Development of a bioartificial pancreas: I. long-term propagation and basal and induced secretion from entrapped betaTC3 cell cultures

Bioartificial pancreatic constructs based on immunoisolated, insulin-secreting cells have the potential for providing effective, long-term treatment of type I (insulin-dependent) diabetes. Use of insulinoma cells, which can be amplified in culture, relaxes the tissue availability limitation that exists with normal pancreatic islet transplantations. We have adopted mouse insulinoma betaTC3 cells entrapped in calcium alginate/poly-L-lysine/alginate (APA) beads as our model system for a bioartificial pancreas, and we have characterized the effects of long-term propagation and of glucose concentration step changes on the bioenergetic status and on the metabolic and secretory activities of the entrapped cells. Cell bioenergetics were evaluated nonivasively by phosphorus-31 nuclear magnetic resonance ((31)P NMR) spectroscopy, and metabolic and secretory parameters by assaying cell culture medium. Data indicate that net cell growth occurred between days 3 and 10 of the experiment, resulting in an approximate doubling of the overall metabolic and secretory rates and of the intracellular metabolite levels. Concurrently, a reorganization of cell distribution within the beads was observed. Following this growth period, the measured metabolic and secretory parameters remained constant with time. During glucose step changes in the perfusion medium from a high concentration of 12 to 15 mM to 0 mM for 4.5 h to the same high glucose concentration, the oxygen consumption rate was not affected, whereas insulin secretion was always glucose-responsive. Intracellular nucleotide triphosphates did not change during 0 mM glucose episodes performed early in culture history, but they declined by 20% during episodes performed later in the experiment. It is concluded that the system of APA-entrapped betaTC3 cells exhibits several of the desirable characteristics of a bioartificial pancreas device, and that a correlation between ATP and the rate of insulin secretion from betaTC3 cells exists for only a domain of culture conditions. These findings have significant implications in tissue engineering a long-term functional bioartificial endocrine pancreas, in developing noninvasive methods for assessing construct function postimplantation, and in the biochemical processes associated with insulin secretion.