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.
- Confocal microscopy
- Madin-Darby canine kidney cells
- System A
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