Modulation by presynaptic adenosine A(1) receptors of nicotinic receptor antagonist-induced neuromuscular block in the mouse

We have investigated how altering the activation of adenosine A(1) receptors modifies nicotinic receptor antagonist-induced fade of tetanic contractions in the mouse isolated hemi-diaphragm. Vecuronium-induced tetanic fade was attenuated by an adenosine A(1) receptor antagonist (8-cyclopentyl-1,3-dipropylxanthine, DPCPX, 10(-7) M) and by an inhibitor of the synthesis of extracellular adenosine from ATP (alpha,beta-methylene ADP, MeADP, 5 x 10(-5) M). Conversely, vecuronium-induced tetanic fade was potentiated by an adenosine A(1) receptor agonist (N-6-cyclohexyladenosine, CHA, 10(-7) M) and an inhibitor of the extracellular destruction of adenosine (erythro-9-[2-hydroxy-3-nonyl]adenine, EHNA, 10(-4) M). The ability of an adenosine A(1) receptor antagonist to attenuate vecuronium-induced tetanic fade indicates that a component of this fade is due to endogenous adenosine. Further, the ability of the inhibitor of adenosine synthesis to attenuate vecuronium-induced tetanic fade indicates that this endogenous adenosine is derived from ATP. Hexamethonium-induced tetanic fade was also potentiated by increasing adenosine A(1) receptor activation, albeit with a higher concentration of CHA (10(-4) M). However, unlike for vecuronium, hexamethonium-induced tetanic fade was not attenuated by reducing adenosine A(1) receptor activation. This latter observation suggests that the tetanic fade produced by hexamethonium and vecuronium does not share a common mechanism of action.