Active transport of potassium and oxygen consumption in the isolated midgut of Hyalophora cecropia.

Oxygen uptake is essential for active ion transport across the isolated silkworm midgut as well as across other isolated epithelial membranes such as the frog skin and toad bladder. The study of the chemical relationship between these two processes in the skin and bladder has been rewarding. The short-circuit current across the isolated frog skin and toad bladder is accounted for entirely by the active transport of sodium and a substantial fraction of the oxygen consumption is used for Na-transport. When sodium is being transported, extra energy is utilized and the rate of respiration is increased. The increase in respiration rate is proportional to the amount of sodium transported, and therefore the ratio between extra oxygen consumed and the amount of sodium transported remains constant (Zerahn, 1956; Leaf and Renshaw, 1957; Leaf, Anderson & Page, 1959). After Harvey & Nedergaard (1964) demonstrated that potassium is actively transported from blood side to lumen of the isolated midgut of Hyalophora cecropia (L), it seemed feasible to measure the relationship between K-transport and oxygen uptake. Following the approach of Zerahn (1956), it seemed sufficient merely to measure the oxygen uptake of the midgut when it was transporting potassium and to subtract the oxygen taken up when the transport was stopped. This value for the net oxygen uptake could be compared to the net K-transport as calculated from concurrent measurements of the short-circuit current. Harvey & Nedergaard showed that more than 80 % of the short-circuit current is carried by potassium. Because of the importance of this figure for calculating the net K-flux, and because of the high respiration of the midgut tissue when experiments are performed in oxygen-saturated solutions, the method of short-circuiting the midgut was modified and the relationship between the K-flux toward the lumen and the short-circuit current was verified. The omission of potassium from the blood-side solution stopped the K-transport at once so that oxygen uptake measured during such a period could serve as a blank. Periods of low transport were also produced by the following method. When the lumen exhibits a large positive potential relative to the blood side, the transport should be retarded and the influx of potassium should approach zero. However, measurements of the K influx (toward the lumen) showed that the natural potential