Adaptation of electrolyte handling to low crude protein intake in growing goats and consequences for in vivo electrolyte excretion

In a previous study it was shown that low crude protein (LCP) intake affected the systemic and renal electrolyte handling (EH) in growing goats. The present study aimed to evaluate the consequences of these changes on the electrolyte excretion and its in vivo regulation by functional studies. Growing male Saanen-type goats were fed diets high or low in crude protein content, and nitrogen (N), phosphorus (P) and calcium (Ca) balance trials were performed to investigate electrolyte excretion. Additionally, plasma urea, Ca, phosphate (Pi) and endocrine regulators of electrolyte homeostasis including insulin-like growth factor-1 (IGF-1), calcitriol, cross-linked N-telopeptide of type I collagen and parathyroid hormone (PTH) concentrations were determined. Renal adaptation to LCP intake was investigated by determining the protein expression of the sodium-dependent Pi transporter types IIa and IIc (NaPi IIa and IIc) and the PTH receptor (PTHR) by Western blotting. To demonstrate associations between the measured variables and thereby to identify the possible pathways of electrolyte handling in goats, linear regression analyses were performed. While N metabolism was adapted to LCP intake, the plasma IGF-1 and, by trend, calcitriol concentration decreased and also the renal PTHR protein expression declined. However, Ca and P excretion and retention remained unaffected. Linear regression analyses showed that calcitriol and IGF-1 concentrations were associated with further key factors of systemic and renal EH. In summary, LCP intake in growing goats led to an adaptation of the systemic and renal EH, and IGF-1 and calcitriol appeared to play significant roles. However, these adaptations seemed to be counterbalanced by the whole systemic network regulating the electrolyte handling and therefore did not have any impact on the functional electrolyte excretion.