Effects of ionophore A23187 and lanthanum on pepsinogen secretion from frog esophageal mucosa in vitro

Abstract

The role of Ca 2+ in the mediation of pepsinogen secretion from frog esophagus was investigated by means of ionophore A23187 and LaCl 3. The esophageal mucosa from Asian bullfrog Rana tigerina was mounted in a double-chamber system to preserve its polarity and was incubated in a medium containing 1.5 mM CaCl 2. Pepsinogen secreted was measured and expressed as % of total. The basal secretion averaged 3.5%/h. Bethanechol (25 μM), dibutyryl-cAMP (10 mM), ionophore A23187 (30 μM) and 3-isobutyl-1-methylxanthine (0.1 mM) increased the secretion to 8.7, 7.4, 7.1 and 6.8%, respectively. The stimulatory effect of bethanechol and of dibutyryl-cAMP were not affected by removing the exogenous Ca 2+ with EGTA. The basal secretion was, however, reduced by 50% when Ca 2+ in the incubation medium was lowered to 20 μM. At this low Ca 2+ concentration, ionophore A23187 not only lost its stimulatory effect but also diminished the stimulation caused by bethanechol and dibutyryl-cAMP. While LaCl 3 at 1 mM had no effect on basal and bethanechol-stimulated secretion, at 10 mM it abolished the stimulation evoked by bethanechol or dibutyryl-cAMP. The conclusions are: (1) both Ca 2+ and cAMP are involved in the mediation of pepsinogen secretion from frog esophagus, (2) basal secretion is dependent on extracellular Ca 2+, whereas bethanechol-stimulated secretion is not, (3) in the plasma membranes of peptic cells may exist a distinct Ca 2+ pool (La 3+- and ionophore A23187-sensitive) which is involved in the stimulated pepsinogen secretion.