Diurnal and seasonal variation of cholecystokinin peptides in humans.

Cholecystokinin (CCK) was determined in plasma obtained from 10 female (aged 23.4±SD 2.3 years) and nine male (aged 22.0±SD 1.4 years) healthy volunteers. Blood samples were drawn three times (8.00a.m., 12 noon and 8.00a.m.) on each of two sessions, one in the winter (November–December) and one in the summer (April–July). The participants had fasted (and were nicotine-free) since midnight preceding the sampling. A standardized breakfast was served after the first sampling. CCK was determined by radioimmunoassay.