AJP - Gastrointestinal and Liver Physiology, Vol 258, Issue 1 103-G106, Copyright © 1990 by American Physiological Society
Identification of neurotransmitters by selective protection of postjunctional receptors
J. R. Grider
Department of Physiology, Medical College of Virginia, Virginia Commonwealth University, Richmond 23298-0711.
The neurotransmitter responsible for relaxation of gastric smooth muscle
was identified by a technique involving the use of selective ligands to
protect postjunctional receptors combined with inactivation of all other
receptors with N-ethylmaleimide (NEM). Relaxation in response to field
stimulation (80 V, 1 ms, 0.5-8 Hz) and to maximally effective
concentrations of vasoactive intestinal peptide (VIP; 1 microM), ATP (1
mM), isoproterenol (1 mM), dibutyryl adenosine 3',5'-cyclic monophosphate
(cAMP; 1 mM), and forskolin (1 microM) was measured in muscle strips from
guinea pig gastric fundus before and after treatment with NEM (5 microM)
for 1 h. Protection of VIP receptors with VIP or the VIP antagonist
VIP10-28 fully protected relaxation induced by VIP and by field
stimulation, but did not protect relaxation induced by ATP or
isoproterenol. Protection of ATP receptors with ATP protected only the
response to ATP, but did not protect the response to field stimulation,
VIP, or isoproterenol. Relaxation induced by either forskolin or dibutyryl
cAMP was not altered by treatment with NEM alone or in the presence of
protective ligands, indicating that at the concentrations used NEM
inactivated membrane receptors without affecting intracellular relaxation
mechanisms. These studies indicate that VIP but not ATP is the
neurotransmitter responsible for neurally induced relaxation in the gastric
fundus.
