Porcine islets have been proposed as a donor source for human transplantation, mainly because of both structural and biological similarities of porcine and human insulin. However, the in vitro function of these islets is poorly characterized. In the present study, we first examined insulin release in response to glucose in static incubation experiments. Increasing glucose concentrations up to 8.3 mmol/L stimulated insulin release; however, this elevation was only twofold, and a paradoxical decline was observed at glucose concentrations higher than 8.3 mmol/L. In cultured porcine islets, a greater insulin secretion may be elicited by agents that increase intracellular cyclic adenosine monophosphate (cAMP) levels. To investigate the possible reasons for the porcine islet low response to glucose in vitro, we then evaluated in parallel experiments glucose transport, phosphorylation, and utilization. Glucose transport studies (using 3-O-methyl glucose uptake at 15°C for 15 seconds) indicated the presence of both a high-affinity (Kmr 1.2 ± 0.6 mmol/L) and a low-affinity (Kmr 11.8 ± 1.9 nmol/L, n = 5) component. Glucose phosphorylation, evaluated by measuring the rate of glucose-6-phosphate formation in a fluorimetric assay, indicated that glucokinase activity had a maximum (Vmax) of 7.97 ± 0.94 nmol/μg DNA/h and a Km of 8.3 ± 0.9 mmol/L (mean ± SE, n = 8). Glucose utilization, determined by measuring 3H2O formation from (5-3H)-glucose, increased from 1.79 ± 0.34 nmol/μg DNA/2 h at 5 mmol/L glucose to 5.97 ± 0.16 and 6.19 ± 0.29 at 20 and 30 mmol/L glucose, respectively, with a calculated Vmax of 6.9 ± 0.33 and a Km of 7.0 ± 1.4 mmol/L (mean ± SE, n = 5). These data in porcine islets were similar to data previously obtained in rat islets using the same procedures. In conclusion, this study indicates that glucose transport, phosphorylation, and utilization are not altered in isolated porcine islets, and therefore, the in vitro low response of porcine islets to glucose is not due to alterations in glucose sensing or metabolism.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism