Renal sugar transport in the winter flounder IV. Effect of CA2+ on sugar transport in teased renal tubules

The electrolyte distribution and sugar uptake by teased renal tubules of winter flounder (Pseudopleuronectes americanus) was studied using incubation media with 1.4 mM Ca²⁺ (controls) and without Ca²⁺. The omission of Ca²⁺: (a) produced some cellular swelling, and increase in cell Na⁺ and loss of K⁺; (b) had no effect on the extracellular propylene-glycol space; (c) increased the uptake of non-metabolizable methyl-α-D-glucoside by the tissue: whereas in controls the equilibrium tissue/medium ratio (T/M) for the Na⁺-independent uptake of the sugar was 0.71 ± 0.03 (SEM, n = 9), in Ca²⁺-free media the T/M rose in 1.18 ± 0.06. The increase in sugar uptake seen in absence of Ca²⁺ was abolished by absence of Na⁺ (Li⁺-media), 0.5 mM ouabain and 0.5 mM phlorizin; (d) produced an increase in the galactose phosphate level without affecting that of free sugar; (e) decreased the active uptake of 2-deoxy-D-galactose by the tubules. These results were analyzed on the basis of available information on the transport characteristics of the sugars at the luminal and antiluminal cell faces. It is suggested that absence of Ca²⁺ increases the permeability of the intercellular junctions, thus permitting sugars to enter from the external medium into the tubular lumen. Methyl-α-D-glucoside and D-galactose can then be taken up into the cells by brush-border localized active processes, whereas 2-deoxy-D-galactose is not reabsorbed at this cellular face. At 1.4 mM Ca²⁺, the intercellular junction appears to be relatively impermeable to sugars and the transport properties in teased tubules reflect then events predominantly localized at the antiluminal cell face.