Journal of Applied Physiology, Vol 60, Issue 1 216-224, Copyright © 1986 by American Physiological Society
Physiological antagonism caused by adrenergic stimulation of canine tracheal muscle
A. R. Leff, J. Tallet, N. M. Munoz and N. Shoulberg
We studied the simultaneous alpha- and beta-adrenergic response
characteristics of canine tracheal smooth muscle in 398 strips from 67 dogs
in vitro. Experiments were performed to determine the effects of
beta-adrenergic blockade on the expression of the alpha-adrenoceptor
contractile responses elicited by norepinephrine (NE), phenylephrine (PE),
and clonidine (CLO). Maximal active tension caused by NE increased from
39.1 +/- 27.0 to 241 +/- 75.0 g/cm2 as the concentration of propranolol
(PROP) was increased from 10(-6) to 10(-4) M. Augmentation of tracheal
smooth muscle contraction caused by PE and CLO was also observed with
progressive beta-adrenoceptor blockade; contraction to NE, PE, and CLO was
blocked selectively with 3 X 10(-5) M phentolamine (PA) and
phenoxybenzamine (PBZ). The beta-adrenergic relaxing properties of the same
three agonists were also studied. After alpha-adrenergic blockade with PA
or PBZ, all three agonists caused relaxation (NE greater than CLO greater
than PE) of methacholine-induced contraction of tracheal smooth muscle that
was reversed selectively with PROP. We demonstrate that NE, PE, and CLO
cause simultaneous stimulation of both the alpha- and beta-adrenergic
receptors in tracheal smooth muscle; the net response elicited is the
result of adrenergic physiological antagonism and depends on the relative
degree of alpha- and/or beta-adrenoceptor blockade.
