Ji Soo Kim (@KimneyJ), Detlef Bockenhauer and Wesley Hayes

Welcome back. If you’ve been with us all week, you’ve successfully navigated your way along the tubule (if not, why not? Get back to the start you! Here’s the links for Day 1, Day 2, Day 3 and Day 4). Today, we’re collecting knowledge in the Collecting Duct!

The Collecting Duct

You can think of the collecting duct in three summary points:

1. It is sensitive to Anti-Diuretic Hormone (ADH). Water is given its’ own channel (aquaporins)
2. It is where H+ ions get secreted (think distal renal tubular acidosis)
3. It is also sensitive to Aldosterone, which increases sodium reabsorption through ENaC channels

Let’s talk about what happens when each bit goes ‘wrong’.

1. Water Handling

Let’s talk about Nephrogenic Diabetes Insipidus (NDI)

Primary NDI tends to be X-linked recessive (loss of function in AVPR2 protein), but sometimes can be autosomal recessive or dominant in inheritance. In any case, it’s worth asking about family history (especially the mother, if your patient is male and the mother talks about drinking water a LOT).

So you’ve diagnosed NDI, but how do you manage it? There are two principles:

1. Reduce the salt / osmotic load
2. Free access to water

Reducing the osmotic load is easier said than done in a child who has to grow! Liaise with your friendly paediatric renal dietician to concoct a meal with plenty of calories but not loading the kidneys with salt and perpetuating the cycle of water loss.

TOP TIP: Got an inpatient with NDI who needs to be nil by mouth for surgery? DO NOT give iv fluids with sodium chloride. Just 5% dextrose at their maintenance rate should do (as long as they are not dehydrated). Giving a ‘saltless’ solution means there’s no salt to drive renal water loss.

Below we have an approach for a child presenting with high serum sodium, so you can see how NDI fits into the picture.

Next up we have inappropriate anti-diuresis; retaining too much water.

There are two principles for managing SIADH / NSIAD.

1. Fluid Restrict
2. Increase the osmotic load (to encourage diuresis)

Fluid restriction sounds easy. It’s not easy when you’re a child who needs to grow. Especially, when you are baby and dependent on liquid food: no fluid = no calories = no happy chubby baby!

The alternative would be increasing the osmotic load. If the osmotic load is high in the tubular fluid, it encourages water to stick around instead of getting reabsorbed back into the body. This can be done with urea granules (rather than giving sodium… because that’s going to result in hypertension: remember: the low plasma sodium concentration is not due to a lack of sodium, but an excess of water).

The ONLY time you would consider giving sodium rapidly (with help from your friendly local paediatric nephrologist) is if a child is so acutely hyponatraemic, they are seizing. Otherwise, correct the hyponatraemia oh so s-l-o-w-l-y.

Below is a flow diagram for a child with low serum sodium and how SIADH slots in.

2. Distal Renal Tubular Acidosis

Let’s pause for a moment and think about what ‘renal tubular acidosis’ (RTA) means. There are four types:

1. ‘Classical’ (hypokalaemic) distal RTA
2. Proximal RTA – which we discussed earlier in the week
3. Mixed proximal & distal RTA
4. Hyperkalaemic RTA

If proximal RTA is acidosis due to urine bicarbonate losses at the proximal tubule, ‘classical’ distal RTA is acidosis due to impaired hydrogen ion secretion into the urine at the collecting duct.

This makes an important difference for treatment: whereas tubular losses are really difficult to treat (remember the bucket with a slit on the side”?), a defect in secretion is easy. Thus, in proximal RTA, increasing bicarbonate supplementation primarily results in increased losses (it’s pouring out of that ‘slit’). In contrast, in dRTA, you just need to give sufficient bicarbonate to buffer the excess acid that hasn’t been secreted and everything is hunky dory!

What does ‘classical’ distal RTA look like?

How do you treat it? Treat the acidosis to enable normal growth. Replace the potassium.

Want the best of both worlds? Rather than giving potassium as a chloride – give it as potassium citrate. Citrate is converted by the liver into 3 mmol of bicarbonate, so you get more bang for your buck!

3. Salt Handling

Time to think back to the Renin-Angiotensin-Aldosterone System (RAAS). The system where your kidney tries to handle water – by handling SALT! You have these possible scenarios (when the RAAS goes wrong):

Hyperaldosteronism or Pseudohyperaldosteronism

The situation in all these forms is as follows: Lots of ENaC activity. Lots of sodium reabsorbed. Where sodium goes, water follows – bam! You’re hypertensive.

It’s nice to know that amiloride happens to block ENaC, so you have an elegant diuretic solution to your problem.

Hypoaldosteronism or (type 1) pseudohypoaldosteronism

Now what about if ENaC activity is down, as in hypoaldosteronism, or type 1 pseudohypoaldosteronism:

The classic clinical thumbprint is as follows:

• Low BP
• Hyperkalaemia
• Acidosis

The situation: ENaC, the channel responsible for allowing sodium reabsorption in the collecting duct is not active. You are losing precious sodium into the urine – lots of it. Where sodium goes? Water follows, causing hyponatraemia and hypovolaemia. Without sodium going through the collecting duct cells, you don’t have “exchange currency” to allow secretion of potassium and protons: Bang! Hyperkalaemia and acidosis. That’s a pretty toxic combination: hypovolaemia AND hyperkalaemia AND acidosis!

Therefore, what’s the management? Salt! And, oh yes, salt! For emergency management: intravenous saline and bicarbonate. Lots of it: as you treat the acidosis, the potassium will improve, as well.

TOP TIP: Beware the neonate with hyponatraemia, hyperkalaemia, hypovolaemia and acidosis. Could be congenital adrenal hyperplasia. But always worth doing a kidney ultrasound scan to check for obstructive uropathy.

Summary Table

Here we go, the final line added to the table. A thing of beauty for sure!

And. We. Are. Done….phew! We got to the end of the renal tubule. Armed with all this knowledge you’ll be the envy of all your friends, the smartest kid in class!

Thanks for sticking with me this week. Here are some key messages to take away for all things that are renal tubular:

1. Think about the context: What is the child’s fluid status with their plasma and electrolyte abnormality?
2. Slowly correct electrolyte deficiencies where possible
3. Where sodium goes, water follows.

And that’s the end folks! As Douglas Adams and every other water molecule would have it, “I may not have gone where I intended to go, but I think I have ended up where I needed to be.”