Studies on the guinea pig pancreas. Parallel discharge of exocrine enzyme activities

Abstract

An in vitro system of guinea pig pancreatic lobules convenient for the study of secretory processes is described in this paper. In this system: (a) the over-all glandular architecture of the tissue is preserved: lobules remain morphologically intact through 5 hours; (b) amylase discharge from unstimulated lobules is low (similar to 4%/hour) and linear over the 5 hours tested; (c) response to carbamylcholine chloride (10-5 M) is energy-dependent, rapid, and extensive (92% discharge of amylase by 5 hours); (d) initial rates of discharge remain stable over the first 3 hours; and (e) no autoactivation of zymogens occurs in incubation medium or tissue. The activation of four zymogens, i.e. chymotrypsinogen, trypsinogen, and procarboxypeptidases A and B, was studied using the following criteria for optimal activation: (a) maximal activation attainable under experimental conditions; (b) stability at the level of maximal activation; and (c) linear relationship between amounts of protein activated and enzyme activity elicited by activation. The concentration of activators (trypsin or enterokinase) and secretory protein, the presence or agents (bovine plasma albumin or Triton X-100) which minimize adsorptive losses of secretory protein on glass or plastic surfaces, and the temperature at which activation is carried out were found to be critical and different for each of the zymogens tested. The kinetics of the appearance of three enzyme activities (amylase, lipase, and ribonuclease) and four potential proteolytic activities (chymotrypsinogen, trypsinogen, and procarboxypeptidases A and B) into the incubation medium was studied under different conditions; i.e. rest and stimulation with various secretogogues (carbamylcholine chloride, caerulein, and pancreozymin). All seven activities estimated to represent similar to 75% of the secretory protein output of the exocrine pancreas were discharged in synchrony and in constant proportions and were released from the tissue to the same extent under each experimental condition investigated.