[3H]Caerulein was bound to dispersed acinar cells from rat pancreas in a rapid, reversible, specific, saturable, and temperature-dependent manner. Binding decreased above pH 6.5. Treatment of intact cells with 2, 4-dinitrophenol and oligomycin, p-choloromercuribenzoate, diisopropylfluoro-phosphate and glutaraldehyde impaired [3H]caerulein binding whereas the addition of EGTA inhibited binding. The C-terminal octapeptide of pancreozymin, desulfated caerulein and pentagastrin inhibited binding of [3H]caerulein whereas vasoactive intestinal polypeptide, secretin, bombesin or carbamoylcholine were wothout effect. The good resistance of [3H]caerulein to inactivation by acinar cells at 37 degrees C was reflected in the high proportion of tracer remaining capable of binding to fresh acinar cells. 2. Scatchard plots of [3H]caerulein binding were curvilinear with an upward concavity. The addition of an excess of unlabeled caerulein resulted in the release of as much as 65% of bound [3H]caerulein within 1 min at 37 degrees C. The dissociation of remainder followed much slower kinetics. 3. The results suggested that intact rat pancreatic acinar cells have one class of caerulein binding sites existing in two states: one with high affinity and another with low affinity, the proportion of sites in each state depending on the degree of site occupancy (negative cooperativity), and on the intracellular concentration of nucleotides.
