Specific binding of [3H]prazosin to myocardial cells isolated from adult rats

The cardiac ventricular 5-HT4 receptor is functional in late foetal development and is reactivated in heart failure

A positive inotropic responsiveness to serotonin, mediated by 5-HT₄ and 5-HT_2A receptors, appears in the ventricle of rats with post-infarction congestive heart failure (HF) and pressure overload-induced hypertrophy. A hallmark of HF is a transition towards a foetal genotype which correlates with loss of cardiac functions. Thus, we wanted to investigate whether the foetal and neonatal cardiac ventricle displays serotonin responsiveness. Wistar rat hearts were collected day 3 and 1 before expected birth (days -3 and -1), as well as day 1, 3, 5 and 113 (age matched with Sham and HF) after birth. Hearts from post-infarction HF and sham-operated animals (Sham) were also collected. Heart tissue was examined for mRNA expression of 5-HT₄, 5-HT_2A and 5-HT_2B serotonin receptors, 5-HT transporter, atrial natriuretic peptide (ANP) and myosin heavy chain (MHC)-α and MHC-β (real-time quantitative RT-PCR) as well as 5-HT-receptor-mediated increase in contractile function exvivo (electrical field stimulation of ventricular strips from foetal and neonatal rats and left ventricular papillary muscle from adult rats in organ bath). Both 5-HT₄ mRNA expression and functional responses were highest at day -3 and decreased gradually to day 5, with a further decrease to adult levels. In HF, receptor mRNA levels and functional responses reappeared, but to lower levels than in the foetal ventricle. The 5-HT_2A and 5-HT_2B receptor mRNA levels increased to a maximum immediately after birth, but of these, only the 5-HT_2A receptor mediated a positive inotropic response. We suggest that the 5-HT₄ receptor is a representative of a foetal cardiac gene program, functional in late foetal development and reactivated in heart failure.

Modelling of alpha1-adrenoceptor-mediated temporal dynamics of inotropic response in rat heart to assess ligand binding and signal transduction parameters

BACKGROUND AND PURPOSE

In order to use the transient response to an antagonist (prazosin) to evaluate properties of agonist interactions with the alpha(1)-adrenoceptor system, an integrative mechanistic model of cardiac uptake of prazosin and its competitive interaction with phenylephrine at the receptor site was developed. Based on the operational model of agonism, the aim was to evaluate both the receptor binding and signal transduction process as determinants of the inotropic effect of phenylephrine.

EXPERIMENTAL APPROACH

In Langendorff-perfused rat hearts, prazosin outflow concentration and left ventricular developed pressure were measured, first in the presence of 12.3 micromol x L(-1) phenylephrine following a 1 min infusion of 1.27 nmol [(3)H]-prazosin, and second, when after 30 min the phenylephrine concentration in perfusate was reduced to 6.1 micromol x L(-1), the 1 min infusion of 1.27 nmol [(3)H]-prazosin was repeated.

KEY RESULTS

The kinetic model accounted for cardiac uptake and receptor binding kinetics of prazosin (dissociation constant, mean +/- SD: 0.057 +/- 0.012 nmol.L(-1)), assuming that the competitive displacement of phenylephrine (dissociation constant: 101 +/- 13 nmol x L(-1)) reduced the receptor occupation by the agonist and, consequently, contractility. This competitive binding process appeared to be the rate-determining step in response generation. The relationship between receptor occupancy and inotropic response was described by an efficacy parameter (tau, ratio of receptor density and coupling efficiency) of 4.9.

CONCLUSIONS AND IMPLICATIONS

Mechanistic pharmacodynamic modelling of the kinetics of antagonism by prazosin allows quantitative assessment of the alpha(1)-adrenoceptor system both at the receptor and post-receptor levels.

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.

Improved isolation of cardiomyocytes by trypsination in addition to collagenase treatment

The present study was undertaken to develop an improved and stabilized method for isolating cardiomyocytes from perfused rat heart. Different lots of the commercial collagenases used for isolating cardiomyocytes give variable results both with respect to the total cell yield and the percentage of elongated cells obtained. When trypsin was present both before and during collagenase treatment of the tissue, the performance of the collagenases was improved and stabilized, and a high and stable cell yield (7.5 x 10(6) cells per heart), and a high percentage of elongated cells (about 70%) was regularly obtained. The cells possessed alpha 1-adrenergic binding sites with binding properties (Bmax = 43.5 fmol/mg protein and Kd = 125.5 pmol/l) in agreement with values previously reported. The cells were able to respond functionally, as the cellular uptake of 86Rb+ increased by 18% after alpha 1-adrenoceptor stimulation with phenylephrine. These criteria indicate that the cells were well preserved during the isolation procedure.

Autonomic modulation of action potential and tension in guinea pig papillary muscles

The effects of alpha 1-adrenoceptor and muscarinic acetylcholine receptor stimulation on action potential and tension were studied in guinea pig papillary muscles obtained from both right and left ventricles. Stimulation of muscarinic acetylcholine receptors with carbachol produced a reduction of the action potential duration and a positive inotropic effect in papillary muscles from both ventricles. Both effects were concentration dependent and atropine sensitive. However, differential responsiveness was found upon alpha 1-adrenoceptor activation in muscles obtained from left and right ventricles. In right side papillary muscles, the alpha 1-adrenoceptor agonist, methoxamine, decreased the action potential duration and produced a positive inotropic effect. In contrast, methoxamine decreased the action potential duration but failed to produce a positive inotropic effect in left side papillary muscles. All methoxamine effects were antagonized by prazosin. Responses to maximum concentration of carbachol and methoxamine on the action potential duration and contractility were additive in right side papillary muscles. Phorbol 12,13-dibutyrate (PDB), a direct protein kinase C activator, also decreased the action potential duration in a manner that was additive to both carbachol and methoxamine. However, PDB reversed the positive inotropic effect of carbachol and methoxamine. The methoxamine-induced shortening of the action potential duration was prevented by pretreatment with indomethacin and nordihydroguaiaretic acid, blockers of arachidonic acid metabolism, but not by the protein kinase C antagonist, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7).(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenoceptors in the corneal endothelium

The potent alpha 1-adrenoceptor antagonist, [3H]prazosin, exhibited high affinity, specific and reversible binding to intact rabbit, bovine and human corneal endothelial cells in culture. The binding of 1 nM [3H]prazosin to rabbit cells reached a steady-state level within 10 min at 37 degrees C. Under these conditions, approximately 50% of the [3H]prazosin bound was specific. The level of specific [3H]prazosin binding was concentration-dependent, but Rosenthal analysis indicated that [3H]prazosin bound to at least two sites. One site exhibited a high affinity for [3H]prazosin (Kd = 0.2 nM), but a relatively low binding capacity (Bmax = 175 fmol bound mg-1 protein); the other site showed a relatively low affinity for the radioligand (Kd = 85 nM), but a much higher binding capacity (1280 fmol mg-1). Several known alpha 1-adrenoceptor antagonists and agonists competitively inhibited [3H]prazosin binding at the high affinity site when incubated with the radioligand. The relative potencies of these competing ligands were generally consistent with their binding affinities for alpha 1-adrenoceptors in other tissues. Phenylephrine stimulated the rate of hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate by 63% in these cells. This stimulation was inhibited by 52% if phentolamine was also present during the incubation. These data indicate that corneal endothelial cells have alpha 1-adrenoceptors which can modulate polyphosphoinositide turnover in this tissue.

Effect of furosemide, hydrochlorothiazide and sodium restriction on rat adrenoceptors

The interaction of thiazide diuretics and furosemide with adrenoceptors was investigated in normal rat in vivo by determining the effect of sodium-deficient diet alone or combined with hydrochlorothiazide or furosemide on adrenoceptors in heart, kidney and skeletal muscle. In addition, interaction of diuretics with renal adrenoceptors in vitro was investigated. Sodium-deficient diet alone retarded growth significantly, and this effect was increased by simultaneous treatment with furosemide 10-40 mg/kg body weight and day subcutaneously. No consistent alteration was demonstrated in density or affinity of alpha 1-, alpha 2- and beta-adrenoceptors due to the in vivo treatment. In vitro, furosemide inhibited the equilibrium specific binding of radioligands to alpha 1-, alpha 2- and beta-adrenoceptors to rat kidney particulate fraction concentration-dependently. Furosemide increased Kd (equilibrium dissociation constant) and reduced Bmax (number of specific binding sites) of the radioligands as determined by Scatchard analysis of binding saturation data. The Pseudo-Hill coefficient of the inhibition by furosemide of 125I-iodocyanopindolol binding was 1.47, indicating a complex interaction consistent with a non-competitive inhibition. In conclusion, the results do not support the hypothesis that sodium deficiency alone or with diuretic treatment for one week alters tissue adrenoceptors in the normal rat. The pattern of inhibition by furosemide of ligand binding to adrenoceptors suggests that furosemide binds to an allosteric site on the receptors or to an adjacent macromolecule.

Both alpha 1- and beta-adrenoceptor stimulation determine the time course of the inotropic effect of noradrenaline in rabbit heart

It has been a matter of controversy whether alpha 1-adrenoceptor stimulation contributes to the final inotropic and lusitropic responses in mammalian myocardium to noradrenaline during concomitant and unopposed beta-adrenoceptor stimulation. In the present paper we report studies that compare time courses of the inotropic and lusitropic responses to separate and combined alpha 1- and beta-adrenoceptor stimulation, respectively, in electrically driven rabbit papillary muscles by a submaximal concentration of noradrenaline. Separate alpha 1- or beta-adrenoceptor stimulation (presence of appropriate receptor blocker) showed the characteristic slow and fast development, respectively, of the inotropic responses. Qualitatively, the respective characteristic changes were also observed: alpha 1-adrenoceptor stimulation caused a negative lusitropic effect giving a prolongation of the time to peak tension (TPT), while beta-adrenoceptor stimulation caused a pronounced positive lusitropic effect giving a shortening of TPT. The time course of the inotropic response to combined adrenoceptor stimulation had characteristics that deviated from the respective time courses to separate alpha 1- or beta-adrenoceptor stimulation thus indicating a contribution from both adrenoceptor populations to the final inotropic response. Combined alpha 1- and beta-adrenoceptor stimulation gave a pronounced positive lusitropic response as might be expected due to the obviously dominating role of the beta-adrenergic component. However, the maximal lusitropic effect and the shortening of TPT were both slightly less during combined adrenoceptor stimulation compared to separate beta-stimulation thus indicating an influence of the alpha 1-adrenoceptor mediated negative lusitropic effect. Quantitatively, the separate alpha 1- and the separate beta-adrenoceptor mediated inotropic effects were not additive. In accordance with other recent studies, this indicated an inhibitory interaction between the two adrenergic receptor populations in myocardium.

Effects of catecholamines on protein synthesis in cardiac myocytes and perfused hearts isolated from adult rats. Stimulation of translation is mediated through the alpha 1-adrenoceptor

Protein-synthesis rates in freshly isolated cardiac myocytes from adult rats were acutely stimulated by 20-30% by 1 microM-adrenaline, by 1 microM-noradrenaline or by 1 microM-phenylephrine, but were not stimulated by 1 microM-isoprenaline. Stimulation by 1 microM-adrenaline was completely prevented by 100 nM-prazosin. Yohimbine was much less effective in preventing stimulation, and 20 microM-DL-propranolol was completely ineffective. The stimulation of protein synthesis by adrenaline was still observed after inhibition of transcription by actinomycin D. None of these manipulations affected myocyte ATP contents. In anterogradely perfused hearts, protein-synthesis rates were stimulated by 1-2 microM-adrenaline in the presence of 10 microM-DL-propranolol (to decrease the beta-adrenergic effects of adrenaline). ATP contents were not altered, but phosphocreatine contents were increased. These observations lead us to conclude that cardiac protein synthesis can be stimulated acutely at the level of translation by alpha 1-adrenergic stimulation. We discuss possible roles for protein kinase C and intracellular alkalinization in the mediation of this effect.

Functional alpha-adrenoceptors in human atrial preparations in the presence of beta-receptor blockade

Inotropic effects via cardiac alpha-adrenoceptors were studied in electrically driven auricular strips (1 Hz, 37 degrees C) from patients treated with beta-blockers for months prior to open heart surgery. Marked alpha-mediated positive inotropic effects were demonstrated with adrenaline (A), noradrenaline (NA) and phenylephrine (PHE) in the presence of beta-blocker and with blockers of the muscarinic receptor and of the neuronal and extraneuronal uptake mechanisms for the catecholamines. In the presence of approximately 10(-6) M propranolol the maximal effects as well as the potencies (pD2-values) for A and NA were not significantly different while higher than for PHE. The alpha 1-blocker, prazosin (10(-6) M), markedly reduced the pD2-values but not the intrinsic activities (alpha-values) for A, NA and PHE in the beta-blocked preparations. Methoxamine, however, induced negative inotropic responses at normal and low frequencies (1, 0.5 and 0.1 Hz) of stimulation, suggestive of non-specific, cardiodepressant effects. Other agonists with alpha-effects in other types of tissue (oxymethazoline, xylomethazoline and clonidine) were without effects on the force and velocity of contraction in the auricular strips under the present experimental conditions. The results show alpha 1-type of adrenoceptor-induced inotropic effects for A, NA and PHE during beta-blockade in human auricular strips, indicating that cardiac alpha 1-receptors may have clinical importance by increasing the inotropy of the human myocardium treated with beta-blocking agents.

Enhancement of the alpha-adrenergic inotropic component of noradrenaline by simultaneous stimulation of muscarinic acetylcholine receptors in rat myocardium

The contribution of an alpha-adrenoceptor-mediated component to the final inotropic response to noradrenaline in the absence and presence of muscarinic acetylcholine receptor stimulation (which exerts a 'functional' antagonism of effects mediated through beta-adrenoceptors but not through alpha-adrenoceptors) was evaluated by recording contraction and relaxation in isolated, paced rat papillary muscles. In the absence of muscarinic acetylcholine receptor stimulation, the alpha 1-selective adrenoceptor blocker prazosin (0.12 microM) did not significantly influence the dose-dependent response to noradrenaline with respect to either contractility or to relaxation. In the presence of concomitant muscarinic acetylcholine receptor stimulation by 10 microM carbachol, prazosin reduced by 32% (alpha = 0.028) the maximal increase in contractility (expressed as (dT/dt)max) evoked by noradrenaline compared to the absence of prazosin. Prazosin also did not influence the effect of noradrenaline upon relaxation under these conditions. Carbachol itself did not significantly reduce the maximal contractile response to noradrenaline. Thus cholinergic stimulation increases both relatively and absolutely the alpha-adrenergic inotropic component of noradrenaline. These observations indicate a ternary regulatory system of myocardial contractility through the autonomic receptors.

[Alpha-adrenoceptors in the myocardium: incidence and functional significance]

Alpha-adrenoceptors mediating positive inotropic effects are well established in the heart of various species including human heart. The mechanism by which alpha-adrenoceptor stimulation increases force of contraction is not known. cAMP is unlikely to be involved as a mediator. Evidence has been presented that an increase in magnitude and duration of the slow Ca++ inward current may be partly responsible for the positive inotropic effect. In addition, stimulation of alpha-adrenoceptors may increase Ca++ sensitivity of the contractile proteins. Stimulation of alpha-adrenoceptors by endogenous catecholamines may serve as a reserve mechanism under various conditions of impaired beta-adrenergic influence, e.g. hypothyroidism, bradycardia or ischemia. Furthermore, alpha-adrenoceptors may be involved in the genesis of reperfusion arrhythmias in ischemic heart.