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

Abstract

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 adenosine's 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, l -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.