AJP - Gastrointestinal and Liver Physiology, Vol 258, Issue 4 544-G551, Copyright © 1990 by American Physiological Society
Decreased secretion due to a Ca2+ influx defect in frog peptic cells isolated with EGTA
N. Uemura, K. E. Dickinson, Y. Horiguchi, H. Matsumoto and B. I. Hirschowitz
Division of Gastroenterology, University of Alabama, Birmingham 35294.
Cells prepared from frog esophageal peptic glands by dispersal in low-Ca2+
medium (peptic acini) or 1 mM ethylene glycol-bis(beta-aminoethyl
ether)-N,N,N',N'-tetraacetic acid (EGTA)-containing medium ("EGTA cells")
were compared. EGTA cells were characterized by decreased secretory
responses to agonists [bombesin (BB), acetylcholine, and isoproterenol] and
intracellular messenger activators (forskolin,
12-O-tetradecanoyl-phorbol-13-acetate), decreased relative intrinsic
efficacies of muscarinic agonists, and somatostatin insensitivity.
Decreased BB and muscarinic receptor responses were not associated with
changes in receptor number or characteristics. The time course of BB- and
acetylcholine-stimulated pepsinogen secretion indicated that the marked
reduction was confined largely to the late secretory phase (2-30 min),
dependent on extracellular Ca2+, rather than early phase (less than 2 min)
secretion, which is related to release of intracellular Ca2+. The defect
could be reversed by the Ca2+ ionophore A23187. BB-stimulated intracellular
Ca2+ mobilization measured with fura-2/AM was similar in the two cell
preparations, whereas BB-stimulated 45Ca2+ uptake was reduced threefold in
EGTA cells, and this defect was also reversed by A23187. Somatostatin
inhibited both BB-stimulated secretion and 45Ca uptake by peptic acini, but
it had no significant effect on these parameters in EGTA cells.
Cytochalasin B inhibited BB stimulation in peptic acini but not EGTA cells.
These findings suggest that peptic cells isolated with EGTA exhibit
decreased secretory responses that are due at least in part to impairment
of a mechanism for uptake of extracellular Ca2+.
