EFFECTS OF ADENOSINE ON RATE-DEPENDENT ATRIOVENTRICULAR NODAL FUNCTION - POTENTIAL ROLES IN TACHYCARDIA TERMINATION AND PHYSIOLOGICAL REGULATION

Background. Adenosine is well known to depress atrioventricular (AV) nodal conduction, but the potential interactions between adenosine and functional AV nodal properties have not been explored. The purpose of the present study was to determine (1) whether exogenous adenosine modifies the rate-dependent properties of the AV node, (2) to what extent such changes underlie the actions of adenosine in an in vitro model of AV reentrant tachycardia (AVRT), and (3) the potential role of endogenous adenosine in rate induced AV nodal responses. Methods and Results. The functional properties of AV nodal recovery (defining the conduction delay of a single premature activation), facilitation (effect of short cycles on subsequent nodal recovery), and fatigue (slowly developing AV nodal delay at a rapid rate) were studied selectively in isolated, superfused rabbit and guinea pig cardiac preparations. Exogenous adenosine increased AV nodal fatigue and attenuated facilitation, resulting in tachycardia-dependent increases in AH interval and AV nodal effective refractory period (AVERP). In experimental AVRT, adenosine caused greater increases in tachycardia cycle length (T) and AVERP as tachycardia rate increased. AVRT was sustained when AVERP/T was <1, and adenosine suppressed AVRT by increasing the slope of the AVERP/T versus tachycardia rate relation, causing the critical ratio of 1 to be attained at slower rates. A mathematical model incorporating quantitative descriptors of recovery, facilitation, and fatigue accounted for changes in AH interval, AVERP, tachycardia cycle length, and AVERP/T under control conditions and in the presence of adenosine. In the absence of exogenous adenosine, I-phenyltheophylline (10 mu mol/L), an adenosine receptor antagonist, did not alter recovery or facilitation but significantly reduced rate-related fatigue (by 31+/-8%, mean+/-SEM, P<.05, in rabbit hearts; 46+/-5%, P<.01, in guinea pig hearts). Combined inhibition of adenosine deaminase (with erythro-9-[2-hydroxy-3-nonyl]-adenine hydrochloride, 5 mu mol/L) and adenosine uptake (with dipyridamole, 1 mu mol/L) increased fatigue in the absence of exogenous adenosine by 57+/-20% (P<.05). Conclusions. We conclude that (1) exogenously administered adenosine increases AV nodal fatigue and reduces facilitation, without altering AV nodal recovery; (2) these changes cause rate-dependent AV nodal depression, which plays a role in adenosine's actions on experimental AVRT; and (3) endogenous adenosine receptor activation plays a role in physiological AV nodal fatigue. Adenosine's ability to terminate reentrant supraventricular tachycardia may be due, at least in part, to its ability to enhance the physiological conduction slowing that results from sustained increases in AV nodal activation rate.