Pertussis toxin does not inhibit the alpha 1-adrenoceptor-mediated effect on inositol phosphate production in the heart

Differential inotropic effects of endothelin-1, angiotensin II, and phenylephrine induced by crosstalk with cAMP-mediated signaling process in dog ventricular myocardium

Endothelin-1 (ET-1), angiotensin II (Ang II), and phenylephrine, an alpha1-adrenoceptor agonist, share the common signaling process, resulting in activation of Gq protein-coupled receptor (GqPCR) to activate the hydrolysis of phosphoinositide (PI). They do not elicit any inotropic effect in isolated dog ventricular muscle. In the presence of forskolin or IBMX (3-isobutyl-1-methylxanthine), ET-1 produced a dual effect, that is, a positive inotropic effect (PIE) and/or a negative inotropic effect (NIE) depending on concentrations of forskolin or IBMX present simultaneously with ET-1. Phenylephrine produced a definite PIE and Ang II induced a small and transient PIE in the presence of forskolin or IBMX, but they did not elicit a NIE. Facilitation of Ca2+ influx via L-type Ca2+ channel may play a crucial role in the crosstalk because GqPCR agonists produced, likewise a PIE in the presence of Bay k 8644. GqPCR agonists failed to induce a PIE in the presence of dihydroouabain or elevated [Ca2+]o. These findings indicate that the accumulation of cAMP or activation of L-type Ca2+ channels markedly modulates the inotropic response to GqPCR agonists in a manner that leads to a PIE in dog ventricular myocardium. In addition, ET-1, but not Ang II or phenylephrine, activates the signal transduction process that results in a NIE.

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.

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.

Enhancement of the positive inotropic effect mediated by alpha 1-adrenoceptors in pertussis toxin-treated rabbit papillary muscles

1. In rabbit papillary muscles, pretreatment with pertussis toxin (PTX) significantly increased the positive inotropic response to isoprenaline and abolished the inhibitory action of carbachol on the isoprenaline response. 2. Phenylephrine in the presence of propranolol produced a positive inotropic effect and prolonged action potential duration through activation of alpha 1-adrenoceptors. Both of the effects of phenylephrine were significantly enhanced by PTX pretreatment. 3. Accumulation of [3H]inositol monophosphate (IP1) in papillary muscles prelabeled with myo-[3H]inositol was increased by phenylephrine in a concentration-dependent manner, which was antagonized by prazosin. Although PTX pretreatment significantly elevated the basal level of [3H]IP1 formation, the phenylephrine-induced increase in [3H]IP1 formation was unaffected. 4. It is concluded that the cardiac responses to alpha 1-adrenoceptor stimulation studied in these experiments are not transduced by a PTX sensitive G protein (Gi). However, the positive inotropic effect and prolongation of action potential duration mediated by alpha 1-adrenoceptor may be negatively regulated by Gi.

Receptor-effector coupling pathway for alpha 1-adrenergic modulation of abnormal automaticity in 'ischemic' canine Purkinje fibers

We studied the receptor-effector coupling mechanism responsible for alpha 1-adrenergic receptor-induced increases in abnormal automaticity (AA) occurring at low membrane potentials in "ischemic" Purkinje fibers, superfused with Tyrode's solution containing [K+]o 10 mmol/L, pH 6.8, PO2 < 25 mm Hg. To exclude beta-adrenergic actions, propranolol was added to all solutions. We derived membrane slope resistance (Rsl) from the current-voltage relation obtained with two microelectrodes for intracellular current injection and transmembrane voltage recording. We also measured the membrane time constant, Tm, to assess changes in membrane resistance (Rm). Phenylephrine effects on Rsl in simulated ischemia were studied in the absence or presence of the alpha 1-subtype blockers WB 4101 (WB) or chloroethylclonidine (CEC), both 0.1 mumol/L, and in Purkinje fibers from dogs injected with pertussis toxin (PTX) (30 micrograms/kg i.v., 60 to 72 hours before study). There were no significant differences in mean values of Rsl before phenylephrine superfusion among all groups of Purkinje fibers. Tm increased by 23% during phenylephrine 0.1 mumol/L superfusion, and Rsl increased by 11%. These two results suggest a 23% increase in Rm with no concordant change in longitudinal resistance. In the presence of CEC, phenylephrine increased Rsl by 12%. In contrast, WB blocked phenylephrine effects on Rsl (0.3%). In PTX-treated Purkinje fibers, the levels of PTX-sensitive G protein as well as phenylephrine effects on Rsl (3%) were significantly reduced. In the absence of WB and of CEC, the phenylephrine effects both on Rsl and on the incidence of AA were directly related to the level of PTX-sensitive substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac alpha 1-adrenoceptors stimulate a high-affinity GTPase activity in sarcolemmal membranes from rabbit atrial and ventricular myocytes

The interaction between cardiac alpha 1-adrenoceptors and GTP-binding regulatory proteins was characterized in isolated rabbit cardiac myocytes (thereby avoiding interference by other cell types present in the myocardium) by examining the alpha 1-adrenergic stimulation of GTPase activity in sarcolemma-enriched membrane fractions. Stimulation of membrane-associated GTPase activity in both atrial and ventricular myocyte preparations by the alpha 1-adrenergic agonists 1-noradrenaline and methoxamine (in the presence of propranolol) was observed to be both linear with time and saturable. alpha 1-adrenergic stimulation did not change the Km for GTP (0.14-0.21 microM), but increased the Vmax by 39% and 72% above basal levels in atrial and ventricular membranes, respectively. Stimulation of GTPase activity by alpha 1-agonists occurred in a concentration-dependent fashion and was blocked in the presence of the alpha-adrenoceptor antagonists phentolamine and prazosin, but not yohimbine. Prior treatment of myocytes with pertussis toxin had no effect on the alpha 1-adrenergic stimulation of GTPase activity, but inhibited stimulation by muscarinic-receptor activation with carbachol. Finally, photoaffinity labelling of an approximately 75-kDa membrane-bound protein with [alpha-32P]GTP was enhanced in the presence of the alpha 1-agonist methoxamine and abolished by addition of excess non-labelled GTP, suggesting that this GTP-binding protein may interact with cardiac alpha 1-adrenoceptors; a similar GTP-binding protein which may be coupled to alpha 1-adrenoceptors has been reported in rat liver plasma membranes (Im, M. J. & Graham, R. M. (1990) J. Biol. Chem. 265, 18944-18951).

Activation of ?1-adrenoceptors modulates the inwardly rectifying potassium currents of mammalian atrial myocytes

The selective alpha 1-adrenergic agonist methoxamine (10(-4)-10(-3) M), in the presence of propranolol (10(-6) M), can reduce both the inwardly rectifying K+ background current (IK1) and the muscarinic cholinergic receptor-activated K+ current (IK,ACh) in rabbit atrial myocytes resulting in action potential prolongation during the final phase of repolarization and a depolarization of the resting membrane potential. The reduction of these K+ currents(s) by alpha 1-adrenoceptor stimulation was insensitive to pre-treatment of atrial myocytes with pertussis toxin (0.15-0.5 micrograms/ml) and was irreversible following intracellular dialysis with the non-hydrolysable guanosine triphosphate (GTP) analogue, Gpp(NH)p (1-5 x 10(-3) M). Neither the protein kinase C (PKC) inhibitors, 1((5-isoquinolinesulphonyl)-2-methylpiperoxine (H-7) (5 x 10(-5) M) and staurosporine (1 x 10(-7) M), nor "downregulation" of PKC by prolonged phorbol ester exposure (5 x 10(-7) M, for 7-8 h) had an effect on the alpha 1-adrenergic modulation of this K+ current. Under cell-attached patch-clamp conditions, bath application of methoxamine reversibly decreased acetylcholine-induced single-channel activity, thus confirming the observed reduction of the ACh-induced current under whole-cell voltage clamp. These results demonstrate that the alpha 1-adrenoceptor, once activated, can reduce current through two different inwardly rectifying K+ channels in rabbit atrial myocytes. These current changes are mediated via a pertussis toxin-insensitive GTP-binding protein, and do not appear to involve the activation of PKC.

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.

Alpha 1-adrenoceptors reduce background K+ current in rabbit ventricular myocytes

1. Ventricular myocytes were isolated by enzymatic dispersion of adult rabbit hearts, and voltage clamped using the whole-cell variation of the patch clamp technique. Experiments were carried out at either 35 degrees C or room temperature (21-23 degrees C). 2. In the presence of 10(-3) M-4-aminopyridine to block the transient outward K+ current, and 10(-6) M-propranolol to block beta-adrenoceptors, the alpha 1-adrenergic agonist methoxamine produced action potential prolongation, and a small depolarization of the diastolic membrane potential. Under voltage clamp conditions, methoxamine decreased the magnitude of the inward rectifier K+ current, IK1, in both the inward and outward directions. This effect was dose dependent (10(-5)-10(-3) M) and fully reversible upon wash-out of the agonist. 3. The neurotransmitter noradrenaline (10(-6)-2 x 10(-5) M), in the presence of propranolol (10(-6) M), also reduced IK1 in ventricular cells, and this effect was blocked by the specific alpha 1-adrenoceptor antagonist prazosin. 4. The alpha 1-adrenoceptor-mediated decrease in IK1 in ventricular myocytes was not affected by pre-incubation of the cells with 0.5 micrograms/ml pertussis toxin (8-10 h, 30-32 degrees C). This result suggests that in rabbit ventricular cells, the alpha 1-modulation of IK1 occurs via a pertussis toxin-insensitive guanine nucleotide-binding regulatory protein. 5. These observations demonstrate that IK1 in ventricular myocytes can be modulated by cardiac alpha 1-adrenoceptors. The resulting changes in action potential repolarization and diastolic membrane potential may have significant effects on cardiac performance.

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.

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.

Intracellular mechanisms for alpha 1-adrenergic regulation of the transient outward current in rabbit atrial myocytes

1. The intracellular mechanism(s) underlying the decrease of a transient outward K+ current (It) induced by alpha 1-adrenergic agonists was studied in isolated adult rabbit atrial myocytes using whole-cell voltage clamp and cell-attached patch clamp techniques. Experiments were carried out at 22-23 degrees C. 2. Application of the specific alpha 1-adrenergic agonist, methoxamine, produced a decrease in It which was irreversible after the non-hydrolysable GTP analogues, GTP gamma S and Gpp(NH)p, had been introduced into cells via the recording micropipette. 3. Pre-treatment of cells with 0.1-0.15 microgram/ml pertussis toxin (PT) for 8-9 h at 30-34 degrees C did not prevent the alpha 1-induced decrease in It. Yet, this protocol, as measured by the PT-catalysed incorporation of [32P]ADP-ribose in membrane-associated 40 and 41 kDa proteins, effectively caused the ADP-ribosylation of approximately 70% of the PT-sensitive GTP-binding proteins (i.e. Gi) in these treated cells. After taking into account the proportion of non-viable cells (20-30%), the effectiveness of this treatment probably approaches 100% in the viable myocytes from which electrophysiological recordings were made. 4. Cell-attached patch recordings showed that bath application of methoxamine altered the single-channel events underlying It by decreasing their opening probability. Averaged currents from ensemble single-channel openings recorded in the presence of 0.2 mM-methoxamine outside the patch reproduced the features of alpha 1-adrenergic modulation of the macroscopic It observed during whole-cell voltage clamp measurements. This observation provides evidence for the involvement of a diffusible intracellular second messenger in the alpha 1-adrenergic modulation of It. 5. The protein kinase C (PKC) activators, 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1-oleoyl-2-acetylglycerol (OAG) increased It, when included in the bath perfusate, whereas the inactive analogues, 4 alpha-phorbol and 4 alpha-phorbol 12,13-didecanoate, had no effect on It. 6. Exposure of cells to the PKC inhibitors, staurosporine and H-7, either by bath superfusion or intracellularly, via the recording micropipette, did not block the decrease in It produced by methoxamine. 7. Prolonged stimulation of atrial myocytes for 7-9 h at 22 degrees C with 500 nM-PMA produced a 'down-regulation' of endogenous PKC activity, as well as a physical loss of the immunoreactive enzyme, as measured by an in vitro assay, and an anti-PKC monoclonal antibody, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

The mechanism of positive inotropy induced by adenosine triphosphate in rat heart

When applied extracellularly in the micromolar range, ATP and related compounds induced a positive inotropy in the rat papillary muscle. This was also true in the rat auricle after pertussis toxin treatment. Then, in both tissues, ATP further increased the contraction after a maximal beta-adrenergic stimulation. The increase in contractile force could be related to the increase in the calcium current. The L-type calcium current was measured by whole-cell patch-clamp recording in single cells isolated from the rat ventricle after the sodium and potassium currents were inhibited by tetrodotoxin and cesium, respectively. When added alone, 10 microM ATP increased the calcium current by 60%. Adenosine 5'-O-(3-thiotriphosphate) was also able to increase calcium current. Adenosine was much less effective, and GTP, UTP, CTP, and ITP were without effect. A similar increase in calcium current was observed when ATP was added in addition to a maximal stimulation by a beta-adrenergic agonist or after internal perfusion with cyclic AMP. However, this increase was preceded by a transient decrease whose origin could not be attributed to a P1-purinergic agonistic effect of ATP. The transient decrease was not elicited by adenosine or in a magnesium-free HEPES solution and was not suppressed after pertussis toxin treatment. This effect appeared related to the variations in the holding current also observed upon ATP application. Together with vasodilation, ATP and adenine compounds induced positive inotropy. The latter effect could be attributed in part to the increase in calcium current and was independent of cyclic AMP. Both effects are complementary with the beta-adrenergic stimulation and can help healthy cells to compensate the failing zone from which ATP could be released.

Effects of purinergic stimulation on the Ca current in single frog cardiac cells

Ca current (ICa) was measured by whole-cell voltage clamp in single cells isolated from frog ventricle, in which the Na current was inhibited by tetrodotoxin (0.3 microM) and K currents were blocked by substituting K with 120 mM intracellular and 20 mM extracellular Cs. The influence of stimulation by ATP (0.1-100 microM) was assessed in the presence of propranolol (1 microM) or pindolol (0.1 microM), prazozin (0.1 microM) and atropine (10 microM). ATP, in the micromolar range, had two types of effect. Like other P1-purinoagonists, it antagonized the increase in ICa elicited by beta-adrenostimulation. When added alone, 1 microM ATP could increase ICa up to twofold. An increase in ICa was also observed even after it had been maximally enhanced by intracellularly applied cAMP (50 microM). Voltage dependence and kinetics of ICa were not affected. These effects were considered to be related to P2-purinoceptor activation. At higher ATP concentrations the increase in ICa was less; at 100 microM, ATP reduced ICa. The ATP-induced increase in ICa was prevented by internal perfusion of the cells with GDP [beta-S] or neomycin, respectively, to block signal transduction to phospholipase C or its phosphodiesterase activity on the polyphosphoinositides. We conclude that P2-purinoceptor stimulation increases the Ca current in frog ventricular cells by a pathway that might involve phosphoinositide turnover.

Effects of pertussis toxin on alpha 1-agonist-mediated phosphatidylinositide turnover and myocardial cell hypertrophy in neonatal rat ventricular myocytes

In neonatal rat ventricular myocytes pretreatment with pertussis toxin did not affect 1 microM (-)-norepinephrine stimulation of inositol phosphates or myocardial cell hypertrophy as measured either by protein radiolabelling or by myocardial cell protein content. Thus guanine nucleotide protein(s) ADP-ribosylated by pertussis toxin do not play a role in two alpha 1-adrenoceptor-mediated processes, phosphatidylinositide turnover and induction of myocardial cell hypertrophy.

Function of myocardial alpha-adrenoceptors

In addition to beta-adrenoceptors (beta ARs), cardiac myocytes of animals and man possess alpha 1ARs, but not alpha 2ARs. Norepinephrine and epinephrine have a higher affinity for myocardial alpha 1ARs than for beta ARs. Unlike beta AR stimulation, myocardial alpha 1AR stimulation does not increase the slow inward current. The alpha 1AR-mediated positive inotropic effect seen in isolated heart preparations appears to involve increased Ca sensitivity of myofibrils and production of inositol triphosphate (IP3) and diacylglycerol (DAG), but the functions of IP3 and DAG are not clear. Myocardial alpha 1AR stimulation reduces rate of isolated atria and Purkinje fibers and lengthens refractory period and action potential duration. Hypoxia increases alpha 1AR density in cardiomyocytes. alpha 1AR-mediated arrhythmias occur in isolated Purkinje fibers during hypoxia, following infarction, and in the presence of Ba2+ or high Ca2+. In animals, coronary artery occlusion and/or reperfusion increase myocardial alpha 1AR density and responsiveness, and alpha AR blocking drugs attenuate arrhythmias. However, an antiarrhythmic effect of alpha AR blocking drugs mediated by action on coronary vascular alpha ARs cannot be excluded. Presently available drugs do not differentiate between myocardial and vascular alpha ARs and thus affect the coronary and systemic circulations and, indirectly, the heart. Additional myocardial alpha 1AR-mediated effects include production of cardiac hypertrophy, stimulation of glucose uptake and phosphofructokinase and cyclic AMP phosphodiesterase activity, and release of atrial natriuretic peptide.

Developmental changes in guanine nucleotide regulatory proteins in the rat myocardial alpha 1-adrenergic receptor complex

During development, the cardiac alpha 1-adrenergic chronotropic response changes from positive in the neonate to negative in the adult. The negative chronotropic effect of alpha 1-adrenergic stimulation in the adult depends on maturation of sympathetic innervation and the presence of a pertussis toxin (PT)-sensitive guanine nucleotide-binding (G) protein. To examine the possibility of a developmental change in coupling of a PT-sensitive G protein to the alpha 1-adrenergic receptor, radioligand binding experiments with the iodinated alpha 1-selective radioligand [125I]-I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([ 125I]-IBE 2254) were performed on membranes prepared from control and PT-treated neonatal and adult rat hearts. Scatchard analysis showed fewer alpha 1-adrenergic receptors in the adult than in the neonate (168 +/- 10 fmol/mg protein in the neonate vs. 124 +/- 13 fmol/mg protein in the adult), but similar affinities (equilibrium dissociation constant 124 +/- 29 pM in the neonate vs. 140 +/- 34 pM in the adult). PT treatment did not alter the results. In both the neonate and adult, 5'-guanylylimidodiphosphate [Gpp(NH)p, 500 microM] shifted the l-epinephrine competition curve to the right and increased the slope factor toward unity. PT had no effect on the l-epinephrine competition curve in the neonate. However, in the adult PT itself caused a partial shift in the agonist competition curve, reducing but not eliminating the effect of Gpp(NH)p. Consistent with the results from the binding experiments, PT did not have any effect on the alpha 1-adrenergic-mediated positive chronotropic response in the neonate, whereas in the adult the alpha 1-adrenergic-mediated negative chronotropic response was completely converted to a positive one after PT treatment. These results indicate the presence of a PT-insensitive G protein in the neonatal and adult rat heart and the acquisition of a PT-sensitive G protein linked to the negative chronotropic response during development.

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)

Dependence on extracellular potassium of the positive inotropic response to St 587, a selective alpha-1 adrenoceptor agonist, in Zucker rat heart ventricle

The effects of St 587, a selective alpha-1 adrenoceptor agonist, were investigated in non obese and obese Zucker rat heart ventricles. In both groups, the numbers and affinity constants for alpha-1 adrenoceptors were found to be similar. At 4 or 10 mM [K]o, St 587 failed to increase the developed tension whereas at 14 mM [K]o, St 587 significantly increased it in both groups of rats. This effect was reversed by prazosin; St 587 also increased action potential duration at 14 mM [K]o. [K]o is thus important for the occurrence of the inotropic effect of St 587 in 12 week-old Zucker rats, either non obese or obese with reduced beta-adrenoceptor responsiveness. This suggests the participation of phosphoinositide metabolism in the mechanism of St 587 inotropic effect in the rat.

Further characterization of the myocardial alpha-adrenoceptors mediating positive inotropic effects in the rabbit myocardium

[3H]Prazosin bound with high affinity to the membrane fraction derived from the rabbit ventricular myocardium. Oxymetazoline displaced [3H]prazosin from its binding site, did not elicit a positive inotropic effect but antagonized the positive inotropic effect of phenylephrine mediated by alpha-adrenoceptors in the presence of a beta-antagonist. Naphazoline was more potent in displacing [3H]prazosin and behaved as a weak partial agonist. YM-12617 (5-[2-[[2-(2-ethoxyphenoxy)ethyl]amino]propyl]-2- methoxybenzenesulfonamide HCl), a potent selective alpha 1-antagonist, displaced [3H]prazosin and antagonized the alpha-mediated positive inotropic effect with equal potency. Thus, a good correlation was found between the potency of alpha-antagonists to displace [3H]prazosin and their ability to antagonize the alpha-mediated positive inotropic effect. On the other hand, there was no significant correlation between the Ki and the pD2 value of the alpha-agonists (norepinephrine, epinephrine, phenylephrine and naphazoline), indicating that there is a non-linear relationship between agonist binding to myocardial alpha 1-adrenoceptors and subsequent functional changes. Myocardial alpha 1-adrenoceptors showed some pharmacological characteristics which appear to be different from those in smooth muscle tissues.

Indirect evidence for a role of phosphatidylinositol turnover in the cardiac response to H1-receptor stimulation

The influence of lithium on the positive inotropic effect of the H1-agonist 2-pyridyl-ethylamine (PEA) and of the H2-receptor agonist 4-methylhistamine was studied in isolated guinea-pig ventricular strips electrically stimulated at 1 Hz. Lithium (1-10 mM) was devoid of any effect on cardiac contraction; the positive inotropic effect of 4-methylhistamine was unaffected in the presence of 10 mM lithium. On the other hand, lithium (1-10 mM) dose-dependently shifted the dose-inotropic effect curve for PEA to the right; an antagonistic effect, qualitatively similar to that of lithium, was induced by the myoinositol antagonist 2-2'-anhydro-2-C-hydroxymethyl-myoinositol, at a concentration of 100 microM. Moreover the antagonistic effect of the higher lithium concentration (10 mM) was almost completely prevented in preparations superfused with 10 mM myoinositol. Since it is known that lithium is able to reduce the cellular availability of myoinositol by an interference with the phosphatidylinositol (PI) cycle, these results suggest that the H1-receptor-mediated increase in contractility may be linked to an increased turnover of PI, while the H2-receptor-mediated one is not.

Influence of lithium on the positive inotropic effect of phenylephrine and isoprenaline in guinea-pig heart

The influence of lithium on the positive inotropic effects mediated by alpha- and beta-adrenoceptor stimulation was studied in isolated guinea-pig ventricular preparations stimulated at 1 Hz. Lithium chloride (10(-3)-2 X 10(-2) M) shifted the concentration-response curve for the inotropic effect of phenylephrine to the right in a dose-dependent manner. [3H]Prazosin binding was not inhibited by 10(-2) M lithium. The antagonistic effect of lithium was almost completely prevented by the presence of 10(-2) M myoinositol, and was potentiated by 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol, an antagonist of myo-inositol. The positive inotropic effect of isoprenaline was completely unaffected by either 10(-2) M lithium, 10(-2) M myo-inositol, or 10(-4) M 2-2'-anhydro-2-C-(hydroxymethyl)-myo-inositol. Since it is known that lithium interferes with inositol phosphate metabolism, our results produce further indirect evidence of an involvement of inositol phosphates in the myocardial response to alpha-adrenoceptor, but not to beta-adrenoceptor, stimulation. Our findings also suggest that chronic lithium treatment could interfere with the adrenergic modulation of myocardial contractility in those physio-pathological conditions in which the role of myocardial alpha-adrenoceptors becomes predominant.

Phorbol ester does not mimic, but antagonizes the alpha-adrenoceptor-mediated positive inotropic effect in the rabbit papillary muscle

The phorbol ester 12-O-tetradecanoyl phorbol-13-acetate (TPA) was used to examine the hypothesis that phosphoinositide turnover is involved in the regulation of myocardial contractility mediated by stimulation of alpha-adrenoceptors in the mammalian cardiac muscle. Exposure of the isolated rabbit papillary muscle electrically driven at a rate of 1 Hz at a temperature of 37 degrees C to TPA in concentrations of 10-1000 nmol/l for 30 min did not affect the basal force of contraction. The concentration-response curve for the positive inotropic effect of (-)-phenylephrine mediated by stimulation of alpha-adrenoceptors in the presence of (+/-)-bupranolol (100 nmol/l) was shifted to the right and downward by TPA in concentrations of 30-1000 nmol/l, while the effect of (-)-phenylephrine mediated by stimulation of beta-adrenoceptors in the presence of prazosin (100 nmol/l) was not decreased, but slightly enhanced by exposure of the muscle to relatively low concentrations of TPA (10-100 nmol/l). Incubation of the membrane fraction isolated from the rabbit ventricular muscle with TPA in vitro under the same condition as employed in the physiological experiments decreased the specific binding of [3H]prazosin but not that of [3H]CGP-12177, while the non-tumor promoting phorbol ester, alpha PDD, was ineffective. These results indicate that activation of protein kinase C by TPA does not mimic the positive inotropic effect of catecholamines mediated by activation of myocardial alpha-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic stimulation of vascular prostacyclin: role of alpha 1-receptors in smooth muscle cells

Epinephrine and norepinephrine (1-10 microM) stimulated the release of prostacyclin (PGI2) from the rabbit aorta in vitro. The stimulation was maintained for at least 2 h in the continuous presence of epinephrine. Phenylephrine mimicked this effect, whereas the selective alpha 2-agonist UK-14,304 was completely ineffective. The action of epinephrine was abolished by prazosin (1 microM) and was maintained in the presence of yohimbine. Epinephrine or phenylephrine neither increased the basal release of PGI2 from bovine aortic endothelial cells nor potentiated the stimulatory action of adenine nucleotides, which is mediated by P2-purine receptors. The response to epinephrine was lost in freshly deendothelialized strips of rabbit aorta, possibly because of cyclooxygenase self-inactivation. The response recovered however following overnight incubation of these strips in a cell culture medium. The response to epinephrine was mimicked by neither phorbol 12-myristate,13-acetate nor ionophore A23187. It was not inhibited by pretreatment with pertussis toxin. It is concluded that adrenergic agents stimulate the vascular production of PGI2, by activating alpha 1-receptors located on smooth muscle cells.

Increase in IP3 precedes alpha-adrenoceptor-induced increase in force of contraction in cardiac muscle

In electrically driven left auricles isolated from rat hearts the alpha 1-adrenoceptor agonist phenylephrine increased inositol trisphosphate (IP3) and force of contraction. The increase in IP3 preceded the increase in force of contraction, indicating that the alpha 1-adrenoceptor-mediated increase in IP3 and force of contraction may have been causally related.