Molecular physiology on ion transport sytems in silkworms

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Insect midgut in Lepidoptera (the main part of the alimentary canal) is a central and indispensable tissue to exchange various ionic solutes across the epithelia and to maintain the cellular activity and the tissue functions (s). We have investigated such a mechanism to understand the ionic homeostasis and osmoregulation in the silkworm system. Results ar as follows :(1) Immunocytochemical studies were done with the specific anitibody against V-ATPase. In silkwrms, V-ATPase can be detected in the midgut, the Malpighian tubules, the salivary gland and the silk gland. V-ATPase in each tissue was located at the cell surface, suggesting that silkworm V-ATPase is a plasma membrane-type not an endomenbrane-type.(2) In the silk gland, V-ATPase seems to contribute to produce an acidic milieu in the glandular lumen. This shows highly contrast to the midgut lumen, which produce an extremely high alkalinity (pH 11-12).(3) The silk gland V-ATPase is quite similar to other animal V-ATPases in the common subunits of V-ATPase.(4) K+/amino acid symporter in the midgut was rather labile protein and hard to purify. Its molecular nature still remains unknown.(5) The silk gland is highly active tissue for silk protein syntesis and secretion. This implies that the tissue needs much supply of amino acids from the haemolymph to synthesize the silk protein. Thus, it is coming up as the quite near future subject to survey the K+/amino acid symporter in the silk gland. This would be expected as the breakthrough for identifing the K+/symporter molecule (s).