Hepatocyte inducible nitric oxide synthesis is influenced in vitro by cell density

Hepatocyte plating density is known to affect cell function. Human and rat hepatocytes have been shown to express the inducible nitric oxide synthase (INOS) in response to cytokines plus lipopolysaccharide (LPS). The following studies were performed to determine the effects of hepatocyte plating density on the regulation of INOS. Rat hepatocytes were plated at densities from 10⁴ to 20 x 10⁴ hepatocytes/cm² and stimulated with a combination of LPS, interferon-γ, interleukin-1, and tumor necrosis factor. We found that NO₂/^- plus NO₃/^- released from stimulated hepatocytes declines with increasing hepatocyte density. Similar effects were seen for 3',5'-cyclic monophosphate release into supernatants and in the amount of nonheme iron- nitrosyl signals measured by electron paramagnetic resonance spectroscopy. Limitations of substrate (L-arginine) and 5,6,7,8-tetrahydrobiopterin were excluded as cause of the reduced nitric oxide generation at higher densities. Although mRNA levels for INOS were not influenced when measured at 24 h, there was a marked reduction in INOS enzyme activity and INOS protein detectable by Western blotting at higher cell density. Total protein synthesis decreased as hepatocyte density increased in both nonstimulated and stimulated hepatocytes at higher cell densities. These data suggest that reduced INOS translation may account for the density-dependent reduction in INOS activity in cultured hepatocytes. The importance of this phenomenon remains to be determined in vivo but has important implications for the in vitro study of INOS expression.