoceptor agonist phenylephrine. The maximum contraction amplitude of the human cells, produced by raising extracellular calcium to greater than 10 mM, was 11.4 +/- 1.1% cell length (n = 9), significantly greater than that produced by endothelin (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol trisphosphate kinase activity in hypertrophied rat heart

In the present experiment, we demonstrated that IP3 kinase activity was increased in SHRSP heart compared to WKY heart. IP3 kinase activity in the heart was highest in the cytosolic fraction in both SHRSP and WKY. Its activity progressively increased with age in 5- to 20-week SHRSP. The activity reached about three times the level of 5-week-old SHRSP in 40-week-old SHRSP. On the other hand, in WKY it was 1.3-fold at 40 weeks compared with that at 5 weeks. We determined the effect of divalent cations on IP3 kinase activity. Ca2+ stimulated its activity in a dose-dependent manner at 10(-9) to 10(-6) M. In SHRSP it was enhanced about 2.1-fold at 10(-6) M of Ca2+, but in WKY it was 1.5-fold at 1.0(-6) M of Ca2+. Mn2+ also stimulated IP3 kinase activity in both groups of animals, while, Fe2+, Zn2+, and Cu2+ inhibited IP3 kinase activity. In our experiment IP3 kinase activity was increased in SHRSP and its activity was markedly affected by divalent cations. These data suggest that the accumulations of IP3 and IP4 after hormonal stimulation play a physiologic role, possibly by alteration of Ca2+ levels in cardiac tissue.

Evidence for the existence of inositol tetrakisphosphate in mammalian heart. Effect of alpha 1-adrenoceptor stimulation

The time course of the effects of phenylephrine (10 mumol/l) on force of contraction and on inositol phosphates in electrically driven left auricles from rat hearts labeled with [3H]inositol was studied. All experiments were performed in the presence of propranolol (1 mumol/l) and LiCl (10 mmol/l). Products measured after separation with high-performance liquid chromatography were inositol 1-phosphate (1-IP1), inositol 1,4-bisphosphate (1,4-IP2), inositol 1,3,4-trisphosphate (1,3,4,-IP3), inositol 1,4,5-trisphosphate (1,4,5-IP3), and inositol 1,3,4,5-tetrakisphosphate (1,3,4,5-IP4). All inositol phosphates increased after stimulation with phenylephrine. 1,4,5-IP3 was the first compound to rise maximally within 30 seconds; this rise was followed by an increase in 1,3,4,5-IP4 and 1,4-IP2 beginning within 2 minutes. The increase in 1,3,4-IP3 and 1-IP1 was slower and did not reach steady state within 15 minutes. The positive inotropic effect of phenylephrine was maximal after 5 minutes. It is concluded that the increase in the presumed second messengers 1,4,5-IP3 and 1,3,4,5-IP4 coincides with the positive inotropic effect after alpha 1-adrenoceptor stimulation. Since the increase in 1,4,5-IP3 precedes the increase in force of contraction, 1,4,5-IP3 may initiate the positive inotropic effect of alpha 1-adrenoceptor agonists and 1,3,4,5-IP4 maintains the increase in force of contraction.

Omega-3 fatty acid treatment fails to alter alpha 1-adrenergic receptor mediated phosphoinositide turnover in streptozotocin-induced diabetic rats

1. The effect of omega-3 fatty acid supplementation on alpha 1-adrenergic receptor-mediated phosphoinositide turnover in the hearts of 6-week streptozotocin diabetic rats was studied. 2. The diabetic state was characterized by decreased body weight gain, hypoinsulinemia and hyperglycemia. Omega-3 fatty acid treatment did not have any significant effects on the above parameters. 3. Plasma triglyceride and cholesterol levels were significantly higher in diabetic rats compared with controls. 4. Omega-3 fatty acid treatment reduced the plasma triglyceride levels in the diabetic rats, whereas plasma cholesterol levels remained unaffected. 5. Norepinephrine (10 and 100 microM) stimulated inositol monophosphate (IP1) formation was significantly decreased in the diabetic rat heart compared with controls. Omega-3 fatty acid treatment had no significant effect on the norepinephrine-mediated IP1 formation in these rats. 6. Inositol bisphosphate (IP2) formation in response to 100 microM of norepinephrine was significantly lower in the diabetic rat heart; which was also not affected by omega-3 fatty acid treatment.

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

The positive inotropic effect of the alpha 1-adrenoceptor agonist phenylephrine is accompanied by an increase in the presumed second messengers inositol 1,4,5-trisphosphate (1,4,5-IP3) and inositol 1,3,4,5-tetrakisphosphate (1,3,4,5-IP4). Both 1,4,5-IP3 and 1,3,4,5-IP4 sensitize myocardial contractile proteins in chemically skinned fibers. In addition to the Ca++ releasing effect of 1,4,5-IP3 from the sarcoplasmic reticulum the Ca++-sensitizing effect of the inositol phosphates may play a role in alpha 1-adrenergic positive inotropism. In isolated heart muscle preparations from patients with endstage heart failure (due to dilated cardiomyopathy) beta-adrenergic as well as alpha 1-adrenergic effects are reduced compared to preparations from healthy hearts. The reduced beta-adrenergic effects can in part be explained by an increased content of signal transducing G1-proteins. It is tempting to investigate whether other G proteins are also altered in severe congestive heart failure.

[Inositol trisphosphate, a new "second messenger" for positive inotropic effects on the heart?]

Myocardial alpha 1-adrenoceptors mediate a positive inotropic effect and influence the inositol phosphate cycle. The receptor-stimulated, phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate (PIP2) results in the generation of two novel second messengers, inositol trisphosphate (IP3) and diacylglycerol (DG). This effect is concentration-dependent and precedes the increase in force of contraction. Recently, it has been shown that the alpha 1-adrenoceptor-mediated increase in IP3 and force of contraction exists in the human heart as well. Possible mechanisms for an inositol phosphate-mediated positive inotropic effect are: (i) release of Ca2+ from the sarcoplasmic reticulum, elicited by IP3, (ii) increase in Ca2+ sensitivity of the contractile proteins, elicited by IP3, inositol tetrakisphosphate (IP4) and/or DG, (iii) increase in slow Ca2+ inward current, elicited directly by IP4 and/or indirectly by DG through a phosphorylation of the protein kinase C substrate in the sarcolemma. In ventricular cardiac preparations muscarinic agonists have a weak positive inotropic effect, but in cardiac atrial preparations they have a negative inotropic effect. In both preparations, these different effects coincide with a concentration-dependent increase in IP3. Thus, the possible positive inotropic effect in atrial preparations is probably masked by an activation of a K+ outward current. The relationship between the inositol phosphate cycle and the positive inotropic effect is in some points still speculative because not all of the mechanisms discussed are well settled yet. However, the stimulation of myocardial phosphoinositide breakdown resulting in an increased IP3 may be involved in the mechanism(s) whereby alpha1-adrenergic and muscarinic receptor stimulation exert an increase in myocardial force of contraction.(ABSTRACT TRUNCATED AT 250 WORDS)