Active transport of water by insect Malpighian tubules.

Through most of his career, Arthur Ramsay was fascinated by matters osmotic. In the period after the Second World War, he worked on osmotic relations of the earthworm (Ramsay, 1949a), developing typically novel methods for measuring the melting-point and sodium content of minute quantities of fluids (Ramsay, 1949b, 1950). However, in the early 1950s, Ramsay started working on insect Malpighian tubules. He soon found that the tubules secreted potassium ions into the lumen against an electrochemical gradient, showing that this transport was an active one. This led Ramsay to the attractive idea that “the secretion of potassium (together with some anion) into the tubule will set up an osmotic pressure, which in its turn will promote a passive inward diffusion of water” (Ramsay, 1954). According to this idea “the secretion of potassium is the prime mover in generating the flow of urine; and if the theory is true it follows that the osmotic pressure of the urine should be equal to or greater than, but never less than, the osmotic pressure of the haemolymph” (Ramsay, 1954). However, in the classic paper that is the subject of the present comments, he provided evidence that he thought destroyed this simple idea. Although we now believe him to have been wrong about this, his paper (Ramsay, 1954) is nonetheless held as a classic publication, as it describes his most novel and powerful technique with which he will always be associated.