Printed in Great Britain ACTIVE TRANSPORT OF MAGNESIUM ACROSS THE ISOLATED MIDGUT OF HYALOPHORA CECROPIA

1. The ^Mg-measured net flux of magnesium from lumen-side to haemolymph-side of the isolated and short-circuited midgut was i'97± 0-28 /j-equiv cm~2 h"1 in 8 mM-Mg2*. 2. The magnesium-influx shows a delay before the tracer steady-state is attained, indicating the existence of a magnesium-transport pool equivalent to 6*7 /i-equiv/g wet weight of midgut tissue. 3. Magnesium depresses the short-circuit current produced by the midgut but not the potassium transport, the depression being equal to the rate of magnesium transport. 4. Magnesium transport yields a linear Lineweaver-Burk plot with an apparent Km of 34 mM-Mga+ and an apparent Fmaxof 14-9/j-equivcm-1 h"1. 5. Magnesium is actively transported across the midgut and contributes to the regulation of the haemolymph magnesium concentration in vivo