AJP - Lung Cellular and Molecular Physiology, Vol 268, Issue 2 221-L229, Copyright © 1995 by American Physiological Society
Role of G proteins and KCa channels in the muscarinic and beta-adrenergic regulation of airway smooth muscle
H. Kume, K. Mikawa, K. Takagi and M. I. Kotlikoff
Second Department of Internal Medicine, School of Medicine, Nagoya University, Japan.
We have examined the functional consequences of G protein coupling to
calcium-activated potassium (KCa) channels using isometric tension records
from guinea pig tracheal smooth muscle. After incubation with 1
microgram/ml pertussis toxin (PTX) for 6 h, the contraction response to 1
microM methacholine (MCh) was suppressed by 31.7 +/- 5.0% (n = 10).
Similarly, the contraction was inhibited by 29.1 +/- 5.0% (n = 6) after
application of 0.1 microM AF-DX 116, an M2-selective muscarinic receptor
antagonist. Cholera toxin (CTX, 2.0 micrograms/ml for 6 h), which activates
the stimulatory G protein of adenylyl cyclase (Gs), also suppressed
contraction by 43.9 +/- 3.3% (n = 11). The inhibitory effects of PTX, AF-DX
116, or CTX were reversed in the presence of 100 nM charybdotoxin (ChTX), a
selective KCa channel inhibitor. These findings suggest that disruption of
inhibitory coupling between muscarinic receptor and KCa channels mediated
by PTX-sensitive G proteins, or KCa channel activation induced by
Gs/adenylyl cyclase-linked processes, antagonizes muscarinic contraction.
The isoproterenol concentration-inhibition curves for precontracted trachea
(1 microM MCh) were shifted to the left after perfusion with PTX or AF-DX
116, and the leftward shift of the curve was blocked by ChTX. Thus direct
or indirect regulation of KCa channels mediated by the inhibitory guanine
nucleotide binding protein (Gi) and Gs may play a functionally important
role in the mechanical antagonism by the two receptor agonists.
