RESUMEN
The modulation of the inotropic effect by affinity-purified antibodies against a synthetic peptide corresponding to the second extracellular loop of the human muscarinic M2 receptors was studied in adult rat ventricular myocardium. These anti-muscarinic M2 receptor antibodies shifted the dose-response relationship of the beta-adrenoceptor agonist isoproterenol to higher concentrations whereas preimmune rabbit immunoglobulin G (IgG) or antibodies against the N-terminus of the beta 1-adrenoceptor had no effect. This effect of anti-muscarinic M2 receptor antibodies was fully blocked after preincubation with the antigenic peptide. No significant change of maximal inotropic response to isoproterenol was observed in the presence of anti-muscarinic M2 receptor antibodies. The anti-muscarinic M2 receptor antibodies did apparently not hamper the access of the muscarinic receptor agonist carbachol. The muscarinic receptor antagonist atropine attenuated the effect of the anti-muscarinic M2 receptor antibodies. The present study demonstrates for the first time in intact adult ventricular myocardium a specific stimulatory muscarinic activity of antibodies raised against a part of the muscarinic M2 receptor protein.
Asunto(s)
Agonistas Adrenérgicos beta/farmacología , Anticuerpos/farmacología , Corazón/efectos de los fármacos , Isoproterenol/farmacología , Contracción Miocárdica/efectos de los fármacos , Receptores Muscarínicos/inmunología , Secuencia de Aminoácidos , Animales , Reacciones Antígeno-Anticuerpo/efectos de los fármacos , Atropina/farmacología , Carbacol/farmacología , Relación Dosis-Respuesta a Droga , Inmunoglobulina G/farmacología , Datos de Secuencia Molecular , Agonistas Muscarínicos/farmacología , Antagonistas Muscarínicos/farmacología , Péptidos/farmacología , Ratas , Receptor Muscarínico M2 , Receptores Adrenérgicos beta 1/inmunología , Receptores Muscarínicos/metabolismoRESUMEN
The intracellular mechanisms activated by stimulation of myocardial alpha 1-adrenoceptors are not known. As in several other tissues, however, activation of alpha 1-adrenoceptors in heart has been related to breakdown of phosphoinositides resulting in production of putative intracellular messengers: different inositol phosphates and diacylglycerol. Lithium has been shown to inhibit enzymes hydrolyzing inositol phosphates. In the present paper we report studies on the effect of lithium upon the alpha 1-adrenoceptor mediated inotropic response elicited in electrically driven rat papillary muscles. While there was no shift of the horizontal positioning of the dose-response curve to alpha 1-adrenergic stimulation in the presence of lithium, the alpha 1-adrenoceptor mediated inotropic effect was increased in a concentration dependent manner (0.25 to 3.0 mmol/l lithium). For comparison, the effect of lithium upon the beta-adrenoceptor mediated inotropic response was also studied. At 3.0 mmol/l lithium, the horizontal position of the dose-response curve to beta-adrenoceptor stimulation was shifted significantly to the right (to higher agonist concentrations) and the maximal beta-adrenoceptor mediated inotropic response was slightly although not significantly reduced. Thus the augmenting effect of lithium upon the alpha 1-adrenoceptor mediated response was specific for this receptor type. Although the effect of lithium may be complex, the data are compatible with the hypothesis that the inositol phosphates may be of functional importance during stimulation of myocardial alpha 1-adrenoceptors.
Asunto(s)
Litio/fisiología , Contracción Miocárdica/efectos de los fármacos , Receptores Adrenérgicos alfa/fisiología , Animales , Técnicas In Vitro , Masculino , Músculos Papilares/efectos de los fármacos , Fenilefrina/farmacología , Ratas , Ratas Endogámicas , Receptores Adrenérgicos alfa/efectos de los fármacos , Receptores Adrenérgicos beta/efectos de los fármacosRESUMEN
The classical approach of displacing the dose-response curve by an alpha adrenoceptor blocker has most often failed to demonstrate a contribution of an alpha adrenoceptor-mediated component in the final inotropic response to norepinephrine unless the beta adrenoceptors are extensively blocked. To unmask and quantify the inotropic components in the response to norepinephrine, we took a different approach by studying reversal responses to appropriate adrenoceptor blockers in isolated paced rat papillary muscles maximally stimulated by norepinephrine. The inotropic response to norepinephrine was rapidly reversed by simultaneous addition of the beta adrenoceptor blocker timolol and the alpha adrenoceptor blocker prazosin. When the adrenoceptor blockers were added sequentially, both alpha and beta adrenoceptor-mediated components in the inotropic response to norepinephrine could be demonstrated: one beta adrenoceptor-mediated component (timolol sensitive only) that represented about 75% of the inotropic response and one alpha adrenoceptor-mediated component (prazosin sensitive only) that represented about 25% of the inotropic response. When one adrenoceptor population was eliminated by giving one of the adrenoceptor blockers before norepinephrine, the inotropic response in this situation could be completely reversed by the other adrenoceptor blocker. The expression of both alpha and beta adrenoceptor-mediated components was significantly less during concomitant receptor stimulation than when the respective receptor populations were stimulated separately. Thus, there was apparently a mutual inhibition of one component upon the other. Although the beta adrenoceptor-mediated inotropic component clearly was the dominating one, the present observations will explain the difficulties in demonstrating an alpha adrenoceptor-mediated component during unopposed beta adrenoceptor stimulation in the inotropic response to norepinephrine in earlier studies.
Asunto(s)
Contracción Miocárdica/efectos de los fármacos , Norepinefrina/farmacología , Receptores Adrenérgicos alfa/fisiología , Receptores Adrenérgicos beta/fisiología , Animales , Técnicas In Vitro , Masculino , Prazosina/farmacología , Ratas , Ratas Endogámicas , Timolol/farmacologíaRESUMEN
The alpha- and beta-adrenoceptor mediated inotropic effects of noradrenaline were determined in rat heart papillary muscle. The pure alpha-adrenergic inotropic effect, obtained in the presence of the beta-blocker timolol, was about half the pure beta-adrenergic inotropic effect, obtained in the presence of the alpha-blocker prazosin. The maximal inotropic effect of noradrenaline in the absence of blockers was, however, of the same magnitude as the pure beta-adrenergic effect. The alpha- and beta-adrenergic components of the inotropic response to noradrenaline were determined as those reversed by prazosin and timolol, respectively. They were smaller than the pure effects evoked separately. When beta-stimulation by noradrenaline was attenuated by carbachol, the alpha-adrenergic inotropic component was increased. Thus the alpha-effect is increased when the beta-adrenergic effect is antagonized. The alpha-adrenergic inotropic component of noradrenaline seems to play a variable role depending upon the degree of concomitant beta-receptor stimulation in the heart.