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

The transport of K+ (using 86Rb 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 NO3 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.