Prevention of Ischemia-Reperfusion Injury in the Rat Liver by Atrial Natriuretic Peptide

This work characterizes the protective effects of ANP preconditioning in ischemia-reperfusion injury of the isolated perfused rat liver. It was of particular interest to evaluate the influence of ANP on the mode of cell death occurring during cold ischemia and reperfusion and to elucidate the involved signal transduction pathways. Apoptotic cell death was mainly seen after cold liver storage, whereas necrosis was predominant in the reperfusion period. It could be demonstrated for the first time that preconditioning with ANP was able to reduce both apoptotic as well as necrotic cell death. After cold ischemia, in particular hepatocytes were protected against apoptosis. After reperfusion, protection against necrosis comprised hepatocytes and sinusendothelial cells predominantly in the periportal liver areas. As target molecules for ANP action, the cGMP-dependent protein kinases did not seem to be responsible for the conferred cytoprotection. In the liver, no expression of these kinases could be detected and a functional connection could not be derived. In contrast, the cAMP-dependent protein kinases were identified to promote survival. This was further supported by the ability of ANP to directly activate cAMP-dependent protein kinases in livers and hepatocytes. An early transcriptional induction of HO-1 by ANP independent of cGMP could be demonstrated. The induction of heme oxygenase-1 by ANP might not be responsible for the observed hepatoprotection, since inhibition of HO-1 activity did not abrogate the ANP effect. Interestingly, cell-type specific evaluation detected that induction of HO-1 in livers by ANP is exclusively restricted to Kupffer cells. In summary, this thesis gives new insights into the actions of the cardiovascular hormone ANP in IRI of the rat liver. This data helps to understand the mechanisms of how ANP mediates cytoprotection by illuminating effects and potential pathways, an important prerequisite for a rational application in therapy. This work was supported by the Deutsche Forschungsgemeinschaft (DFG: Ge 576/14-2 and FOR 440/1, TP2)