Ion Activities and Electrochemical Gradients in the Mealworm Rectal Complex

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Abstract
Ion activities and potential differences in cellular and extracellular compartments of the rectal complex of Tenebrio molitor L. larvae have been recorded simultaneously using double-barrelled ion-sensitive microelectrodes. On average, the tubule lumen (TL) was 44mV positive to the haemolymph. Values of aK in the posterior rectal complex exceeding 2700 mmol l−1 were measured, sufficient to account for much of the osmolality of 6.8 osmol kg−1 driving uptake of water from the rectal lumen. The mean value of 797 mmol l−1 exceeded the Nernst equilibrium activity more than 75-fold, indicating active transport of K+. Intracellular potassium activities in the tubules (153 mmol l−1) were high relative to the values in other insect cells, but moderate relative to TL values. Tubule lumen Na+ activities as high as 400 mmol l−1 and pH values of 6.8 were well above the equilibrium values of 11 mmol l−1 and 7.9, respectively, indicating active transport of these cations as well. The ease and frequency of impaling a perinephric space (PNS) surrounding the tubules established it as a functional compartment. On average, the PNS was 22 mV negative to the haemolymph. Potassium activities in the PNS were close to electrochemical equilibrium with the haemolymph, whereas mean aNa and pH were reduced fivefold and 0.5 units, respectively, below the corresponding Nernst equilibrium values. The results suggest that cations move from haemolymph to PNS, and that the PNS is the immediate source for cation transport into the tubule lumen. Cl− was close to electrochemical equilibrium with the haemolymph in both compartments, and presumably enters the tubule lumen as a passive consequence of positive potential differences (PDs) in the tubule lumen.

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