Signal transduction in gastric and intestinal smooth muscle is mediate
d by receptors coupled via distinct G proteins to various effector enz
ymes, including PI-specific PLC beta 1 and PLC-beta 3, and phosphatidy
lcholine (PC)-specific PLC, PLD and PLA(2). Activation of these enzyme
s is different in circular and longitudinal muscle cells, generating C
a2+ mobilizing (IP3, AA, cADPR) and other (DAG) messengers responsible
for the initial and sustained phases of contraction, respectively. IP
3-dependent Ca2+ release occurs only in circular muscle. Ca2+ mobiliza
tion in longitudinal muscle involves a cascade initiated by agonist in
duced transient activation of PLA(2) and formation of AA, AA dependent
depolarization of the plasma membrane and opening of voltage-sensitiv
e Ca2+ channels. The influx of Ca2+ induces Ca2+ release by activating
sarcoplasmic ryanodine receptor/Ca2+ channel and stimulates cADPR for
mation which enhances Ca2+-induced Ca2+ release. The initial [Ca2+](i)
transient in both muscle cell types results in Ca2+/calmodulin-depend
ent activation of MLC kinase, phosphorylation of MLC20 and interaction
of actin and myosin. The sustained phase is mediated by a Ca2+-indepe
ndent isoform of PKC, PKC-epsilon. DAG for this process is generated b
y PLC- and PLD-mediated hydrolysis of PC. Relaxation is mediated by cA
MP- and/or cGMP-dependent protein kinase which inhibit the initial [Ca
2+](i) transient and reduce the sensitivity of MLC kinase to [Ca2+](i)
. Relaxation induced by the main enteric relaxant neurotransmitters, V
IP and PACAP, involves two cascades, one of which reflects activation
of adenylyl cyclase. A distinct cascade involves G-protein dependent s
timulation of Ca2+ influx leading to Ca2+/calmodulin-dependent activat
ion of a constitutive eNOS in muscle cells; the generation of NO activ
ates soluble guanylyl cyclase. The resultant activation of PKA and PKG
is jointly responsible for muscle relaxation. (C) 1997 Elsevier Scien
ce Inc.