Involvement of cyclic GMP in nitric-oxide-induced gastric relaxation Comparison of the actions of cyclic GMP and cyclic AMP.
BACKGROUND
Smooth muscle relaxation induced by various agents that increase the cellular levels of cyclic nucleotides (cAMP and cGMP) is accompanied by a decrease in intracellular Ca2+ concentration. However, little is known about the differences between the inhibitory effects of cAMP and cGMP on the contraction of smooth muscle.


OBJECTIVE
To compare the effects and underlying mechanisms of cAMP and cGMP on the inhibition of gastric smooth muscle contraction, cyclic nucleotide promoting agents, as well as cell membrane permeable cyclic nucleotides were used.


METHODS
Isometric contraction was measured from circular muscle strips prepared from the fundus of cat stomach in a cylinder-shaped chamber filled with Krebs-Ringer solution (pH 7.4, temperature 36 degrees C) bubbled with 5% CO2 in O2. The level of inositol phosphates (IPs) was measured.


RESULTS
Forskolin and sodium nitroprusside significantly inhibited acetylcholine (ACh)-induced gastric smooth muscle contraction and increased the cellular levels of cAMP and cGMP, respectively. Direct application of 8-Br-cAMP and 8-Br-cGMP also significantly inhibited ACh-induced contraction. Both verapamil and TMB-8 inhibited ACh-induced contraction. The combined inhibitory effect of verapamil and TMB-8 was significantly greater than the effect of either one, separately. Forskolin or sodium nitroprusside similarly augmented the effect of verapamil. However, the inhibitory effect of TMB-8 was augmented only by 8-Br-cGMP or sodium nitroprusside but not by 8-BrcAMP or forskolin. Forskolin and 8-Br-cAMP significantly inhibited the formation of inositol phosphates stimulated by ACh.


CONCLUSIONS
cAMP inhibits the contraction mechanism associated with intracellular Ca2+ mobilization as well as extracellular Ca2+ influx, while cGMP inhibits contraction by inhibiting the mechanism associated with extracellular Ca2+ influx.
Sim
S S
SS
Dept of Physiology, College of Medicine, The Catholic University of Korea, Seoul.
Kim
Y C
YC
Shim
H S
HS
Choi
J C
JC
Min
D S
DS
Rhie
D J
DJ
Yoon
S H
SH
Hahn
S J
SJ
Kim
M S
MS
Jo
Y H
YH
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D002415:Cats; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology / Q000502:physiology*; D006152:Cyclic GMP / Q000494:pharmacology / Q000502:physiology*; D005260:Female; D007295:Inositol Phosphates / Q000096:biosynthesis; D007537:Isometric Contraction / Q000187:drug effects / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects* / Q000502:physiology; D009569:Nitric Oxide / Q000502:physiology*; D009599:Nitroprusside / Q000494:pharmacology; D013270:Stomach / Q000187:drug effects* / Q000502:physiology
D007295:Inositol Phosphates; D009599:Nitroprusside; D005576:Colforsin; D009569:Nitric Oxide; D000242:Cyclic AMP; D006152:Cyclic GMP; D000109:Acetylcholine