Acute activation of the renal betaine/GABA transporter by low extracellular calcium

Abstract

The betaine/GABA transporter (BGT1) is important for osmoprotection in kidney medullary cells. We previously reported an acute (30 min) increase in extracellular Ca caused dose‐dependent inhibition of BGT1 in renal MDCK cells (AJP Renal 291:F305‐12, 2006). To determine if extracellular Ca might be a local regulator of BGT1, we tested the response to low Ca serum‐free growth medium (LCM, 0.09 mM Ca2+). Na+/GABA cotransport in MDCK monolayers was activated within 30 min after transfer from standard culture medium to LCM. Activation was significant after 60–90 min (n=3–5) and was independent of medium osmolarity. Peak transport was increased 50% in isotonic LCM and 100% in hypertonic (500 mOsm) LCM over controls. Hypertonic LCM also activated Na+/betaine cotransport by 400% within 60 min (n=3). The system A amino acid transporter showed a similar 4‐fold activation in hypertonic LCM. Perfusion of Fura‐ 2‐loaded MDCK cells with LCM decreased intracellular Ca2+ by 31% (n=22) within 6–7 min. Confocal imaging of ZO‐1 protein showed a marked change in distribution within 45 min exposure to LCM, consistent with disruption of tight junctions. Trafficking pathways may be disrupted by loss of tight junctions and cell polarity, and activation of BGT1 and system A transport by LCM could be due to increased membrane insertion or decreased retrieval.