Short Communication Water Transport across the Short-circuited Midgut of the American Silkworm

The midgut of the American silkworm transports potassium actively from the blood-side to the lumen (Harvey & Nedergaard, 1964) and when the midgut is shortcircuited, the short-circuit current (I^.) is approximately equivalent to the active K transport. The rate of movement of water has been considered to be low. It has been measured as 9±70/ilh~ (N=3) from blood-side to lumen in an open system (Nedergaard, 1972). In the closed system described by Harvey, Haskell & Zerahn (1967) the water flow could be monitored from the shape and size of the midgut and it was obvious that the movement of water was small. In this paper the movement of water across the short-circuited gut is compared with the active K flux; the water flux follows the K flux from the blood-side to the lumen and depends on the K and sucrose concentrations in the bathing solution. When the K flux is stopped by lack of oxygen in the solution the water flux also stops. Fifth instar larvae (weight about 10 g) of Hyalophora cecropia (L) grown on artificial diet (Riddiford, 1968) or willow were used. The larvae were chilled on ice for at least 1 h before the gut was removed and placed in the apparatus described by Harvey & Zerahn (1972). The lamen side was open, so it was easy to take samples and get circulation with a slow flow of oxygen saturated with water vapour. Control experiments in which the gut was substituted with a piece of plastic tubing showed that no change of ^Na concentration in the lumen took place during 1 h, provided that there was no evaporation or addition of water during the experimental period. The blood-side was closed and stirred with a magnetic flea. All bathing solutions contained (in mmoir) NaCl, 30; CaCl2, 1; MgCl2, 1; KHCO3, 2; plus different concentrations of KC1 and sucrose. The concentrations of K were 16, 32, 64 and lZSmmoll". The concentrations of sucrose were 100, 166 and 222mmo\\~. All experiments were performed on short-circuited guts, and the active K flux was assumed to be equal to the short-circuit current (IK). The water flux was measured by monitoring the decrease in concentration of Na in the lumen solution. This radioisotope (half-life 2-6 years) was chosen because it emits gamma rays as well as hard beta particles (/J). Both kinds of radiation can be easily