Expression pattern of heme oxygenase isoenzymes 1 and 2 in normal and stress-exposed rat liver.

Heme oxygenase (HO) catalyzes the oxidative cleavage of the alpha-mesocarbon of Fe-protoporphyrin-IX yielding equimolar amounts of biliverdin-IXa, iron, and carbon monoxide. The HO-system consists of two isoenzymes, namely HO-2 and the inducible isoform HO-1, also referred to as heat shock protein (hsp) 32. Although both parenchymal and non-parenchymal liver cells participate in heme metabolism, the expression pattern of the isoenzymes in normal and stress exposed liver is unknown. To study this, rats underwent either endotoxin (lipopolysaccharide [LPS]) challenge, hemorrhagic hypotension, glutathione (GSH) depletion, or cobalt chloride injection, all known to provoke oxidative stress. HO-2 messenger RNA (mRNA) and protein were constitutively expressed in hepatocytes, Kupffer/endothelial-, and stellate (Ito-) cell enriched fractions. Although both non-parenchymal cell fractions expressed HO-1 transcripts, HO-1 immunoreactive protein was restricted to Kupffer cells in the normal liver. In contrast to HO-2, a significant increase in HO-1 on the whole organ level was noted by hemorrhagic hypotension, GSH depletion, and cobalt chloride injection. However, the distinct stress models led to a strikingly different cell-type specific and sublobular expression pattern of HO-1 gene expression. HO-1 was inducible in sinusoidal lining cells (hemorrhagic hypotension, LPS challenge), in periportal (cobalt chloride), or pericentral (GSH depletion, hemorrhagic hypotension) hepatocytes. The blockade of protein translation before hemorrhage by cycloheximide reduced upregulation of HO-1/hsp32 mRNA significantly (65.4% reduction, P < .05), whereas the inducibility of hsp70 transcript was maintained. In addition to transcriptional regulation, HO-1 seems to be subject to posttranscriptional control in particular in non-parenchymal cells.

Maturational differences in hepatic microhemodynamics in rats.

Age-related changes in the hepatic microcirculation may contribute to the increased susceptibility of the immature liver to microvascular injury. We quantified sinusoidal and acinar diameters, sinusoidal red cell velocities (VRBC), and sinusoidal volume flows to characterize microhemodynamics of weanling and adult rat livers with and without hepatic artery (HA) ligation using intravital fluorescence videomicroscopy. Despite a 20% faster heart rate and a nearly 20% lower mean arterial and portal vein pressure in weanling rats relative to those in the adults, weanling periportal and pericentral sinusoidal velocities were approximately 30 and 25% faster, respectively, than those in adults. Furthermore, the HA was found to contribute more to maintenance of sinusoidal VRBC in the immature liver as demonstrated by a significant decrease in both periportal and pericentral VRBC following HA ligation. HA ligation had no effect on VRBC of either zone in adults. Zonal volume flow (HA intact), however, was maintained independent of age. These results suggest a lower extrasinusoidal resistance in the weanling. The 25% shorter acinar diameter that we found in weanling livers likely contributes to a lower extrasinusoidal resistance by allowing a higher ratio of inflow vessels to volume of tissue. Shorter sinusoidal pathways in weanling livers also decreases sinusoidal resistance 1.3-fold relative to that in the adult, countering the approximately 1.5 times increase in resistance due to the smaller caliber of sinusoidal vessels so that overall sinusoidal resistance is not age-dependent.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium-free reperfusion prevents mitochondrial calcium accumulation but exacerbates injury.

Excessive accumulation of intracellular calcium has been proposed as a mediator of cell injury during ischemia and reperfusion. To test whether restriction of extracellular calcium might ameliorate hepatic injury following hypothermic ischemic preservation, isolated perfused rat livers were reperfused with a medium with normal extracellular calcium (mM-Ca) or no added calcium (microM-Ca) following 26 hr preservation at 4 degrees C. During reperfusion mitochondria from mM-Ca livers accumulated calcium and respiratory activity declined. Although mitochondrial calcium accumulation was prevented by microM-Ca reperfusion, damage to respiratory activity was exacerbated. Edema formation and loss of gluconeogenesis were similarly exacerbated in microM-Ca. This accelerated damage was not reversed by subsequent restoration of calcium. These results demonstrate that rather than ameliorating cell injury, calcium deprivation during reperfusion exacerbates damage to mitochondrial and cellular functions. Thus, cellular or mitochondrial calcium accumulation does not appear to be a sine qua non for hepatocyte injury during reperfusion following hypothermic ischemia.