Modulation of renal calcium and phosphate transporting proteins by dietary nitrogen and/or calcium in young goats

摘要

In young goats, a reduction in dietary nitrogen (N) had an impact on mineral homeostasis although ruminants are able to recycle N effectively due to rumino-hepatic circulation. A solitary calcium (Ca) reduction stimulated calcitriol synthesis and Ca concentrations remained unchanged, whereas a dietary N reduction led to a decrease in calcitriol, which could not be prevented by a simultaneous reduction of N and Ca. In a previous study, it was shown that a reduced dietary N intake caused a decrease in intestinal Ca absorption due to a reduction of intestinal Ca transporting proteins. As no data on the potential role of the kidneys are available, it was the aim of the present study to evaluate whether an N- and/or Ca-reduced diet had an impact on renal Ca and phosphate (Pi) transporting protein expression in young goats. The animals were divided into 4 feeding groups, each receiving an adequate N and Ca supply, a reduced N supply, a reduced Ca supply, or a combined N and Ca reduction for 6 to 9 wk. The protein expression of the renal Ca channel transient receptor potential cation channel subfamily V member 5 (TRPV5) was diminished in N-reduced fed goats (P = 0.03), whereas in Ca restricted animals, the expression remained unaltered. The mRNA and protein expression of the Ca-binding protein calbindin-D28K (CaBPD28K) and the sodium–Ca exchanger 1 (NCX1) were significantly decreased due to the N-reduced feeding (mRNA, P = 0.003; P 0.0001; protein, P = 0.002; P = 0.02), whereas dietary Ca reduction increased the CaBPD28K and NCX1 mRNA expression (P = 0.05; P = 0.01). The mRNA and protein expression of the parathyroid hormone receptor (PTHR) decreased due to the N-reduced feeding (P = 0.02; P = 0.03). These results confirm that a reduced dietary N intake led to decreased TRPV5, CaBPD28K, PTHR, and NCX1 expression levels, contributing to low levels of calcitriol and plasma Ca. In contrast to this, sodium–phosphate cotransporter type IIa expression and plasma Pi concentration were increased during dietary N reduction, thus indicating that Pi homeostasis is modulated in a calcitriol-independent manner. In conclusion, the modulation of Ca transporting proteins expression in the kidney is not able to prevent changes in mineral homeostasis in young goats receiving an N-reduced diet.