This study shows that specialized contractile endothelial cells exist in rat liver sinusoids which may be involved in the local control of hemodynamics and which are sensitive to vasoactive agents, including the vasorelaxant hormone relaxin. Male rats were treated with 10 microg relaxin for 4 days; phosphate-buffered saline (PBS)-treated rats were the controls. For comparison, rats treated with relaxin together with the NO-synthase inhibitor N(omega)-nitro-l -arginine methyl ester (L-NAME), and rats treated with the vasodilator taurodeoxycholic acid or the vasoconstrictor ethanol were investigated. Liver fragments were studied morphologically and morphometrically. In the control rats, peculiar contractile cells were present in the endothelial lining. These cells had abundant myofilaments and formed cytoplasmic blebs projecting into and often occluding the lumen. In the ethanol-treated rats, sinusoids were constricted and filled with cytoplasmic blebs. In the relaxin-treated rats, sinusoids were markedly dilated and the cytoplasmic blebs nearly disappeared. Similar findings were observed in the taurodeoxycholic acid-treated rats. The effects of relaxin were blunted by L-NAME, suggesting that the relaxin action involves an NO-mediated mechanism.

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