Single channel recording experiments show the presence of K+ channels in the hSTB microvillous membrane sensitive to Tetraethylammonium (TEA) and Barium (Ba+2). Patch current activity was diminished 50% and 70% by 20 mmol/L TEA and 5 mmol/L Ba+2 respectively. The more frequent conductance was approximately 73 pS, however several levels of current were detected suggesting the presence of more than one type of K+ channel. In addition, sodium (Na+) sensitivity was detected in the patch current thus, over 10 mmol/L Na+ reduced the seal current to 38%. These results were corroborated by the total current experiments where the K+ current elicited in injected oocytes with apical purified membrane was blocked by Ba+2 and TEA. The total current was also affected by Na+, becoming larger when a Na+-free solution Was used. Our results show the existence of at least two types of Ba+2-sensitive K+ channels including a TEA sensitive sub-population, and some of them N+ sensitive K+ channels. These channels could be the conductive pathways proposed previously for this cation in placental hSTB. Our novel contribution has been to successfully obtain K+ channel recordings in systems suitable for electrophysiological studies of isolated apical membranes. (c) 2008 Elsevier Ltd. All rights reserved.