Proline transport in MDCK cells expressing a mutant regulatory subunit of cAMP-dependent protein kinase

Inhibition of the renal betaine transporter by calcium ions

Chronic upregulation of the renal betaine/GABA transporter (BGT1) by hypertonic stress has been well documented, but it is not known whether BGT1 can be regulated acutely after insertion in the basolateral plasma membrane. Related transporters, such as the rat brain GABA transporter, can be rapidly removed from the plasma membrane through activation of G protein-coupled receptors. The goal of the present study was to determine whether acute changes in extracellular and/or intracellular Ca2+will regulate BGT1 transport activity at the plasma membrane level in Madin-Darby canine kidney cells subjected to 24-h hypertonic stress. After brief pretreatment with a Ca2+-free solution, the addition of extracellular Ca2+in the transport assay produced dose-dependent inhibition of Na+-GABA cotransport. Maximum inhibition was 49% at 2 mM Ca2+( P < 0.05). Fura 2 imaging confirmed that addition of 2 mM Ca2+produced a transient increase in intracellular Ca2+that preceded transport inhibition. Acute inhibition of Na+-GABA cotransport was reproduced by addition of thapsigargin (5 μM) and ionomycin (10 μM). Amino acid transport system A, assayed as a control, was not inhibited. Brief treatment with phorbol esters reproduced the specific inhibition of Na+-GABA cotransport, and the inhibition was blocked by staurosporine. Surface biotinylation confirmed that the response to phorbol esters was accompanied by loss of BGT1 protein from the plasma membrane, and immunohistochemistry showed a shift to an intracellular distribution. We conclude that BGT1 can be inhibited acutely by extracellular Ca2+through a mechanism involving BGT1 protein internalization, and protein kinase C may play a role.

Characterization of sulfate, proline, and glucose transport systems in anterior cruciate and medial collateral ligament cells

The present study was undertaken to define the nature of key transport processes for sodium, glucose, proline, and sulfate in primary culture of canine anterior cruciate ligament (ACL) and medial collateral ligament (MCL) cells. Uptake studies using radiolabeled isotopes were performed and Na,K-ATPase activity was determined in cell lysates. At 25 degrees C both ACL and MCL cells showed a significant uptake of 86Rb. Ouabain inhibited Rb uptake by 55% in ACL cells and by 60% in MCL cells. The transport activity of Na,K-ATPase in intact cells was calculated to be 57 and 71 nmol.(mg protein)-1.(15 min)-1, respectively. The enzymatic activity of Na,K-ATPase in cell lysates was observed to be 104 for ACL cells and 121 nmol.(mg protein)-1.(15 min)-1 for MCL cells. Cytochalasin B, a known inhibitor of sodium-independent D-glucose transport, completely inhibited D-glucose uptake in ACL and MCL cells. Removal of Na+ or addition of 10-5 mol/L phlorizin, a potent inhibitor of the sodium-D-glucose cotransporter, did not alter D-glucose uptake, suggesting that glucose entered the cells using a sodium-independent pathway. Both ACL and MCL cells exhibited high sulfate uptake that was not altered by replacement of Na+ by N-methyl-D-glucamine, whereas DIDS, an inhibitor of sulfate/anion exchange abolished sulfate uptake in both cell types. Thus, neither cell type seems to possess a sodium-sulfate cotransport system. Rather, sulfate uptake appeared to be mediated by sulfate/anion exchange. Proline was rapidly taken up by ACL and MCL cells and its uptake was reduced by 85% when Na+ was replaced by N-methyl-D-glucamine, indicating that proline entered the cells via sodium-dependent cotransport systems. The data demonstrate that both ACL and MCL cells possess a highly active sodium pump, a secondary active sodium-proline cotransport system, and sodium-independent transport systems for D-glucose and sulfate.