IONIC MECHANISMS OF THE EFFECT OF ADENOSINE ON SINGLE-RABBIT ATRIOVENTRICULAR NODE MYOCYTES

The ionic mechanisms underlying the negative dromotropic effect of ade nosine were studied in calcium-tolerant myocytes isolated from the reg ion of the rabbit atrioventricular (AV) node. Action potentials and me mbrane currents were recorded by using the whole cell patch clamp tech nique. Adenosine (1 to 50 mu M) abolished the spontaneous activity of AV node myocytes with hyper polarization of the membrane potential. Vo ltage clamp experiments showed that adenosine induced an inwardly rect ifying, time-independent potassium current. These effects were antagon ized by 8-cyclopentyl-1,3-dipropylxanthine and produced by ribose 5-ph osphate isomerase A, indicating that they were mediated by the A(1) ad enosine receptor. Adenosine also had a small direct inhibitory action on the inward calcium current (I-Ca) but had a more marked indirect ac tion following stimulation of the calcium current by isoprenaline. The isoprenaline-induced increase in I-Ca was abolished in the presence o f adenosine 10 mu M. In cells pretreated with the nitric oxide synthas e inhibitor N(<(omega)over bar>)-nitro-L-arginine methyl ester (L-NAME ), the isoprenaline-induced increase in I-Ca was not reduced by the ad dition of adenosine. Coincubation of the cells with L-NAME plus L-argi nine (the endogenous substrate of nitric oxide synthase) restored the adenosine-induced attenuation of I-Ca. A membrane permeable analogue o f cGMP, 8Br cGMP, an inhibitor of cGMP-stimulated phosphodiesterase, p revented the antiadrenergic effect of adenosine. These results suggest that adenosine activates guanylyl cyclase following the production of nitric oxide, and the subsequent stimulation of phosphodiesterase enh ances the breakdown of isoprenaline-elevated cAMP leading to a reducti on in the stimulated I-Ca. In conclusion, the important ionic mechanis ms of the actions of adenosine on AV nodal cells are a direct effect, with activation of a potassium conductance and an indirect antiadrener gic effect on I-Ca, which is mediated by nitric oxide production and p hosphodiesterase stimulation.