Time courses of plasma magnesium concentrations and urinary magnesium excretion in cows subjected to acute changes in potassium intake

Abstract

Hypomagnesaemic tetany in cows develops occasionally after an acute increase in K intake such as can occur when cows are transferred to spring grass. There is evidence that under these conditions plasma Mg concentrations are only transiently decreased. In this study the questions addressed were whether the plasma Mg concentration, indeed adapts to a high K intake as only dietary variable, and whether urinary Mg excretion is associated with this adaptation. Dry cows were fed rations containing either 26 or 50 g K/kg dm, the extra K being in the form of KHCO3. When the cows were acutely transferred from the low to the high K ration, plasma Mg concentrations fell slightly, but significantly from 0.86 to 0.76 mmo1/1 within five days, but rose again to 0.80 mmo1/1 after another 23 days, this rise being also statistically significant. None of the animals developed tetany. The decrease in plasma Mg concentration in individual animals after five days on the high-K ration ranged from 6 to 21 %. The time course of urinary Mg excretion resembled that of plasma Mg concentration; minimum Mg excretion was seen after four to six days on the high-K ration with a subsequent increase thereafter. To explain the transient lowering of plasma Mg concentration, it is suggested that the K-induced decrease in Mg status caused a delayed increase in the carrier-mediated component of Mg absorption, which in turn caused an increase in urinary Mg excretion. When the cows were acutely switched from the high to the low K ration, plasma Mg concentration and urinary Mg excretion rose, but no transient changes were seen.

Magnesium Absorption by Lactating Dairy Cows on a Grass Silage-Based Diet Supplied with Different Potassium and Magnesium Levels

“…The urine excretion of Mg and plasma Mg were positively associated in a curvilinear action. Schonewille et al (2000) have also shown such an association in dry cows. However, in the present study with lactating cows, the urine excretion was markedly higher.…”

“…The average K level in the diets was 16 g/kg of DM. However, in many areas with intensive dairy production, K content in forage is often >30 g/kg of DM (Fisher et al, 1994;Schonewille et al, 2000). The forestomachs are the main site of Mg absorption in adult ruminants (Tomas and Potter, 1976;Greene et al, 1983).…”

Ureterosygmoidostomy-associated quadriparesis, non-traumatic rhabdomyolysis, and tetany in an adult

“…Although the patient had mild hypocalcemia at admission, she developed tetany on the third day of replacement therapy. It may, therefore, have been a secondary acute increase in potassium intake [9] and/or bicarbonate loading [10].…”

Evaluation of L-carnitine in the treatment of experimentally induced hypomagnesemia in sheep

“…The relationship of K, Ca, and Mg in forage is an important factor in the development of hypomagnesemia. Heavy K fertilization can cause hypomagnesemia even when Ca and Mg levels are within the normal range (Grunes et al, 1984;Schonewille et al, 2000). Potassium ingestion by ruminants may be an enhancement of the urinary excretion of Mg, the major effect on Mg metabolism is a substantial reduction of absorption of Mg from the reticulorumen (Tomas and Potter, 1976;Schonewille et al, 2000) The classical acute (tetanic) form of hypomagnesaemia is due to the critical role that magnesium plays in neuromuscular transmission.…”

“…Heavy K fertilization can cause hypomagnesemia even when Ca and Mg levels are within the normal range (Grunes et al, 1984;Schonewille et al, 2000). Potassium ingestion by ruminants may be an enhancement of the urinary excretion of Mg, the major effect on Mg metabolism is a substantial reduction of absorption of Mg from the reticulorumen (Tomas and Potter, 1976;Schonewille et al, 2000) The classical acute (tetanic) form of hypomagnesaemia is due to the critical role that magnesium plays in neuromuscular transmission. Clinical signs initially include depression and dullness, and later progress to stiffness, excitability, tremors, chewing, and hypersalivation, blinking of the third eyelid, twitching of the pinnae (ear flapping), collapse, tetanic muscle spasms, coma and death.…”