Decreased secretion due to a Ca²⁺ influx defect in frog peptic cells isolated with EGTA

¹ Division of Gastroenterology and Department of Dermatology, University of Alabama at Birmingham, Birmingham, Alabama 35294

Cells prepared from frog esophageal peptic glands by dispersal in low-Ca²⁺ medium (peptic acini) or 1 mM ethylene glycol-bis(-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 (<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⁺.

bombesin; somatostatin; isoproterenol; forskolin; phorbol ester; calcium ionophore; muscarinic and gastrin-releasing peptide receptor binding; fura-2/AM; cytosolic calcium; cytochalasin B; electron microscopy

Submitted on February 21, 1989
Accepted on November 13, 1989