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

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Abstract
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.