In vitro Phosphate Transport in Chick Ileum: Effect of Cholecalciferol, Calcium, Sodium and Metabolic Inhibitors

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Abstract
Three parameters of phosphate transport were monitored in everted gut saes prepared from the ileum of rachitic chicks injected with cholecalciferol 48 hours before use and untreated rachitic chicks by measuring tracer 32P movement: transfer from the outside (mucosal) compartment to inside (serosal) compartment; total transfer out of mucosal compartment, i.e., into intestinal tissue plus serosal compartment; and retention in the tissue. All three were significantly increased by prior administration of cholecalciferol. The elevated rates of 32P transport due to the vitamin were independent of the presence of Ca24 ion in the buffer medium as judged by studies in which the buffer Ca was omitted or a Ca chelator was included. Experiments in which ouabain was included in the incubation medium, or Na was omitted, indicated the possible involvement of the (Na+-K+)-ATPase and related Na pump in the serosal extrusion of phosphate and also the need for some Na+ for the phosphate uptake and storage steps. When glycolysis was inhibited with metabolic inhibitors the uptake and storage of phosphate was reduced by everted gut sacs from both rachitic and vitamin D-treated chicks. The inhibitor of intestinal alkaline phosphatase, L-phenylalanine, did not inhibit phosphate transport; contrariwise, it significantly increased tissue uptake and storage of phosphate by a mechanism not elucidated in these studies.