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Resorption of phosphate, calcium and magnesium in the in vitro locust rectum Andrusiak, Edward William


The ability of the locust rectum in vitro to resorb Ca⁺⁺, Mg⁺⁺ , and PO₄ was studied. The rectum has a low permeability to Ca⁺, but Ca⁺ is not accumulated against concentration differences. Mg⁺ was not accumulated in the basal compartment of the rectal sac even when Mg⁺ concentration gradients were favourable for net diffusion. Phosphate was found to be accumulated by the rectum against a threefold concentration difference. The rectal tissue incorporated inorganic phosphate into organic forms, but phosphorus transferred into the basal compartment was found to be in the form of inorganic phosphate. Uptake into the basal compartment can be described by Michaelis-Menton saturation kinetics. The resorption of water by the rectum did not increase (by solute drag) the amount of PO₄ accumulated in the basal compartment, except at very high PO₄ concentrations in the apical bathing medium. Metabolic poisons such as KCN and IAA inhibited PO₄ accumulation in the basal compartment but did not inhibit PO₄ entry into the tissue from the apical bathing medium. Arsenate, a competitive inhibitor of PO₄ uptake in other systems, inhibited PO₄ entry into the tissue. A mechanism for PCL uptake into the tissue is proposed.

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