Physiological significance of secretin in the pancreatic bicarbonate secretion. I. Responsiveness of the secretin-releasing system in the upper duodenum.

In nine normal subjects intraduodenal pH was measured by means of a glass electrode placed in the passage from the first to the second part of the duodenum. The physiological variations in pH were simulated by intraduodenal injection of 2.5, 5, or 10 ml of 0.1 mol x 1(-1) HCl and subsequent neutralization by injection of bicarbonate. A total of 26 injections of acid was followed by a pH spike from a median pH of 7.0 to a median of pH 2.1 Median spike duration was 45 sec. The concentration of secretin in plasma increased from a median of 1.2 pmol x 1(-1) to a peak value of 2.2 pmol x 1(-1) after 4 min. It is concluded that the secretin response to a brief acidification of the first 4--6 cm of the duodenum is sufficient to explain the physiological variations in the concentration of secretin in human plasma.