Insulin content and insulinogenesis by the perfused rat pancreas: effects of long term glucose stimulation

The dynamic response of the perfused pancreas differed between pancreases from fed and fasted rats. Insulin secretion was significantly lower in pancreases from fasted rats during the first 40 min of perfusion at glucose levels of 200 and 300 mg/dl. Thereafter, from 40-90 min, insulin secretion was similar by pancreases from both fed and fasted rats. The typical biphasic insulin secretory profile, consisting of a transient spike of insulin secretion followed by a slowly rising secretory phase, was observed in pancreases from fasted rats. In contrast, the transition from first to second phase secretion was accelerated in pancreases from fed rats. This suggests that transport of intracellular insulin stores may be accentuated due to the fact that insulinogenic sites (e.g. Golgi) in pancreases from fed rats may be fully primed for optimal secretion. Total pancreatic insulin measurements support this concept. Total pancreatic insulin content was determined under fed and 24-h fasted conditions after various times of perfusion (0, 60, and 90 min and 6 h) and in response to various glucose levels (0, 200, and 300 mg/dl). Fasting resulted in a significant decrease in insulin content at zero time compared with pancreases from fed rats (39.2 +/- 2.4 vs. 61.6 +/- 9.8 micrograms). In the fed rat pancreases, total insulin content decreased slightly after a 60-min glucose stimulus of 300 mg/dl, but returned to the basal level after 90 min and remained at that level during a 6-h period of perfusion. In the fasted state, insulin content remained constant as a function of time until 60 min, but increased by 90 min to a level comparable to that in pancreases from fed rats. The response to lower levels of glucose stimulation (200 mg/dl) was qualitatively similar by pancreases from fed and fasted rats compared to the response to a higher glucose dose (300 mg/dl), except that secretion was less. Insulin content remained relatively constant for periods of perfusion up to 60 min. Insulinogenesis (defined as de novo synthesis and conversion of existing preproinsulin and proinsulin to insulin, less intracellular degradation of insulin) was increased as a function of glucose concentration and differed temporally as a function of the food intake of the animal. At no time of perfusion with any level of glucose stimulation did the insulin content exceed the zero time value in pancreases from fed rats. This suggests that insulin secretion is the rate-limiting step for insulinogenesis.