AJP - Heart and Circulatory Physiology, Vol 257, Issue 1 132-H140, Copyright © 1989 by American Physiological Society
Alpha 2-adrenoceptor stimulation can augment coronary vasodilation maximally induced by adenosine in dogs
M. Hori, M. Kitakaze, J. Tamai, K. Iwakura, A. Kitabatake, M. Inoue and T. Kamada
First Department of Medicine, Osaka University School of Medicine, Japan.
To determine whether alpha 2-adrenoceptor stimulation can augment
adenosine-induced coronary vasodilation, 34 open-chest dogs were studied.
When a small dose of clonidine (up to 0.24 micrograms.kg-1.min-1 ic) was
administered under beta-adrenoceptor blockade, coronary blood flow [312 +/-
16 (SE) ml.100 g-1.min-1] maximally induced by intracoronary infusion of
adenosine was further increased (P less than 0.05) by 66 +/- 16 ml.100
g-1.min-1, despite no significant changes in coronary perfusion pressure,
myocardial oxygen consumption, and coronary venous adenosine concentration.
However, when a larger dose of clonidine (0.36-0.60 micrograms.kg-1.min-1)
was infused, adenosine-induced flow progressively decreased. This biphasic
action of the alpha 2-adrenoceptor activity was also observed when the dose
of norepinephrine was increased during alpha 1-adrenoceptor blockade with
prazosin. Norepinephrine up to 0.24 micrograms.kg-1.min-1 (ic) further
increased adenosine-induced coronary blood flow by 24 +/- 5% (P less than
0.001), whereas hyperemic flow was decreased by a larger dose of
norepinephrine. In contrast to the alpha 2-adrenoceptor stimulation, the
alpha 1-adrenoceptor stimulation (norepinephrine with yohimbine)
progressively decreased coronary blood flow. Furthermore, with a small dose
of clonidine, reactive hyperemic flow significantly increased compared with
that without clonidine (303 +/- 13 vs. 355 +/- 13 ml.100 g-1.min-1, P less
than 0.001), but a larger dose of clonidine adversely reduced reactive flow
(254 +/- 18 ml.100 g-1.min-1, P less than 0.001). Adenosine release during
reactive hyperemia with and without intracoronary infusions of clonidine
were not altered significantly.(ABSTRACT TRUNCATED AT 250 WORDS)
