Urine formation by the malpighian tubules of Calliphora. II. Anions.

The current concept of fluid transport in vertebrates is that water moves passively down the osmotic gradient created by an active transport of solute. Earlier suggestions that these tissues could transport water in the absence of a concomitant movement of solute have been rejected (Curran, 1965; Diamond, 1965; Robinson, 1965, 1966). Interest is now focused on the mechanism by which water movement is coupled to active solute transport. A simple explanation would be that active solute transport across an epithelium might increase the total solute concentration on one side and thus provide the osmotic gradient for a passive flow of water. This simple hypothesis, however, cannot account for water transport against an osmotic gradient. Curran (i960) extended the hypothesis by postulating that active solute transport takes place into an enclosed compartment within which a large osmotic pressure gradient could be maintained. Long narrow channels (e.g. lateral intercellular spaces and basal infoldings), which are a constant feature of secretory or reabsorptive epithelia, appear to be the structural analogues of the enclosed compartment in Curran's model (Diamond & Tormey, 1966a, b; Kaye, Wheeler, Whitlock & Lane, 1966; Berridge & Gupta, 1967; Davis & Schmidt-Nielsen, 1967). A mathematical treatment of solutelinked water transport indicates that standing osmotic gradients within long narrow channels can adequately account for the coupling of water to solute transport (Diamond & Bossert, 1967). Urine formation by the Malpighian tubules of insects resembles fluid transport in vertebrate epithelia in that water flow depends on an active secretion of potassium (Ramsay, 1953, 1955 b, 1956). However, details of the exact linkage between solute and water transport have not been determined. This paper attempts to extend Ramsay's observations on the nature of cation transport and to consider the structural basis for the mechanism of solute-linked water transport in Malpighian tubules.