Differential effects of locally-applied capsaicin on distension-stimulated gastric acid secretion in the anesthetized rat

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Abstract
The effects induced by the local administration of capsaicin on acid production have been investigated in the continuously perfused stomach of the anesthetized rat. Basal acid secretion was not influenced by 10 min intragastric perfusion with capsaicin (300 μg min−1). Acid responses elicited by distension of the stomach with increases in intragastric pressure of 5 and 10 cm H2O were not modified after a 10 min intraluminal infusion with 80 or 300 μg min−1 of capsaicin. H+ output stimulated by higher intraluminal pressure (20 cm H2O) were significantly decreased by intraluminal infusion of capsaicin (20, 80, 300 and 600 μg min−1). Acid responses to carbachol (4 μg kg−1, i.p.) were not influenced by intragastric (300 μg min−1), or systemic neonatal, treatment with capsaicin. Intraluminal infusion of the neurotoxin tetrodotoxin (0.12 μg min−1, 10 min) decreased acid responses to an increase in intragastric pressure of 20 cm H2O but not those elicited by distention with a pressure of 10 cm H2O. Neonatal systemic treatment (s. c.) with capsaicin or local gastric serosal application of either capsaicin or tetrodotoxin abolished acid responses to gastric distension (+ 20 cm H2O). Capsaicin (80 μg min−1) and tetrodotoxin (0.12 μg min−1) infused concurrently into the lumen did not inhibit gastric acid secretion stimulated by an increase of 20 cm H2O in intragastric pressure to any greater extent than did either drug given alone. The inhibition of acid secretion cannot be attributed to an increase in H+ loss, since intragastric infusion of capsaicin (300 μg min−1) did not cause any significant acid loss in rats pretreated with omeprazole and undergoing luminal perfusion with acid saline during gastric distension (+ 20 cm H2O). Neither the intragastric perfusion nor the serosal application of capsaicin modified the H+ production induced by electrical stimulation of the vagus. This acid response was, however, abolished following 10 min intraluminal perfusion with tetrodotoxin (0.12 μg min−1). These observations confirm that capsaicin-sensitive sensory fibers located in the stomach wall mediate the acid secretory responses to intragastric distension. Such fibers are susceptible to the effects of capsaicin when administered systemically or through the gastric serosa, but only partially if capsaicin is infused intraluminally.