AJP - Gastrointestinal and Liver Physiology, Vol 241, Issue 5 365-G375, Copyright © 1981 by American Physiological Society
Involvement of extracellular calcium in gastric stimulation
C. D. Logsdon and T. E. Machen
We have tested whether external Ca2+ is required for either initiation or
maintenance of secretory parameters, including membrane elaboration of
oxyntic cells, in frog gastric mucosa. Ca2+ was removed from in vitro
mucosal preparations [by washing repeatedly in Ca2+-free Ringer solution
and adding 0.1 mM ethylene
glycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid to the serosal
solution] either before (i.e., resting tissues) or after addition of
stimulants. Electrophysiology [transepithelial potential difference (PD)
and resistance], morphology (morphometric analysis of transmission electron
micrographs), and transport (H+ secretion) were monitored. La3+ (1 mM) was
added to the mucosal solution to help maintain resistance and PD. La3+
decreased tissue shunt conductance during Ca2+-free conditions, as
evidenced by a decreased mucosal-serosal flux of 22Na+, presumably by
preserving tight-junction integrity. Secretion was elicited by histamine
alone or in combination with dibutyryl cAMP and isobutylmethylxanthine (a
phosphodiesterase inhibitor). External Ca2+ is not required for the
initiation of H+ secretion or the accompanying morphological changes when
the combined stimulants are used, whereas H+ secretion and the
morphological change showed some Ca2+ dependency when histamine alone was
used. Thus, histamine-elicited secretion seems to be more sensitive to Ca2+
removal than that brought about by the combined stimulants. Long-term
effects of Ca2+-free solutions on resistance, PD, and H+ secretion can
largely be explained by disruptive effects on tight junctions.
