Abstract
Phosphorus (P) deficiency in early lactating dairy cows is receiving increased attention because of incentives aiming at curtailing environmental pollution with P by reducing dietary P in ruminant diets. An in-vitro study using bovine hepatocytes incubated for 7 days with phosphate (Pi) concentrations of 0.9, 1.8 or 2.7 mmol/L, and an in-vivo study feeding late pregnant dairy cows diets with either adequate (0.28% and 0.44% in DM ante-partum and post-partum respectively) or low P content (0.15% and 0.20% in DM ante-partum and post-partum respectively) from 4 weeks before to 4 weeks after calving were conducted to explore effects of P deprivation on liver function. In vitro the relative abundance of mRNA of key enzymes of the carbohydrate metabolism in incubated hepatocytes and liver metabolites in culture medium were determined. In vivo health and productivity of experimental cows on low and adequate dietary P supply were monitored, and liver tissue and blood samples were obtained repeatedly. Liver tissue was assayed for its triacylglycerol-, mineral and water content as well as for the relative abundance of mRNA of enzymes of the carbohydrate-, fat- and protein metabolism. Reduced Pi-availability was not associated with altered enzyme transcription rates or metabolic activity in-vitro. The most prominent clinical finding associated with P deprivation in-vivo was feed intake depression developing after the first week of lactation. Accordingly cows on low P diets had lower milk yield and showed more pronounced increases in liver triacylglycerol after calving. Although the liver P content decreased in P deficient cows, neither negative effects on enzyme transcription rates nor on blood parameters indicative of impaired liver metabolic activity or liver injury were identified. These results indicate the P deprivation only indirectly affects the liver through exacerbation of the negative energy balance occurring as P deficient cows become anorectic.
Highlights
LDH activities of low Pi medium (LPi), intermediate [Pi] (IPi) and high Pi (HPi) media ranged between activities determined in HG and 0G
The results of these studies confirm that pronounced hypophosphatemia can be induced by dietary P deprivation in transition cows
Most prominent effect of sustained P deprivation is the development of anorexia that occurred after parturition and resulted in pronounced liver TAG accumulation
Summary
Ethics statementLiver tissue required for the in-vitro study (part 1) was obtained from cows sacrificed for an unrelated project (permit no. 33.12-42502-04-15/2024, State Office for Consumer Protection and Food Safety, Lower Saxony, Germany). Primary bovine hepatocytes (PBH) were collected from liver tissue obtained immediately after slaughter from healthy cows. Hepatocytes were collected into 100 mL ice-cold Williams’ Medium E containing 20% fetal bovine serum (FBS, PAN BioTech, Aidenbach, Germany). The cell solution was filtered through gauze, centrifuged (60xg for 3 min at 4 ̊C), and washed with ice-cold Williams’ Medium E with 10% FBS. After 15h of incubation in serum-free medium the culture medium was replaced by experimental culture media with different concentrations of inorganic phosphorus (Pi) and glucose (Table 1). These media contained no insulin but 1.25 mmol/L propionate, 1.0 mmol/L pyruvate and 1% non-essential amino acids were added. The concentrations of Pi ([Pi]) used in this study were arbitrarily chosen with the objective to mimic states of hypo-, normo- and hyperphosphatemia normally encountered in vivo in dairy cows [19]
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