Cellular mechanisms of salt secretion by the Malpighian tubules of insects.

Malpighian tubules secrete primary urine and the hindgut and rectum perform the fine regulation of its composition. The system plays an important role in the regulation of the salt and water content in insects : after eclosion and before flight some insects must lose a lot of water in order to reduce their weight ; or after a meal the blood sucking bug Rhodnius, for instance, or the female mosquito need to eliminate a high load of NaCl and water; the mealworm and the desert beetle on the other hand use a cryptonephric system, built by the Malpighian tubules and the rectum, to reabsorb practically 100 \% of the water present in the excreta. Formica, the species described in this paper, is an omnivorous, continuously feeding species. K+ secretion is essential in the formation of primary urine by the Malpighian tubules of Formica. In the present paper a model for this K+ secretion is discussed in detail and a brief comparison is made with other species living in different conditions and/or exposed to different salt and water loads. The prime mover for salt secretion in the tubule is a V-type H+ ATPase in the luminal membrane in parallel with a Na+/H+ or K+/H+ antiporter. Uptake mechanisms for K+ and/or Na+ at the haemolymphal side may differ according to the species : in tubules of Formica uptake of K+ through high conductance channels occurs in the presence of a high K+ concentration. At lower K+ concentrations a K/Cl and a Na/K/2Cl cotransporter also become functional in these tubules. In some species an appreciable Na+ conductance is present (e.g. Aedes aegypti) or a Na+/K+-ATPase may play a role (e.g. Rhodnius). The pathway followed by the accompanying anion (mostly Cl-) is still controversial.