MACROPHAGE-GENERATED NITRIC-OXIDE AS CYTOTOXIC FACTOR IN DESTRUCTION OF ALGINATE-ENCAPSULATED ISLETS - PROTECTION OF ARGININE ANALOGS AND OR COENCAPSULATED ERYTHROCYTES/

Isolated rat islets were microencapsulated in alginate beads of about 1.5 mm in diameter. These were cocultured with activated or resident p eritoneal macrophages of syngeneic rats for 24 hr. Examination of the encapsulated islets by transmission electron microscopy showed that th e islets were lysed by activated (80.0+/-12.8% of islets lysed), but n ot by resident, macrophages (17.5+/-12.2% lysis) despite encapsulation . Islet lysis was inhibited in a concentration-dependent manner by a s pecific nitric oxide-synthase inhibitor (0.5 mM N-G-methyl-L-arginine: 5.9+/-3.9% lysis) in an L-arginine-reversible manner (0.5 mM N-G-meth yl-L-arginine + 10 mM L-arginine: 55.1+/-16.6% lysis). Incubation of e ncapsulated islets with 3 different nitric oxide-generating compounds also resulted in a concentration-dependent islet lysis. Coencapsulatio n of autologous erythrocytes was found to be an effective and easy way of protection from macrophage-mediated lysis. Protection was dependen t upon the number of erythrocytes coencapsulated. This in vitro study demonstrates that nitric oxide secreted by activated macrophages is ab le to destroy islets despite encapsulation in alginate, and that both, inhibition of nitric oxide formation using enzyme inhibitors and scav enging of nitric oxide once formed exploiting the hemoglobin of autolo gous erythrocytes, protect encapsulated islets from destruction.