Distribution of [Na+,K+]ATPase on the cell surface of canine hepatocytes was investigated quantitatively by incubating prefixed and dissociated liver cells with ferritin antibody conjugates against canine kidney holo[Na+,K+]ATPase. We found that [Na+,K+]-ATPase exists bilaterally both on the bile canalicular and sinusoid-lateral surfaces. The particle density on the bile canalicular surface was much higher (approximately 2.5 times) than that on the sinusoid-lateral surface. In the latter region, the enzyme was detected almost equally both on the sinusoidal and lateral surfaces. On all the surfaces, the distribution of the enzyme was homogeneous and no clustering of the enzyme was detected. Total number of the enzyme on the sinusoid-lateral surface was, however, approximately three times higher than that on the bile canalicular region, because the sinusoid-lateral surface represents approximately 87% of the total cell surface of a hepatocyte. We suggest that the [Na+, K+]ATPase on the bile canalicular surface is responsible for the bile acid-independent bile flow and the other transport processes on the bile canalicular cell surface, while that on the sinusoid-lateral surface is responsible not only for the active transport of Na+ but also for the secondary active transport of various substances in this region.

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