Human pancreatic polypeptide: studies of fasting and postprandial plasma concentrations.

Circulating human pancreatic polypeptide (HPP) was studied in 16 human subjects during two 24-h test periods, one fasting and another eating meals at 0830, 1230, and 1730. In six subjects the tests were repeated with propantheline administration. Blood was sampled every 30 min; additional blood samples were drawn 15 and 45 min postprandially. Mean HPP concentrations increased gradually during the day in 13 out of 16 fasting subjects, reaching an average peak at 2100. By 0200 mean HPP concentrations returned to initial fasting concentration. Propantheline eliminated this circadian rhythm. Biphasic plasma HPP responses were recorded after each of the three meals during the test. Calculation of integrated HPP outputs revealed similar outputs to the three meals; the percentage contribution of the first release phase increased with successive meals. Propantheline caused a 60% decrease in integrated HPP outputs. The first release phase was practically absent after the 0830 meal; however, distinct biphasic responses were present after the 1230 and 1730 meals. These studies demonstrate that fasting plasma HPP displays a cholinergic-dependent circadian rhythm, that postprandial plasma HPP responses are quantitatively similar throughout the day, and that different cholinergic activities are involved in the maintenance of these fasting and postprandial plasma HPP patterns.