Ito Cells: A Putative Cellular Component Responsible for Carbon Monoxide-Mediated Microvascular Relaxation in the Rat Liver

Ito cells are microvascular pericytes occurring specifically in the liver. They are characterized by abundant fat droplets and constitute a major storage pool of vitamin A in the liver. These cells encircle the outer surface of microvascular walls and constitute a well-organized meshwork of intercellular connection by using their unique neuron-like dendritic structure. Ito cells have thus been considered to be a putative machinery controlling sinusoidal blood flow. Carbon monoxide (CO) generated by the heme oxygenase reaction serves as an endogenous relaxing factor that actively relaxes hepatic sinusoids. Although such a CO-dependent vasorelaxing mechanism seems to involve cyclic guanosine monophosphate- (cGMP-) dependent relaxation of Ito cells, it is still unknown whether Ito cells can exhibit cell relaxation in response to CO through unidentified cGMP-independent mechanisms. This chapter provides an overview of mechanisms for CO-dependent Ito cell relaxation in vivo and in vitro.