Antagonism of the Positive Dromotropic Effect of Isoproterenol by Adenosine: Role of Nitric Oxide, cGMP-dependent cAMP-phosphodiesterase and Protein Kinase G

A. Zima, A. E. Martynyuk, C. N. Seubert, T. E. Morey, C. Sumners, R. F. Cucchiara and D. M. Dennis. Antagonism of the Positive Dromotropic Effect of Isoproterenol by Adenosine: Role of Nitric Oxide, cGMP-dependent cAMP-phosphodiesterase and Protein Kinase G. Journal of Molecular and Cellular Cardiology (2000) 32, 1609–1619. We hypothesized that nitric oxide (NO) plays an important role in mediating the anti-adrenergic effect of adenosine on atrioventricular (AV) nodal conduction. In guinea-pig hearts instrumented for measurement of AV nodal conduction time (atrium-to-His bundle, A–H, interval), the NO synthase (NOS) inhibitor, l-NMMA (100 μ m), reversibly inhibited 80% (P=0.009, n=6) of adenosineʹs anti-adrenergic action on the positive dromotropic effect of isoproterenol (0.01 μ m). In parallel studies carried out in rabbit AV nodal myocytes, intracellular mechanisms whereby NO mediates the inhibitory effect of adenosine on isoproterenol-induced A–H interval shortening were studied. Adenosine (3 μ m) inhibited isoproterenol-stimulated (0.1 μ m) ICa,L(β -ICa,L) by 46±6% (P<0.001, n=17). Consistent with isolated heart data, the NOS inhibitors, -NMMA (100 μ m) and L-NNA (500 μ m) attenuated the effect of adenosine onβ -ICa,Lby 69±8% (P<0.001, n=16) and 69±7% (P<0.001, n=10), respectively. An inhibitor of NO-stimulated guanylyl cyclase LY83538 (40 μ m) reduced the inhibitory effect of adenosine on β -ICa,Lby 97±6% (P=0.004,n =15). Similarly, the non-specific inhibitor of cAMP-phosphodiesterases IBMX (50 μ m) decreased the anti-adrenergic effect of adenosine by 60% (P=0.02, n=6), whereas the extracellular application of the non-hydrolyzeable cAMP analog 8-Br-cAMP (500 μ m) prevented this action of adenosine. Activation of cGMP-dependent protein kinase (PKG) by CPT-cGMP (300 μ m) diminished β -ICa,L, but to a significantly smaller degree (16±4%, P=0.025, n=12) than that caused by adenosine. NO mediates the anti-adrenergic effect of adenosine on AV nodal conduction by a mechanism predominately involving activation of cGMP-dependent cAMP-phosphodiesterase and to a lesser extent activation of PKG.