Active transport of magnesium across the isolated midgut of Hyalophora cecropia

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Abstract
The 28Mg-measured net flux of magnesium from lumen-side to haemolymph-side of the isolated and short-circuited midgut was 1.97 + 0.28 µ-equiv cm−2 h−1 in 8 mM-Mg2+. 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 µ-equiv/g wet weight of midgut tissue. Magnesium depresses the short-circuit current produced the midgut but not the potassium transport, the depression being equal to the rate of magnesium transport. Magnesium transport yields a linear Lineweaver-Burk plot with an apparent Km of 34 mM-Mg2+ and an apparent Vmax of 14.9 µ-equiv cm−1 h−1. Magnesium is actively transported across the midgut and contributes to the regulation of the haemolymph magnesium concentration in vivo.