Effect of Age on Proinsulin and Insulin Secretory Patterns in Isolated Rat Islets

Abstract

Although isolated islets from older rats store ample insulin, they do not respond as efficiently as do islets from young animals to a glucose or leucine challenge. The current study was conducted to determine if this decline in insulin secretion is associated with abnormalities in islet content or conversion of proinsulin, or in the mobilization of insulin that is newly synthesized. Isolated islets from 2-, 12-, and 18-mo-old rats were assayed for total islet proinsulin and insulin content, and after stimulation with glucose the incubation medium was assayed for secreted proinsulin and insulin. In addition, islets from 2-and 12-mo-old donors were pulse-labeled with 3 H-leucine, after which the temporal conversion and pattern of release of newly synthesized hormone was compared. The results show that the ratio of proinsulin to insulin declines progressively with age for both the stored and secreted hormone. Incorporation of 3 H-leucine into proinsulin during glucose stimulation also decreases with age. However, the onset, time course, and endpoint of the conversion of this labeled proinsulin to insulin did not change with age. Although the fractional secretion rate of newly synthesized hormone (whether proinsulin or insulin) is similar in 2-and 12-mo-old rats, it exceeds that of the unlabeled, or stored, insulin. When the wave of labeled protein enters the secretory pool it apparently equilibrates with a portion of the insulin pool, which is more rapidly secreted than the total pool of insulin. We considered this the glucose responsive pool, and estimate that this pool is actually smaller in islets from 12-as compared with 2-mo-old rats. The reduction in size of this responsive compartment of insulin in the older rats may, in part, explain

Heterogeneity and compartmental properties of insulin storage and secretion in rat islets.

“…Thus, immature storage components containing large amounts of unprocessed proinsulin were readily secreted. At 30 min, newly synthesized, secreted hormone was not quite as rich in proinsulin as was stored hormone, as observed (2,5,8). This could indicate either that B granules require partial maturation before secretion or that a large proportion of the radioactive proinsulin continues to reside in the lumen of the rough endoplasmic reticulum and golgi membranes rather than within the secretory granules at 30 min.…”

“…Thus, all lines of evidence indicate that the conversion process occurs in the B granules. From 30 min onward (presumably upon entering the B granule) proinsulin converts to insulin in our system in a pseudo-first-order process with a t1/2 of 50 min, as reported (2,8,18).…”

“…Hence, islets function as though they store a portion of their total insulin in a labile compartment that is both rich in newly synthesized hormone and preferentially mobilized for secretion in response to glucose. This compartment has been reported to diminish in size in islets isolated from older rats (8), but little else is known about its properties and its effect on B cell function. For example, what are the characteristics of newly synthesized proinsulin and insulin filling this compartment and their exchange with the remainder of the stored hormone?…”

Evidence That Glucose "Marks" β Cells Resulting in Preferential Release of Newly Synthesized Insulin

“…rich prey [for instance, a mid-water shrimp, or selective feeding on its hepatopancreas (8)]. The low 210Po family, the Gonostomatidae, must have a completely different, low 210Po diet-copepods, perhaps.…”

Diabetic Peripheral Neuropathies: A Morphometric Overview

“…Alloxan exerts its diabetogenic action when administrated intravenously, intraperitoneally or subcutaneously. The dose required for diabetes induction depends on the animal species, route of administration and animal nutritional status, age and gender (Gold et al, 1981). The most frequently used intravenous dose used to induce diabetes in rats is 65mg/kg (Gruppuso et al, 1990, Boylan et al, 1992 but the effective dose for intraperitonealy or subcutaneously injections must be 2 to 3 times higher (Szkudelski).…”