Liver phosphorus content in Holstein-Friesian cows during the transition period

Hepatic lipidosis and hypophosphatemia are frequently observed in high-yielding periparturient dairy cows. Objectives of this study were to investigate the association of the liver P content with the degree of liver fat accumulation and serum P concentration and to characterize the change in liver P content throughout the transition period. In a cross-sectional study, liver biopsies obtained from 33 Holstein-Friesian cows 14 d postpartum (p.p.) were assayed for total lipid (TLip), triacylglycerol, DNA, P, Mg, K, Na, and Ca content. Serum samples obtained at the time of biopsy were analyzed for indices of liver function and injury and the serum P concentration was determined. From this cross-sectional study, 6 cows were selected for a longitudinal study and liver tissue obtained from the 6 cows on d -65, -30, -14, 1, 14, 28, and 49 relative to calving was assayed. The amounts of P, K, Mg, Na, and Ca were expressed as amount in dry weight (DW), wet weight (WW), nonfat wet weight (NFWW), and indexed to DNA. In the cross-sectional study, P_DW and P_WW decreased with increasing TLip, whereas P_NFWW and P_DNA were independent of TLip. Values for P_DNA varied widely, whereas P_NFWW varied within a narrow range. Stepwise regression analysis revealed the strongest associations between P_DW and the amount of tissue water (partial R² = 0.74) and the log to the base 10 of triacylglycerol (partial R² = 0.05). The P_WW was associated with the log to the base 10 of triacylglycerol (partial R² = 0.20), but no associations were found for P_NFWW. These findings indicate that decreased electrolyte content in dry and wet liver tissue with increased liver lipid content is predominantly due to the decrease in tissue water and therefore the distribution volume of electrolytes. In the longitudinal study, P_DW, P_WW, and P_NFWW were decreased on d 14 p.p. Similar directional decreases were found for K, Mg, and Na, but P was the only electrolyte that was significantly decreased in liver tissue at d 14 p.p. This finding indicates that the P content of liver tissue decreases in early lactation due to a reduction in hepatocellular cytosol volume as well as a decrease in cytosolic P concentration, with the latter having biological relevance. The clinical significance of decreased cytosolic P concentration in the hepatocytes of dairy cows in early lactation remains to be determined.