The aim of this study was to investigate the effects of A(1) and A(2) adeno
sine-receptor activation on the sympathetic nervous system. The effects on
efferent renal nerve activity of selective A(1) (CCPA; 2-chloro-N-6-cyclope
ntyladenosine) and A(2) (2HE-NECA; 2-hexynyl-5'-N-ethylcarboxamidoadenosine
) adenosine-receptor agonists were studied in anesthetized rats either with
intact baroreflexes (intact rats) or with bilateral sinoaortic denervation
and vagotomy (denervated rats). After a control period of 5 min, A(1) or A
(2) agonist or vehicle were intravenously infused for 8 min in separate gro
ups of intact or denervated rats, in which arterial pressure and heart rate
were continuously recorded. CCPA (5.0 mu g/kg/min) and 2HE-NECA (0.7 mu g/
kg/min) were selected to obtain comparable blood pressure changes over the
period of observation. Arterial pressure significantly and equally decrease
d during the A(1) (-41 +/- 8%), and A(2) (-35 +/- 5%) agonist administratio
n. Heart rate significantly decreased during A(1) agonist infusion, but it
did not change during A(2) agonist administration. Bilateral sinoaortic den
ervation and vagotomy did not modify the hemodynamic responses to both drug
s. The A(1) and A(2) administration caused a large and significant increase
in efferent renal nerve activity (+66 +/- 22% and +76 +/- 15%, respectivel
y), and this effect was entirely abolished in denervated rats. A linear rel
ation with a significant negative slope between changes in arterial pressur
e and changes in neural discharge was observed for each treatment. The comp
arison of the regression slopes showed that the reflex increase of efferent
sympathetic activity caused by the administration of both agonists was sig
nificantly smaller than the increment induced by equipotent hypotensive dos
e of sodium nitroprusside (10 mu g/kg). These data show that the selective
activation of A(1) and A(2) receptors elicits a reflex increase in efferent
renal nerve activity. This neural activation is smaller as compared with t
he effect of equihypotensive doses of sodium nitroprusside, thus indicating
a blunting effect of both adenosine agonists on baroreceptor sensitivity.