Iwona Schiander

Anti-M2 muscarinic receptor antibodies inhibit β-adrenoceptor-mediated inotropic response in rat myocardium

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.

106: BOTH |[alpha]|- AND |[beta]|-ADRENORECEPTOR |[lpar]|AR|[rpar]| MEDIATED INOTROPIC COMPONENTS |[lpar]|IC|[rpar]| OF NORADRENALINE |[lpar]|NA|[rpar]| ARE REVEALED BY REVERSING THE RESPONSE IN MYOCARDIUM BY ADRENERGIC BLOCKERS |[lpar]|AB|[rpar]|

The contribution of α-AR stimulation (S) to the total inotropic effect (IE) of NA during full β-ARS has been questioned. The present study reveals a way of demonstrating an α-AR effect in the presence of full β-ARS by studying the reversal responses to AB at supramaximal NA stimulation in rat papillary muscles. The response was rapidly reversed (t50 = 2.8 ± 0.2 min) by simultaneous addition of the β-blocker timolol and the α-blocker prazosin. When AB were added sequentially (5-10 minutes apart), two IC in the inotropic response to NA could be demonstrated: one IC which was sensitive to timolol (73.3 ± 6.9 % of total response) and was taken as β-AR mediated and one IC which was sensitive to prazosin (26.7 ± 5.7 % of total response) and was taken as α-AR mediated. Thus, there is a significant contribution also from an α-AR effect. There is a mutual inhibition of one component upon the other as the expression of both α-AR and β-AR effects was less during combined AR stimulation than when the receptor populations were stimulated separately.