Rectal water transport in vivo in the cockroach, Leucophaea maderae: Tritiated water uptake and solute effects

An in vivo perfusion technique with tritiated water was used to study the efflux of rectal lumen water and the influx of haemolymph water in the cockroach, Leucophaea maderae . No isotopic effects on tritiated water absorption through the rectal wall were observed. Concentrations of a non-diffusible solute trehalose ranging from 0·15 to 0·6 M did not differ in their effect on absorption of initial lumen water which was very rapid in the first 30 min. Influx of unlabelled haemolymph or tissue water was also rapid, preventing any net reductions in lumen volume in this time. When no solutes were perfused into the lumen, absorption was increased but influx of water still occurred at a reduced rate, apparently against a concentration gradient. The rapid turnover and cycling of water through the rectal wall and chamber in contrast to the long time necessary to dry the faecal pellet may be a way to facilitate reabsorption of small, metabolically useful, solute molecules including amino acids and monosaccharides filtered from the haemolymph by the Malpighian tubules or occurring as digestion residues. The process is relatively independent of a wide range of osmotic pressures in the lumen.