Chloride transport by human placental microvillous membrane vesicles

Unidirectional uptake of chloride by microvillous membrane vesicles prepared from human term placentas was studied over a range of membrane potentials in the presence and absence of chloride transport inhibitors alone and in combination at maximally effective concentrations. At 0 mV, inhibition of chloride uptake by 0.1 mM DIDS, 0.5 mM DPC, and 0.5 mM DPC plus 0.1 mM DIDS was similar, suggesting a common action upon an anion exchanger; neither 0.1 mM furosemide nor 0.1 mM bumetanide alone had any effect. An inside-positive membrane potential was created by imposing an inwardly directed potassium ion gradient in the presence of valinomycin. Total chloride uptake increased with increasing membrane potential (0, 4.6, 17.3, 25.8 and 32.0 mV). The inhibition of uptake by DPC and DPC/DIDS increased with the membrane potential. The effect of DPC compared to DPC/DIDS was significantly different at 4.6, 17.3 and 25.8 mV, suggesting a degree of additivity of inhibition. Neither furosemide nor bumetanide had any effect at any potential. There was a significant increase in inhibition due to DIDS alone until the membrane potential reached 25.8 mV. But there was no significant difference between the level of inhibition at 32 mV as compared to 0 mV, providing evidence of a DIDS-sensitive conductance similar to that previously seen in patch clamp studies. We suggest that uptake of chloride across the microvillous membrane of the human placenta may be by at least three different pathways; an electroneutral, DIDS-sensitive anion exchanger, a DPC-sensitive chloride conductance and a DIDS-sensitive chloride conductance.