The organochlorine insecticide analogs mirex and chlordecone impair biliary excretion of readily excretable organic anions in rats (i.e., polar metabolites of PCB's and imipramine, phenolphthalein glucuronide, and exogenous taurocholate). Hepatobiliary dysfunction is not associated with marked cholestasis or hepatocellular necrosis but diminished capacity to concentrate organic anions in bile. Inhibition ATPase activities in various subcellular fractions by these agents (particularly the Na+, K+- and oligomycin-insensitive Mg2+-ATPase (OIMATPase) of liver plasma membranes) indicates energy transduction as a site of toxicity. Na+, K+-ATPase provides energy for Na+-coupled uptake of bile salts from the circulation into the liver and is probably a """"""""pump"""""""" responsible for electrolyte and accompanying water movement into bile. The role of the OIMATPase present in high activity in bile canaliculi enriched fractions (BCEF) is not understood. A major objective of this research is to determine whether inhibition of BCEF OIMATPase is a primary event in mirex and chlordecone-induced hepatobiliary dysfunction and development of a mechanistic model for this toxicity. Definition of the role of this enzyme in biliary excretion and the bile formation process per se is an integral part of this goal. The working hypothesis is that BCEF OIMATPase activity represents a H+-ATPase which provides energy for canalicular transport of organic solutes and/or a """"""""myosin-like"""""""" ATPase which plays a role in contraction of bile canaliculi and maintenance of """"""""tone"""""""" in the biliary tree. The above general objective will be approached by physiological and biochemical methods. The potential roles of hepatic blood flow and oxygenation, bile salt composition and concentration, and plasma membrane lipid composition in mirex- and chlordecone-induced hepatobiliary dysfunction will be evaluated. Intact rat studies will examine the sequence of impaired organic anion biliary excretion, inhibition of BCEF OIMATPase, and increased permeability of the biliary tree in both time course and pharmacological reversal experiments. Vesicular transport studies in BCEF aimed at determining the role of OIMATPase in coupled transport of organic anions are ongoing. Studies aimed at determining the site and mechanism of organochlorine action at the biomembrane level (BCEF) are proposed. These will involve the effects of mirex and chlordecone on BCEF physical properties as determined in differential scanning calorimetry and spin labeling (ESR) studies.
