Effect of microencapsulation on oxygen distribution in islets organs.

In islet transplantation, limitation of oxygen supply may restrict graft function, particularly when encapsulated tissue is used. Therefore, oxygen tensions (PO2) in isolated islet organs (Brockmann bodies) of Osphronemus gorami were measured. In a thermostatically (37 degrees C) controlled measuring chamber, PO2 values were recorded at subsequent microelectrode positions on a radial track toward the center of the organ. In 2 independent groups, we studied the effect of fluid convection (n = 12) and microencapsulation (n = 12). In both groups, sigmoidal PO2 profiles were found, which permit differentiation in an oxygen-depleted zone surrounding the surface, a steep decline inside the tissue corresponding to the oxygen-consuming rim, and a plateau in the center without oxygen consumption which reflects necrosis. The PO2 values decreased (P < 0.001) when convection was stopped. Compared with starting values, PO2 levels at the surface were 61 +/- 3% with and 41 +/- 4% without convection. Surface values for encapsulated tissue were 44 +/- 5% compared with 64 +/- 4% in nonencapsulated tissue. In the tissue, center oxygen dropped to 27 +/- 5% with convection and to 6 +/- 3% without, and to 11 +/- 3% for encapsulated tissue compared with 22 +/- 4% for nonencapsulated tissue. The thickness of the outer oxygen-depleted zone was 81 +/- 16 microns with and 196 +/- 57 microns without convection (P < 0.001), and 188 +/- 16 microns for encapsulated and 94 +/- 14 microns for nonencapsulated tissue (P < 0.001). The oxygen-consuming rim was 295 +/- 22 microns with and 235 +/- 36 microns without convection (NS), and 216 +/- 15 microns for encapsulated and 315 +/- 24 microns for nonencapsulated tissue (P < 0.01). These results illustrate the special distribution of oxygen in isolated islet tissue and indicate that barium alginate encapsulation may worsen oxygenation mainly by expanding the "unstirred water layer" surrounding the tissue.