Through direct cellular effects, low levels of vitamin D induce loss of podocytes and development of glomerulosclerosis, damaging the integrity of the glomerular filtration membrane. Indirectly, vitamin D suppresses renin transcription contributing to a reduction in proteinuria by hemodynamic effects. As previously described, the conversion of vitamin D into biologically active form is tightly regulated by several factors, including FGF-23. In addition to promote renal phosphate excretion, FGF-23 suppresses the production of vitamin D by inhibition of 1-a-hydroxylase and stimulation of 24-hydroxylase. Recent findings have been supporting an increasingly and important role of FGF-23 as the initial event in the development of CKD. The first step for that is featured by increased levels of FGF-23 preceding changes in calcium, phosphorus, PTH, or even calcitriol levels, that is, its respective regulatory factors. Curiously, in our study we found decreased levels of FGF-23 in both VDD and VDD+IRI groups. According to Rodriguez-Ortiz et al, this decreased FGF-23 levels could act as a compensatory response to prevent further reductions in calcitriol levels, which could exacerbate the hypocalcemia already expected by the evolution of CKD. So, a plausible explanation for our results is that we evaluated FGF-23 levels in a very early development of renal disease, without loss of renal function. Moreover, we must consider the diet used in our study for feeding the VDD and VDD+IRI animals. Besides being totally depleted of vitamin D, the diet also presented low levels of VE-821 ATM/ATR inhibitor calcium and phosphorus, which may have contributed for the low levels of FGF-23 found in VDD and IRI+VDD groups. An important partnership between FGF-23 and Klotho has been described. Klotho proteins form binary complexes with FGF receptors, increasing Klotho affinity and selectivity for FGF23. During kidney disease progression, there is reduced expression of Klotho, a finding also confirmed in our study. We found a significant reduction in Klotho expression in VDD, IRI and VDD+IRI groups when compared to Control group. In addition, our data showed that vitamin D alone reduced Klotho expression in VDD animals, followed by a similar profile of gene expression for a-klotho. It is described that renal ischemia/ reperfusion injury is related to Klotho deficiency. Ming-Chang Hu et al, using a murine model, showed that ischemic AKI was able to induce an acute and transient state of Klotho deficiency with recovery levels after seven days of injury. Most important, epidemiological studies have shown that increased levels of FGF23, PTH and low levels of 1,252D3 are features that precede hyperphosphatemia during progression to CKD. Moreover, such alterations in FGF-23, PTH and vitamin D levels is usually followed by a progressive decrease in secreted Klotho protein in urine of CKD patients.