Hepatic transport mechanisms for bivalent organic cations. Subcellular distribution and hepato-biliary concentration gradients of some steroidal muscle relaxants

In order to characterize the hepato-biliary transport of bivalent cations in more detail, the subcellular distribution of three steroidal muscle relaxants, that differ physicochemically and kinetically, was studied by differential centrifugation of liver homogenates. Binding of the muscle relaxants to macromolecular compounds was measured in Krebs-albumin solution, in cytosolic fraction of liver homogenate and in bile, to estimate the unbound concentrations in the particular fluids. Cytosol/plasma concentration ratios increased in the order pancuronium less than Org 6368 less than vecuronium, but for all of the compounds did not exceed the value that would be attained by passive equilibration according to the membrane potential. The subcellular distribution patterns of the three substances indicated that the mitochondrial fraction is a major storage compartment in the liver. Yet Org 6368 was bound to the particulate fraction of liver homogenate to a larger extent than pancuronium and vecuronium. The high bile/cytosol concentration ratios indicate that for all of these cations an active transport system is involved in the biliary excretion process. For Org 6368 and vecuronium the bile/cytosol concentration ratios are in the same range (about 30) and substantially higher than for pancuronium (about 6). This suggests that for Org 6368 and vecuronium the transport across the canalicular membrane is more efficient than for pancuronium. The combined data indicate that the extensive binding of Org 6368 to particles within the cell is a major factor in the relative efficient hepatic uptake and the modest biliary excretion of this agent. The limited hepato-biliary transport of pancuronium appears to be due to a relatively small net transport, both at the sinusoidal land at the canalicular membrane.