Differential effects of harmaline and ouabain on intestinal glucose transport in the pigeon

Effects of harmaline and ouabain on intestinal transport in vitro of D-glucose in the pigeon are investigated. Harmaline inhibits glucose influx and affects intestinal Na⁺-K⁺-ATPase activity though the substrate uptake is more sensitive than the enzyme activity. Low concentration of harmaline while drastically inhibiting glucose uptake, does not affect intracellular concentration of Na⁺ and K⁺. In contrast, ouabain, though has no significant effect on glucose uptake, alters substantially the ionic balance of cells. Harmaline also affects that component of nutrient influx which is left unaffected ouabain. Mucosal-serosal flux of glucose is reduced harmaline when it is present only on the mucosal side of everted intestinal sacs. On the contrary, similar effect is produced ouabain when it is placed only on the serosal side. It appears that harmaline possibly inhibits glucose transport in the pigeon intestine two ways: first, irreversible binding Na⁺-K⁺-ATPase-a feature shared ouabain, and second, reversible binding Na⁺-binding sites of enterocyte membrane-an effect not shared ouabain.