Absorption and transport of ferritin and exogenous horseradish peroxidase in the opisthonephric kidney of the sey lamprey II. The tubular nephron.

The tubular nephrons in the opisthonephric kidney of the sea lamprey, Petromyzon marinus L., were demonstrated to absorb and transport exogenous protein. The proximal convoluted segment is the major site of protein absorption from the tubular lumen and the process involved is similar to that described for the proximal convoluted segment in the kidneys of other vertebrates. The lateral intercellular spaces are a possible channel for the transport of intact proteins in this segment. The cilated neck segment, the intermediate segment, and the collecting segment demonstrate only limited involvement with protein tracers and likely are not involved in the absorption of protein from the glomerular filtrate. A substantial involvement of the distal segment with the absorption and transport of protein is suggested. Diffusion of tracers from the peritubular capillaries and sinusoids through the basement membrane into the lateral intercellular spaces and smooth vesicles of cells in both the proximal and distal segments indicates that retrograde transport likely occurrs. This also demonstrates that the cytoplasm of these cells contains a complex endomembranous system continuous with the plasma membrane. The significance of these findings is discussed in relation to possible functions of the kidney in osmoregulation of the sea lamprey in both freshwater and saltwater environments.