Caffeine is known to stimulate gastric acid secretion, but, the effects of caffeine on gastric mucus secretion have not been clarified. To elucidate the action of caffeine on gastric mucin-producing cells and its underlying mechanism, the effects of caffeine on mucus glycoprotein secretion and agonist-induced [Ca 2+] i mobilization were examined in human gastric mucin secreting cells (JR-I cells). The measurement of [Ca 2+] i using Indo-1 and the whole cell voltage clamp technique were applied. Mucus glycoprotein secretion was assessed by release of [ 3H]glucosamine. Caffeine by itself failed to increase [Ca 2+] i and affect membrane currents, while it dose-dependently inhibited agonist (acetylcholine (ACh) or histamine)-induced [Ca 2+] i rise, resulting in inhibiting activation of Ca 2+-dependent K + current (I K.Ca) evoked by agonists. The effect of caffeine was reversible, and the half maximal inhibitory concentration was about 0.5 mM. But, caffeine did not suppress [Ca 2+] i rise and activation of I K.Ca induced by A23187 or inositol trisphosphate (IP 3). Theophylline or 3-isobutyl-1-methyl-xanthine (IBMX) did not mimic the effect of caffeine. Caffeine failed to stimulate mucus secretion, while it significantly decreased ACh-induced mucus secretion. These results indicate that caffeine selectively inhibits agonist-mediated [Ca 2+] i rise in human gastric epithelial cells, probably through the blockade of receptor-IP 3 signaling pathway, which may affect the mucin secretion. © 1997 Elsevier Science B.V. All rights reserved.