Determination of plasma cholecystokinin (CCK) concentrations by bioassay and radioimmunoassay in man. A critical evaluation.

The present investigation was designed to perform a direct comparison of a rat pancreatic acini bioassay system and a specific CCK radioimmunoassay (antiserum G-160) for the measurement of fasting and meal-stimulated plasma CCK in the presence and absence of the CCK receptor antagonist loxiglumide. The G-160 CCK antiserum is directed against the C-terminal O-sulfated tyrosine residue of the CCK molecule which is essential for full bioactivity of CCK peptides. For plasma extraction prior to bioassay measurement, hydrophobic reverse-phase chromatography on octadecylsilane cartridges was employed and resulted in simultaneous adsorption and elution of both CCK peptides and loxiglumide with recoveries of 87.5 +/- 9% and 75.0 +/- 5.9%, respectively. In the absence of loxiglumide, fasting and meal-stimulated values for CCK-like bioactivity and CCK-immunoreactivity (IR-CCK) were nearly identical (basal values: 1-2 pmol/l; meal-stimulated plateau levels: 4-6 pmol/l). After intravenous infusion of loxiglumide (30 mg/kg/h for 10 min, 10 mg/kg/h thereafter), resulting in plasma steady state levels of 200-300 mumol/l, meal-stimulated CCK-like bioactivity was undetectable, whereas IR-CCK levels were augmented 6.5-fold. In the bioassay system, standard samples containing 50 mumol/l loxiglumide produced complete inhibition of acinar lipase release in response to 50 pmol/l synthetic CCK-8. We conclude, that postprandial circulating non-CCK-like factors do not contribute significantly to the direct receptor-mediated stimulation of exocrine pancreatic secretion. The good agreement of CCK-like bioactivity and IR-CCK levels in the absence of loxiglumide confirms the sensitive and specific recognition of bioactive CCK peptides by the G-160 antiserum and suggests that this antibody exerts binding characteristics probably similar to a pancreatic acinar receptor.