The Potentiation of the negative dromotropic effect of adenosine
by free radicals
Surender Rajasekaran, MD; Timothy Morey, MD; Anatoly E Martynyuk,
PhD; Donn Dennis, MD
Dept of Anesthesiology, University of Florida
Background: The negative dromotropic effect of adenosine
underlies its therapeutic value for the treatment of
supraventricular tachyarrhythmias and its role in prolonging AV
nodal conduction during myocardial ischemia. We propose that
that free radicals present during reperfusion following
myocardial ischemia or cardiopulmonary resuscitation
significantly enhance the negative dromotropic effect of
adenosine.
Methods:
Guinea pig hearts were isolated and prepared according to the
Langendorff method. Electrograms were acquired with the use of
unipolar electrodes placed on the surface of the left atrium and
in the His bundle position. The stimulus-to-His bundle interval
was used as a measure of AV nodal conduction time. To determine
if a potentiation existed, we performed an experimental protocol
as detailed in fig. 1 with adenosine (2 ?M) and pyrogallol (30
?M), a free radical generator. To determine which adenosine
receptor subtype was operative, a second series of experiments
were performed, but adenosine’s effect was antagonized with CPX
(100 nM), an A1
receptor
blocker (fig. 2). To determine if this effect of adenosine was
mediated by NO, a third set of experiments was performed in the
presense of a baseline infusion of L-NNMA (100?M), a nitric
oxide synthase inhibitor (fig. 3), To provide additional
evidence of the role of free radicals, another set of
experiments similar to fig. 1 was performed except that
superoxide dismutase 200 U/ml, a free radical inhibitor, was
infused into the system. One way repeated measures ANOVA was
used for statistical analysis.
Results: The accompanying figures demonstrate the outcome of our
experiments.
Discussion: This series of experiments clarify the interactions
between adenosine and
free
radicals on AV nodal conduction. It is well documented that both
are present in certain pathophysiological conditions such as the
reperfusion that follows myocardial ischemia and low flow states
such as sepsis. We showed that this interaction is not simply
additive, but synergistic (P<0.001). The effect is mediated by
the A1 receptor and is reversible by CPX (P<0.001). Our
experiments also suggest that these effects are nitric
oxide-dependent as L-NNMA prevented the potentiation (P<0.001).
We also found that when the free radicals were inhibited by SOD
the potentiation was significantly reduced. Eventually the
knowledge of these interactions could lead to design of possible
interventions towards minimizing tissue damage and the
arrythmogenic potential of this reactions.
