“…However, this transport process apparently is not saturable or blocked by the usual metabolic inhibitors (114). Nevertheless, it does appear to be shared by amide-containing compounds (e.g., acetamide) but not by thiourea, to be sodium linked, and to be suppressed by inhibitors of sodium transport and phloretin (245,259). Although net urea absorption by elasmobranch tubules may involve some sort of active step across the luminal membrane, the inability to define it precisely and the elaborate anatomical structure of the nephrons have led to the suggestion of a passive mechanism (28).…”