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Vol. 54, Issue 1, 122-128, July 1998
Departments of
Physiology (K.S.M.) and
Medicine (G.M.M.), Medical
College of Virginia, Virginia Commonwealth University, Richmond,
Virginia 23298-0711
The characteristics of inhibitory regulation of adenylyl cyclase V/VI
by Ca2+ and G proteins were examined in dispersed gastric
smooth muscle cells. The mechanisms were evoked separately,
sequentially, or concurrently using ligand-gated and G protein-coupled
receptor agonists and receptor-independent probes (e.g, thapsigargin). During the initial phase of agonist stimulation, 
,
-methylene-ATP, UTP, and ATP inhibited forskolin-stimulated cAMP formation in a
concentration-dependent fashion. Inhibition by ,-methylene-ATP, which activates ligand-gated P2X receptors, was abolished
by zero Ca2+, whereas inhibition by UTP, which activates
P2Y2 receptors coupled to Gq/11 and
Gi3, was not affected by zero Ca2+ but was
abolished by pertussis toxin (PTX). Inhibition by ATP, which activates
both P2X and P2Y2 receptors, was not affected by zero Ca2+ alone; but after inhibition mediated by
G
i3 was blocked with PTX, inhibition by Ca2+
influx was unmasked and was abolished by zero Ca2+.
Inhibition by cholecystokinin-8 was observed only during the phase of
capacitative Ca2+ influx and was blocked by zero
Ca2+. Inhibition by UTP during this phase was not affected
by zero Ca2+ alone; but after inhibition mediated by
Gi3 was blocked with PTX, inhibition by Ca2+
influx was unmasked and was abolished by zero Ca2+.
Inhibition of adenylyl cyclase V/VI activity in smooth muscle can be
mediated independently by inhibitory G proteins and Ca2+
influx but is exclusively mediated by inhibitory G proteins when both
mechanisms are triggered.
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