AJP - Gastrointestinal and Liver Physiology, Vol 248, Issue 3 332-G336, Copyright © 1985 by American Physiological Society
Jejunal-ileal differences in dopaminergic but not alpha-adrenergic antisecretory effects
R. Wahawisan, T. S. Gaginella and L. J. Wallace
Clonidine, an alpha-adrenergic agonist, and apomorphine, a dopaminergic
agonist, inhibit water secretion in rat intestine induced by dibutyryl cAMP
(DBcAMP) in situ. Apomorphine reversed DBcAMP-induced secretion in ileum
and jejunum with ED50 values of 90 and 200 nM, respectively. ED50 values
for clonidine's antisecretory effect were 32 and 2 microM in ileum and
jejunum, respectively. The antisecretory effects of both clonidine and
apomorphine were blocked by the alpha-adrenergic antagonist phentolamine
and by the dopaminergic antagonists halo-peridol and sulpiride.
Phentolamine was about 10-fold more potent against clonidine than against
apomorphine in the ileum and the jejunum. Haloperidol and sulpiride were at
least 10-fold more potent against apomorphine than against clonidine in the
ileum. In contrast, in the jejunum, haloperidol was equally potent in
blocking effects of both agonists, while sulpiride did not block the
apomorphine effect. The data are consistent with the presence of both alpha
2-adrenergic and DA2-dopaminergic receptors that stimulate absorption in
the ileum. In the jejunum, antisecretory effect appears to be mediated by
an action on either an alpha 2-adrenergic or a dopaminergic receptor with
characteristics different from those of peripheral DA1- or DA2-receptors.
