Induced Hypocalcaemia in Cows and Sheep. I. Changes in Plasma Inorganic Phosphorus Levels

Abstract

Mean plasma Pi concentrations were lowered significantly ( P = 0.03) in a group of seven cows during the induction of hypocalcaemia (to the recumbent-comatose stage) by the infusion of 4.7% Na 2 EDTA solution, compared to the results obtained in the same cows with a control infusion of 0.9% saline. The regression coefficient of plasma Pi concentration on plasma calcium concentration calculated from the pooled results of the eight cows during the induction of hypocalcaemia was 0.400 ( P < 0.01) with a correlation coefficient of 0.843. Analysis of results from the total period of induction and recovery in six of the cows, which recovered spontaneously from hypocalcaemia, showed a similar relationship and yielded a regression coefficient of 0.448. Results in a second group of six cows in which hypocalcaemia was induced to a stage just preceding recumbency supported the above relationships but in this group, where sampling was more frequent, it was found that the regressions of plasma Pi on plasma calcium differed significantly between cows. No significant differences were found in the response of plasma Pi between a group of six wethers in which hypocalcaemia was induced by the infusion of Na 2 EDTA and six other wethers infused with 0.9% saline although changes in plasma Pi concentration tended to follow those of plasma calcium in the former group and significant differences occurred between the individual regressions of Pi on calcium concentration. Plasma Pi showed a highly significant mean increase of 0.46 mmol/l in six cows with induced hypocalcaemia immediately after treatment with calcium borogluconate. Enforcing sternal recumbency for three hours in a group of eight cows resulted in a significant fall ( P < 0.001) in plasma Pi concentrations. Possible reasons for the observed changes in plasma Pi observed in these experiments are discussed.