Which of the following statements regarding human gastric acid secretion is/are true?

a. Fasting acid secretion, normally 2 to 5 mEq/h, is due to ambient vagal tone and histamine secretion

b. Truncal vagotomy decreases basal secretion by 80%

c. Histamine2 receptor antagonist administration can decrease basal acid secretion by 80%

Both vagal tone and locally secreted histamine are presumed to be the determinants of basal acid secretion in humans. Gastrin does not have a role in basal acid secretion in normal individuals. Parietal cell activation and the resultant acid secretory response is greater to a combination of agonists than the sum of the responses to the agents used singly. This increase in responsiveness is termed potentiation. Potentiating interactions are most apparent when the stimulants use different second messenger systems, for example, acetylcholine and histamine. Conversely, blockade of receptors to one stimulant also blocks responsiveness to the other agonist. Because of this interaction, blockade of histamine receptors by agents like cimetidine decreases responsiveness to acetylcholine. Blockage of acetylcholine release by vagotomy decreases responsiveness to histamine secreted by gastric mast cells. Both vagotomy and histamine2 receptor antagonists decrease basal acid secretion by approximately 80%.