Effect of Ca on Na-D-glucose cotransport across isolated renal brush-border membranes.
Brush-border membrane vesicles were prepared from rat kidney cortex by Mg precipitation. Using quin2 (free acid), intravesicular [Ca2+] was found to be 44 microM and less than 300 nM when vesicles were incubated in 0.2 mM CaCl2 or Ca-free buffer, respectively. In Ca-loaded vesicles, the initial D-glucose uptake, measured in the presence of 150 mM Na+ and 0.1 mM D-glucose inward gradients, was reduced to 30% of the control uptake. This reduction persisted when the extra-vesicular Ca2+ was chelated by ethylene glycol-bis(beta-amino-ethyl ether)-N,N,N',N'-tetraacetic acid but was abolished in the presence of saturating concentrations of D-glucose. Whereas KG0.5 (g) for D-glucose at constant [Na] in the Ca-loaded membranes increased by approximately 50% of the control value (0.5 +/- 0.1 mM), no significant change in Jmax was observed. In contrast, both Jmax and KG0.5 (Na) for glucose, measured as a function of [Na] in the extravesicular fluid, were found to be significantly reduced. Na uptake, determined in the presence of 0.5 mM amiloride, was found to increase by approximately 30% of the value for control membrane. This increase was abolished when vesicles were preincubated with 0.5 mM neomycin or 0.5 mM phlorizin. The results suggest that the effect of Ca2+ on Na entry may be mediated in part by activation of phospholipase C and are consistent with a model of cotransport in which Ca2+ increases the mobility of the binary Na-sugar-translocator complex, thus leading to uncoupling of Na transport from glucose uptake ("slipping") and in part with Ca-induced Na entry by nonmediated leakage.
Lin
J T
JT
Department of Physiology, Cornell University Medical College, New York, New York 10021.
Xu
Z J
ZJ
Lovelace
C
C
Windhager
E E
EE
Heinz
E
E
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000494:pharmacology*; D005947:Glucose / Q000378:metabolism*; D066298:In Vitro Techniques; D007668:Kidney / Q000378:metabolism* / Q000648:ultrastructure; D008871:Microvilli / Q000378:metabolism; D008954:Models, Biological; D010539:Permeability; D010695:Phlorhizin / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism*; D010738:Type C Phospholipases / Q000502:physiology
D012964:Sodium; D010695:Phlorhizin; D010738:Type C Phospholipases; D005947:Glucose; D002118:Calcium