EVIDENCE FOR FUROSEMID E‐SENSITIVE Na + ‐K + ‐Cl #x2212; CO‐TRANSPORT IN LACTATING RAT MAMMARY TISSUE

The transport of K + (using 86 Rb as tracer) by lactating rat mammary tissue slices has been studied in order to seek evidence for Na + ‐K + ‐Cl − co‐transport. Potassium transport was inhibited by furosemide; the locus of inhibition was at a site other than the Na + :K + pump. Replacing medium Cl − with NO 3 reduced the bidirectional movement of K + ; moreover, furosemide was without effect in a Cl − ‐free medium. Sodium replacement by N ‐methyl‐D‐glucamine acted to reduce the loop diuretic‐sensitive component of K + uptake whilst concomitantly increasing K + influx via an ouabain‐and furosemide‐resistant pathway. Potassium efflux was found to be transiently stimulated by Na + ions; this was attenuated by furosemide. Potassium egress was markedly increased via a furosemide‐insensitive pathway when salicylate was used to replace Cl − . The results are consistent with furosemide‐sensitive Na + ‐K + ‐Cl − co‐transport.