Pepsinogen secretion by the frog esophagus in vitro

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Abstract
Mucosa from the lower esophagus of the bullfrog, R. catesbeiana, permitted the study of pepsinogen secretion without attendant acid secretion. Basal release of pepsinogen was 1.33% of total tissue stores (mucosal + secreted) over a 150 min period. Pentagastrin, histamine, isoproterenol and norepinephrine neither increased nor inhibited pepsinogen release. External Ca and Mg were similarly ineffective. Bethanechol stimulated pepsinogen secretion in a concentration-dependent fashion with a maximum release of 4.85%/150 min at 125 .mu.M. The supramaximal concentration of 1.25 mM bethanechol decreased secretion to 50% of maximum, an inhibition that could be reversed by 10 mM dibutyryl cyclic AMP (dB-cAMP), indicating desensitization at the receptor rather than inhibition by an excess of intracellular cAMP. Theophylline did not affect secretion stimulated by the maximal or supramaximal concentrations of bethanechol or by 10 mM dB-cAMP (5.68%/150 min). Secretion at the half-maximal concentration of bethanechol, 12.5 .mu.M, became maximal (4.99%) and the threshold for stimulation was lowered to 1.25 .mu.M. While dibutyryl cyclic GMP increased secretion slightly (< 12% of dB-cAMP-stimulated secretion), other nucleotides were without effect. As pepsinogen release could be fully stimulated only by the muscarinic agonist, bethanechol, or by dB-cAMP, in this tissue cAMP may act as a 2nd messenger for cholinergic stimulation.

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