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Am J Physiol Gastrointest Liver Physiol 254: G513-G521, 1988;
0193-1857/88 $5.00
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AJP - Gastrointestinal and Liver Physiology, Vol 254, Issue 4 513-G521, Copyright © 1988 by American Physiological Society

ARTICLES

Temperature dependence of high-affinity CCK receptor binding and CCK internalization in rat pancreatic acini

J. A. Williams, A. C. Bailey and E. Roach
Cell Biology Laboratory, Mount Zion Hospital and Medical Center, San Francisco 94121.

125I-labeled cholecystokinin (CCK) binding and internalization were studied as a function of temperature in isolated rat pancreatic acini. At 37 degrees C, acini readily bound and degraded 125I-CCK. When labeled hormone binding was inhibited by increasing amounts of unlabeled CCK, competition-inhibition curves were biphasic, consistent with both high- (Kd, 18 pM) and low-affinity (Kd, 13 nM) binding sites. At 4 degrees C, acini bound only one-third as much 125I-CCK and degradation was essentially abolished. At 4 degrees C, CCK competition curves were consistent with a single class of low-affinity binding sites (Kd, 19 nM). Internalization of 125I-CCK was evaluated by three washing procedures utilizing acid, base, and trypsin. All were shown to remove membrane-bound 125I-CCK, and this finding was validated for trypsin by electron microscope autoradiography. After 1 h at 37 degrees C, washing showed 67% of bound 125I-CCK to be internalized and autoradiography showed 54% to be internalized. At 4 degrees C, internalization of bound CCK was greatly reduced but not abolished. When internalization of 125I-CCK was evaluated as a function of the medium concentration of CCK, both high- and low-affinity components were observed. These results suggest that high-affinity CCK binding and CCK internalization are separate temperature-sensitive processes. Moreover, internalization is not uniquely associated with high-affinity binding.


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