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Department of Physiology, School of Medicine of Ribeirão Preto, University of São Paulo, 14049-900 Ribeirão Preto, São Paulo, Brazil
The possible
involvement of adenosine A1 receptors in neurotransmission
of the sympathoexcitatory component of the chemoreflex in the nucleus
tractus solitarii (NTS) of awake rats was evaluated. Unilateral
microinjection of increasing doses of adenosine (0.01, 0.06, 0.12, 1.25, 2.5, and 5.0 nmol/50 nl) into the lateral aspect of the
commissural NTS produced a long-lasting increase in baseline mean
arterial pressure (MAP) and no changes in baseline heart rate (HR).
Microinjection of adenosine at 1.25 nmol/50 nl (ED50) into
the NTS (n = 9) produced a significant increase in
baseline MAP (119 ± 3, 122 ± 4, and 117 ± 4 mmHg at
30 s, 1 min, and 2 min, respectively) compared with control
(102 ± 3 mmHg) but no significant changes after previous
microinjection of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an
adenosine A1 receptor antagonist (107 ± 3, 107 ± 3, and 106 ± 3 mmHg at 30 s, 1 min, and 2 min,
respectively) compared with control (102 ± 3 mmHg).
Microinjection of adenosine before and after DPCPX into the same site
of the lateral commissural NTS produced no changes in baseline HR. In
another group of rats (n = 8), microinjection of DPCPX
(0.285 nmol/50 nl) into lateral and midline aspects of the commissural
NTS produced no significant changes in pressor (+46 ± 4 vs.
+47 ± 2 mmHg) or bradycardic responses (
216 ± 9 vs.
226 ± 12 beats/min) to chemoreflex activation with intravenous
potassium cyanide compared with control responses. These data show that
microinjection of adenosine into the NTS produced a small and
long-lasting pressor response by activating A1 receptors
and that blockade of these receptors produced no changes in
cardiovascular responses to chemoreflex activation. We conclude that
adenosine A1 receptors are not involved in processing of
the chemoreflex afferents at the NTS level.
8-cyclopentyl-1,3-dipropylxanthine; purinergic receptors; sympathoexcitation; cardiovascular reflexes
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