Quantitative electron microscope autoradiographs of 125I-cholecystokinin in pancreatic acini

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Abstract
To morphologically evaluate the interaction of cholecystokinin (CCK) with its receptors on pancreatic acinar cells isolated mouse acini were incubated at 37.degree. C with radioiodinated CCK and quantitative EM autoradiographs were prepared. Specific binding of CCK to acini was half maximal at 2 min of incubation and maximal after 10 min. The cell-associated radioactivity was extracted and analyzed on Sephadex G-50. After 2 min, 90% of the total cellular radioactivity remained as intact CCK; after 30 min, the intact radioactivity decreased to 65% of total. At 2 min, the fraction of bound hormone that fixed to acini was 84% of total; this amount decreased to 78% after 30 min. The majority of radioactivity in the autoradiographs at both time points was intact CCK; however, at 30 min, a small amount was also degraded hormone. After both 2 and 30 min of incubation, Ag grains were highly concentrated over the basolateral plasma membrane. A significant number of grains were in the cell interior at both time points, increasing from 13% of total grains at 2 min to 42% at 30 min. At both times, the largest fraction internalized grains was localized over the endoplasmic reticulum. At 30 min, a significant concentration of CCK grains was observed over multivesicular bodies. Evidently, CCK binds to specific receptors on the basolateral surface of pancreatic acinar cells. After binding, the hormone is internalized, locates predominantly on the endoplasmic reticulum, and is then degraded.