ABSTRACT
Urate is transported against an electrochemical gradient into the lumen of the lower Malpighian tubule of Rhodnius. In vivo, higher urate transport rates are induced by the increase in haemolymph urate concentration that follows feeding. The induced rate of transport is sufficient to account for the amount of urate eliminated. In vitro experiments with the tsetse fly Glossina suggest a possible induction of transport as a direct response of the tubule cells to an increase in urate concentration in the surrounding fluid.
Urate transport has been shown for Malpighian tubules of insects from several different orders, suggesting that the mechanism is widespread. Insect urate transport differs from that of vertebrates; it is ouabain-insensitive and results in the precipitation of free uric acid instead of urate salts.
The basis for these calculations is the formula, derived by Ramsay (1958), S/M = b/a+b, where S/M is the ratio of the concentration in the secreted fluid to that in the bathing fluid, b is the permeability of the tubule wall in nl min−1 mm−2 and a is the rate of fluid secretion in nl min−1 mm2 of tubule wall. The areas of upper tubule wall are 7 mm2 and 12·5 mm2 for 5th instar larvae and adults respectively. 1 nl min−1 mm−2 is 1·67 × 10−6 cms−1.
For example, in the case described of an adult insect with its tubules secreting fluid at 3 nl min−1, bathed in haemolymph containing 2mm-urate, and each having a permeability to urate of l·16nl min−1 mm−2, then S/ 2 = 1·16/[(3/12·5) + 1·16], whence S = l·66mm, and the rate of urate elimination by each tubule is 3 nl min−1 × 1·66mm = 5 pmol min−1.
