Preservation of Insulin Release Periodicity from Individual Islets of Obese Zucker Rats

Abstract

Utilizing individual islets of Langerhans, Bessman, Beigelman and have demonstrated a pulsatile periodicity of insulin release. More recent data, have correlated this pulsatile pattern of insulin release with bursts of electrical activity in single microdissected islets exposed to glucose (Atwater, Rojas and Scott 1979;Beigelman and Ribalet 1980). Materials and MethodsIslets from obese and lean Zucker rats were isolated by the collagenase method of Shibata, Ludvigsen, Naber, McDaniel and Lacy (1976). Individual large islets (0.4 to 0.8 mm diameter) from obese rats were perifused in Hanks' solution adjusted to a pH of 7.4 and gassed continuously with 95 % 0 2 and 5 % C0 2 (Hayek and Woodside 1979).Flow rate, maintained at 1 ml/min, allowed the collection of 250 µl aliquots every 15 seconds. Aliquots were immediately frozen and simultaneously assayed at a later time for insulin by radioimmunoassay. Islets from lean rats (diameter < 0.1 mm) served as control. Results and DiscussionWe have previously shown that when groups of islets from obese and lean rats are perifused, the observed pattern of insulin release in response to stimulation is closely correlated to islet size (Hayek and Woodside 1979). Perifusion of groups of islets however, obscures the synchrony of insulin secretion in discreet short pulses exhibited by individual, normal islets (Beigelman, Thomas, Slavin, Shu and Bessman 1973). Fig. Insulin release from individual islets of fat and lean Zucker rats in a perifusion system. Prior to glucose stimulation, a total of 50 individual islets were perifused for 45 min in order to achieve steady-state insulin secretion. The figure shows 6 examples of the pattern of insulin release for large and 5 from small islets.

Coordinate pulsatile insulin secretion by chronic intraportally transplanted islets in the isolated perfused rat liver.

“…Intrahepatic transplanted islets become vascularized within 7 d of transplantation (12). The present data therefore appear to lend additional argument against the hypothesis that coordinate pulsatile insulin secretion in vivo is due to entrainment of islets by either oscillations in circulating glucose (8) or an intrinsic metabolic cycle present in pancreatic islets (26). It has been suggested that coordinate secretory burst activity is achieved through the intrapancreatic nerve network (10,27).…”

Pulsatile Insulin Secretion in Isolated Rat Islets

“…In addition, direct sampling from the portal vein in vivo supports the contention that serum insulin oscillations are primarily a secretory phenomenon (3). Studies published in single isolated rat (11,12) and mouse (13) islets have demonstrated high-frequency insulin secretory cycles, but whether these bear any relationship to the fluctuations observed in vivo is unclear. It is not clear, however, whether the pacemaker for oscillatory insulin secretion is within the pancreatic nervous system or whether the islets have an intrinsic pulsatile rhythm that is coordinated by the extensive neurogenic connections of the pancreas.…”

The in vivo regulation of pulsatile insulin secretion

“…However, the net contribution of the secretory bursts is important, because the presence of concentration oscillations does not exclude the dominance of a time-invariant (basal) release pattern. Furthermore, the relation between the in vivo observed secretory bursts and the in vitro observed pulsatile release from the isolated pancreas [9,10,101] and the isolated peri-perfused islets [102,103], as well as episodic beta-cell depolarization [104±106], beta-cell glycolysis [105,107], and beta-cell increase in calcium remains to be established [104, 105, 108±111]. The coupling of cyclic metabolism to pulsatile in vitro release has been addressed in an excellent review by Tornheim [112].…”