Cold acclimation in insulin secretion of isolated perfused pancreas of the rat

Characterization of secretory responses in exocrine pancreas of genetically obese Zucker rats

Secretory responses of the exocrine pancreas in the obese Zucker rat with an inherited genetic disorder were compared with those in lean control rats. Amylase concentration in pancreatic acinar cells of obese rats was reduced to 62% of the control value, whereas trypsinogen concentration was increased to 269%. The activity ratio of amylase to trypsinogen in the pancreatic acini of obese rats was about one-fourth of that in controls. The activity ratio of amylase to trypsinogen in pancreatic juice released by stimulation with varying concentrations of CCK8 or carbachol in the obese rats was also reduced to one-fourth of that in control rats. The concentration of the secretagogs inducing half-maximal secretory responses (EC50) in the obese rats was almost identical to that in the controls. The downward shift of the dose-response relation for these secretagogs in the acini of obese rats was analogous to that in streptozotocin-induced diabetic rats, cold-exposed rats, or lactating rats, as demonstrated previously. The present results may be explained by the changes in enzyme content and secretory responses found in pancreatic acini of obese Zucker rats, which would be attributable mainly to congenital disturbance in "insulo-acinar axis."

Suppression of secretagogue-induced amylase secretion in pancreatic acini of cold-exposed rats

1. The influence of prolonged exposure to a low ambient temperature on the pancreatic exocrine function was investigated in rats. Amylase concentration in acini prepared from rats housed at 5 degrees C for 2 weeks was specifically and dramatically reduced. The enzyme activity relative to acinar DNA concentration was only 48% of that of control acini obtained from rats reared at the thermoneutral temperature of 25 degrees C. Amylase content relative to acinar protein concentration also decreased by 35%. In contrast, acinar trypsinogen content relative to protein concentration increased to 161% of that of control acini. No comparable change was found in the parotid glands. 2. In acini from cold-exposed rats, amylase release in response to cholecystokinin octapeptide or carbamylcholine was simultaneously reduced when expressed relative to acinar DNA or protein concentration and the dose-response curve shifted downward. However, when the secretory responsiveness of amylase was represented as a percentage of initial acinar enzyme content, the observed decrease disappeared and the dose-response curve was unaffected by cold exposure. 3. Secretagogue-stimulated trypsinogen release was augmented in acini from cold-exposed rats when expressed per acinar protein concentration. Conversely, its percentage release was slightly lowered but the release per acinar DNA concentration remained unaltered. 4. Pancreatic insulin concentration normalized to tissue wet weight or protein concentration was unaffected by prolonged cold exposure but the plasma insulin concentration significantly decreased by 45% as compared with that of thermoneutral rats. 5. Chronically administered exogenous insulin partially restored the decreased acinar amylase concentration in cold-exposed rats. The dose-response curve for amylase normalized by acinar DNA or protein content also tended to be shifted upward by insulin treatment. Acinar trypsinogen concentration in these rats was not significantly altered from that in cold-exposed, non-insulin-treated rats. Percentage release of each enzyme was decreased by exogenous insulin. 6. Cold-induced suppression of pancreatic acinar amylase concentration, resulting in a proportional decrease in secretory responsiveness, was confirmed at the cellular level. Insulin deficiency at a low ambient temperature is likely to be a major cause of cold-induced suppression of amylase biosynthesis and its release via an insulin-pancreatic acinar axis.

Augmentation of secretagogue-induced amylase secretion in pancreatic acini of heat-exposed rats

1. The effects of prolonged heat exposure on pancreatic exocrine secretion were investigated to evaluate the involvement of the insulin-pancreatic acinar axis. Rats were kept at 34 degrees C and a relative humidity of 40% for 2 weeks with or without insulin administration. Control rats were housed at the thermoneutral temperature of 25 degrees C. By using isolated acini, secretory function was examined at the cellular level. 2. Without insulin treatment, acinar amylase concentration, expressed per microgram of cellular protein, was increased by heat exposure, while trypsinogen concentration, expressed per microgram of acinar DNA, decreased. The ratio of acinar amylase to trypsinogen increased significantly from 2.82 to 5.69 by prolonged heat exposure. With insulin treatment, the heat-induced increase in amylase activity was lessened but the decrease in trypsinogen remained unchanged. The ratio was somewhat lessened to 4.66. 3. In acini from saline-treated rats, amylase release in response to varying concentrations of cholecystokinin octapeptide or carbamylcholine was significantly augmented by prolonged heat exposure. As a result, the dose-response curve shifted upwards. However, in acini from insulin-treated rats, the increase in secretory response was lessened, similar to the effect on acinar content. On the other hand, changes in trypsinogen release were not as notable as those in amylase release. The ratio of amylase to trypsinogen in pancreatic juice released by 100 pM-cholecystokinin octapeptide increased from 1.56 to 3.04 in saline-treated rats and from 1.60 to 2.34 in insulin-treated rats. 4. In heat-exposed, saline-treated rats the plasma concentrations of glucose and insulin were significantly elevated, while in insulin-treated rats these increases were lessened and the elevation of plasma insulin concentration was no longer significant. 5. It is suggested that heat exposure elevates resting plasma glucose concentration, probably because of decreased metabolic activity, which results in a concomitant rise in plasma insulin concentration. This hormonal change most likely is a major cause of augmentation of acinar amylase synthesis and of the resultant potentiation of stimulus-secretion coupling via an insulin-pancreatic acinar axis. Heat exposure modified pancreatic exocrine function in the opposite direction to what occurred in cold-exposed animals. It was confirmed that changes in pancreatic exocrine function via a modification of the insulin-pancreatic acinar axis can actually occur under normal physiological circumstances. From the nutritional point of view, this modification by ambient temperature should be taken into consideration.