Md. Amirul Islam, Hiroshi Nojima and Ikuko Kimura (*)
Department of Chemical Pharmacology, Faculty of Pharmaceutical Sciences,
Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama 930-0194,
Japan
(*) To whom correspondence should be addressed.
Abstract: Several lines of evidence suggest the molecular and
functional entity of muscarinic M1 receptors in mammalian heart.
We have reported that acetylcholine (ACh) reduces the maximum upstroke velocity
of action potential (Vmax) through activation of muscarinic M1
receptors, which is followed by a muscarinic M2 receptor-mediated
increase. The present study sought to determine whether activation of beta-adrenergic
receptors modulates the muscarinic M1 and M2 receptor-mediated
effects on Vmax in isolated mouse right atria. Intracellular
recordings of spontaneous action potential were done using the conventional
glass microelectrode technique. Isoproterenol (3 nM) completely antagonized
ACh (5 microM)-induced reduction in Vmax. The antagonism was
accompanied by a subsequent increase in Vmax. Propranolol (0.3
microM) abolished the effects of isoproterenol on ACh-induced changes in
Vmax. Isoproterenol antagonized McN-A-343 (4-(m-chlorophenyl-carbamoyloxy)-2-butynyltrimethylammonium
chloride) (300 microM, a muscarinic M1 receptor agonist)-induced
reduction in Vmax. Oxotremorine (0.03 microM), a muscarinic M2
receptor agonist, did not affect Vmax by itself, but significantly
increased it in the presence of 3 nM isoproterenol. The effects of isoproterenol
were mimicked by cholera toxin (100 nM, 1 hr), a Gs-protein activator,
and forskolin (10 nM), a direct activator of adenylyl cyclase. H-89 (N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide,
1 microM), a selective protein kinase (PK)-A inhibitor, abolished the antagonism
by isoproterenol of ACh-induced reduction in Vmax. The present
results suggest that activation of the beta-adrenergic-Gs-adenylyl
cyclase system antagonizes ACh-induced reduction (muscarinic M1-mediated)
and potentiates the subsequent increase (muscarinic M2 receptor-mediated)
in Vmax. The beta-adrenergic antagonism of ACh-induced reduction
in Vmax may involve cross-talk between PK-A and PK-C signaling
pathways.
Keywords: Muscarinic M1 receptor, beta-Adrenergic-muscarinic
interaction, Vmax, Action potential, Right atrium