RE: blood calcium levels and urine specific gravity

Beats — Tue, 22 Jan 2019 11:22:53 GMT

you want to read Chapter 12 of Jill Madissons book "Clinical Reasoning in Small Animal Practice" [there is an e-book version, so if you have library access, say through a CertAVP module, then you should be able to get this]

Mechanism of hypercalcemia induced polyuria:

"Hypercalcemia impairs the action of ADH on the collecting duct, although the exact mechanism has not been identified. A protein, apical extracellular calcium-sensing receptor, is believed to be involved. When luminal calcium increases, CaSR decreases ADH-induced permeability of the collecting duct. In addition, there may be down-regulation of the formation of water channels (aquaporin 2) in the collecting duct. The effect may be partial or total. Other mechanisms proposed include impaired NaCl transport in the loop of Henle anddirect stimulation of the thirst centre.

Hypercalcemia will also decrease the glomerular filtration rate by causing vasoconstriction of afferent arterioles, which results initially in reversible azotemia. Eventually, tubular function may become permanently impaired causing azotemia due to nephrocalcinosis."

In essence, the relevance is that by separate mechanisms, can cause both azotemia and PUPD, thus can have a patient without clinical signs of dehdration (eg skin tent) with both azotemia and PUPD and this may be mistaken for CKD in the case of a dog with primary hyperparathyroidism for instance.

(other than addison's) most other conditions that cause dilute urine in the presence of azotemia will only result in azotemia in a dehydrated patient, so less likely that a case of pyometra or whatever will get confused with CKD.

The argument would then be that checking electrolytes and calcium in the dog is a relevant thing to do if one believes that a patient may have kidney disease based on PUPD, azotemia and inappropriate lack of concentraiton of urine. (In cats, CKD is much more common than addison's or hypercalcemia, so less likely to miss a case of the latter)

Where confusion can then occur is that secondary renal hyperparathyroidism can result in elevated calcium... but technically, this should be total calcium only and the ionised calcium should be normal.

I believe primary hypoparathyroidism can also cause PUPD in dogs, but I'm not sure of the mechanism of this.

EDIT - I believe on p.204 of the above named text it should read "hyperparathyroidism" rather than "hypoparathyroidism"; if anyone else looks this up can they confirm that I've got this the correct way round and this is a typo?

RE: blood calcium levels and urine specific gravity

Robin Grimmer — Tue, 22 Jan 2019 11:00:02 GMT

Hypercalcaemia is definitely a cause of polyuria/polydipsia. Not sure about hypocalcemia.

RE: blood calcium levels and urine specific gravity

vetbl.locum — Tue, 22 Jan 2019 10:55:48 GMT

Learn something every day!

Sorry I cannot usefully answer your question

RGds