RAPID DOWN-REGULATION OF SUBSTANCE-P BINDING TO GUINEA-PIG PANCREATICACINAR-CELLS DURING HOMOLOGOUS DESENSITIZATION

1. Binding of I-125-labelled peptides, cytoplasmic Ca2+ concentration ([Ca2+](i)) and amylase release were studied in guinea-pig pancreatic acinar cells during exposure to substance P (SP), and cholecystokinin octapeptide (CCK-8). 2. Pre-incubation of cells at 22 degrees C with 0 .03 nM to 1 mu M SP for 10 min or at 37 degrees C for 5 min followed b y acid or neutral washes reduced subsequent binding of I-125-Bolton-Hu nter reagent-labelled SP (I-125-BH-SP) in a biphasic manner by up to 9 5 %. 3. Incubation at 4 degrees C eliminated high-affinity binding of I-125-BH-SP and concentrations of SP above 1. nM were required for inh ibition of subsequent tracer binding. 4. Pre-incubation of cells at 37 degrees C with 1 nM to 1 mu M CCK-8 for 10 min followed by neutral wa shes reduced subsequent binding of I-125-BH-CCK-8 by up to 85 %. 5. In cell suspensions, the [Ca2+], response to SP was gradually reduced by pre-exposure to increasing agonist concentrations from 0.2 to 20 nar. 6. Pre-incubation with high SP concentrations for 10 min caused profo und reduction of subsequent amylase responses to SP, whereas secretion was little affected in corresponding experiments with CCK-8. 7. Down- regulation of receptor binding is not important during short exposure to CCK-8, but it is a pronounced and rapid phenomenon during SP exposu re, which explains tachyphylaxis of[Ca2+], and amylase responses.