Bioassay of plasma cholecystokinin in rat and human: inhibition of protein synthesis prevents the decrease in the sensitivity and responsiveness of isolated rat pancreatic acini to CCK-8.

Isolated rat pancreatic acini were most sensitive and responsive when stimulated directly with cholecystokinin octapeptide (CCK-8) without preincubation. Both the responsiveness and sensitivity of acini to CCK-8 decreased time dependently with prolonged preincubation. When acini were stimulated with CCK-8 following pulse labeling with radioactive leucine, old protein was discharged together with newly synthesized (labeled) protein. However, the sensitivity and responsiveness of these acini for release of labeled protein were primarily reduced with preincubation, indicating that newly synthesized protein was contributing to the time-dependent loss of sensitivity and responsiveness. Then isolated acini were treated with 300 microM cycloheximide for 2 h. This treatment prevented the decreases in the sensitivity and responsiveness of amylase release to CCK-8 stimulation and made these alterations in one series of experiments negligible. Using these acini, a sensitive and specific bioassay for the measurement of CCK in human and rat plasma was developed.