Nephron Handling and a Diuretic
A 60-year-old woman with hypertension is prescribed a drug that acts on the ascending limb of the loop of Henle, blocking the sodium-potassium-chloride cotransporter. Before treatment: plasma sodium 140 mmol/L, potassium 4.2 mmol/L, blood pressure 158/96 mmHg. After four weeks: plasma sodium 136 mmol/L, potassium 3.1 mmol/L, blood pressure 138/84 mmHg, and her urine output has increased. She reports muscle weakness and cramps. Her glomerular filtration rate is unchanged.
1. Explain why blocking this cotransporter increases urine output, using the normal function of the ascending limb.
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The ascending limb of the loop of Henle is impermeable to water but actively transports sodium, potassium and chloride out of the tubule into the medullary interstitium. This does two things:
- It dilutes the tubular fluid (water stays behind as solute leaves).
- It builds the medullary concentration gradient that the descending limb and collecting duct rely on to reabsorb water.
Blocking the cotransporter means less solute is moved out. The medullary gradient becomes weaker, so the collecting duct cannot reabsorb water as effectively even under antidiuretic hormone. More water remains in the urine, so output rises. Solute that stays in the tubule also holds water osmotically, adding to the diuresis.
The judgement is that the drug's effect on urine volume is a consequence of disrupting gradient formation, not a direct effect on water channels.
2. Account for the fall in potassium and the fall in sodium, and explain why the potassium change is the more clinically significant of the two.
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Potassium (4.2 → 3.1 mmol/L): the cotransporter normally reabsorbs potassium along with sodium and chloride. When it is blocked, more sodium reaches the distal nephron. Distal sodium delivery drives sodium reabsorption in exchange for potassium secretion, so potassium is lost in urine. The fall is substantial and symptomatic - muscle weakness and cramps are consistent with hypokalaemia affecting muscle membrane excitability.
Sodium (140 → 136 mmol/L): this is a modest fall, within the range influenced by water balance as well as sodium loss. Increased urine output can concentrate or dilute the plasma depending on intake, so the small fall is not the main concern.
Why potassium matters more: a change from 4.2 to 3.1 is a large proportional shift and moves the value below the normal range, where cardiac and neuromuscular effects occur. Sodium at 136 is only just below the reference range and is unlikely alone to cause symptoms. The judgement is to prioritise the potassium - clinically, the symptomatic and larger deviation is the one requiring action, even though sodium is the electrolyte most people think of first in diuretic use.