Involvement of extracellular calcium in gastric stimulation

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(β-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.