Somatostatin and lanthanum discriminate two Ca2+ mobilizing mechanisms regulated by bombesin receptors in peptic cells

Nonselective cation channel as a Ca(2+) influx pathway in pepsinogen-secreting cells of bullfrog esophagus

In pepsinogen-secreting cells of bullfrog (Rana catesbeiana), recent evidence suggests that Ca(2+) release from internal stores followed by Ca(2+) influx across the plasma membrane elicits pepsinogen secretion. Such a Ca(2+) influx could be carried by a background current, potentiated by bombesin, that was found in these cells using the whole cell patch-clamp technique. The permeability ratio of Cs(+)-Rb(+)-K(+)-Na(+)-Li(+)-N-methyl-D-glucamine(+)-Ca(2+) was 1.01:1:1:0.86:0.72:0.54:0.34. The current was almost totally blocked by the nonselective cation channel blockers La(3+) (0.1 mM) and Gd(3+) (0.1 mM) and was activated by intracellular Ca(2+). These properties demonstrated that the current, which was activated by bombesin, was a nonselective cation current. At the same time, Gd(3+) suppressed pepsinogen secretion by 29 +/- 5.6% in isolated pepsinogen-secreting glands. These results are in accord with the idea that a nonselective cation channel in pepsinogen-secreting cells plays a role as a Ca(2+) influx pathway leading to secretion of pepsinogen in bullfrog esophageal mucosa.

Effects of cholinergic, histaminergic, and peptidergic stimulation on pepsinogen secretion by isolated human peptic cells

BACKGROUND

Whereas pepsin secretion by the human stomach has been extensively investigated, the characteristics of the peptide cell cannot be fully understood from in vivo studies. We therefore studied isolated human peptic cells to test directly cellular responses to different agents.

METHODS

Eight endoscopic biopsy specimens yielded 10(6) cells, > 90% pure and > 95% viable. Secreted pepsinogen was measured with a sensitive hemoglobin digestion method at pH 2.

RESULTS

Pepsinogen secretion was concentration-dependently stimulated by acetylcholine (ACh) (EC50 = 0.3 microM), cholecystokinin (CCK)-8 (2 nM), histamine (2 microM), and gastrin-I (30 nM) but not by bombesin or pentagastrin. ACh stimulation was inhibited 40 times more potently by atropine (IC50 = 12 nM) than by pirenzepine (IC50 = 0.5 microM). Histamine was inhibited by 10(-4) cimetidine. CCK-8 stimulation was inhibited 80% by the CCK-A-selective antagonist L364,718 (IC50 = 12 nM) but not by the CCK-B-selective antagonist L365,260.

CONCLUSION

Isolated human peptic cells from endoscopic biopsy specimens secrete pepsinogen in response to ACh > CCK-8 > histamine > gastrin-I. The human peptic cell muscarinic-cholinergic receptor is not of the M1 subtype, and the CCK-8 response is predominantly mediated by a CCK-A receptor subtype.

Gastric chief cells: receptors and signal-transduction mechanisms

Elucidation of receptors and mediators regulating gastric pepsinogen secretion has lagged behind understanding of the factors that control acid secretion. During the past decade, as a consequence of the development of in vitro models for studying the control of pepsinogen secretion at the cellular level, much information about chief cell receptors and signal-transduction mechanisms has been obtained, including the identification and characterization of receptors for secretin, vasoactive intestinal polypeptide, cholinergic agonists, gastrin, cholecystokinin, peptide YY, and cholera toxin. Moreover, these cell preparations have permitted secretagogue-induced changes in chief-cell calcium concentration, protein kinase C distribution, and phosphoinositide and cyclic nucleotide content to be measured and related to changes in pepsinogen secretion. This article reviews these advances, discusses areas of uncertainty and controversy, and indicates areas for future investigation.