In industrialized countries, uptake of highly processed food results in increased phosphate load which may enhance the cardiovascular risk not only in patients with impaired kidney function, but also in the general population. It is discussed that hyperphosphatemia and the phosphate-induced increased levels of the phosphaturic hormones parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) cause cardiac complications, which are responsible for higher morbidity and mortality rates. The exact pathological mechanisms are still poorly understood and it is not clear whether hyperphosphatemia itself or the resulting high FGF23 and/or PTH mediate cardiac remodeling and dysfunction.
To investigate pathological cardiac changes due to chronic high phosphate load, male C57BL/6 wild type mice received a high phosphate diet containing 2% inorganic phosphate (HPD) or a 0.8% normal phosphate (NPD) for six months. Long-term chronic phosphate overload resulted in significantly increased level of serum and urinary phosphate, as well as increased level of plasma PTH and FGF23 compared to NPD controls. HPD mice showed impaired heart function with reduced ejection fraction, increased end-systolic volumes and an enlarged systolic left ventricular internal diameter compared to NPD accompanied with impaired left ventricular relaxation as seen in Doppler echocardiography. Although, HPD mice revealed a mere slightly elevated heart weight-to-body weight ratio, cardiomyocyte area and length were significantly increased as seen in WGA staining of heart sections indicating cellular hypertrophy. However, those mice showed a blunted activation of the hypertrophic gene program as the classical prohypertrophic NFAT target genes e.g. ANP, BNP, αMHC and βMHC, respectively, were unchanged compared to NPD group. To find relevant signaling pathways altered in HPD mice, we next performed bulk RNA sequencing of whole heart tissue. Thereby, genes of the calcium signaling pathways were differentially expressed. Besides, immunoblot analysis showed alterations in Phospholamban activity after six months of HPD. To rule out the effect of high phosphate levels on cardiomyocyte contractility and calcium handling, sarcomere movements and changes of intracellular calcium levels in single cardiomyocyte isolated from NPD and HPD mice were measured with the IonOptix system. Results will be presented here.
In vitro stimulation of isolated neonatal rat ventricular cardiomyocytes with phosphate could induce a hypertrophic response as seen in cell size measurements, possibly affecting calcium handling and revealing the direct effect of phosphate on the heart.
In conclusion, our data indicate that chronic high phosphate load in mice cause a dilated cardiac phenotype including altered contractility, cellular hypertrophy, and differential gene expression of calcium signaling pathways.