Posttranscriptional mechanisms regulate ontogenic changes in rat renal sodium-phosphate transporter

The present investigation sought to characterize the relationship between ontogeny and Na⁺-P(i) transporter expression in the rat kidney. Results showed that the maximal reaction rate (nmol · mg protein^-1 · 10 s^-1) of Na⁺-P(i) transport was highest in 21-day-old rats (2.26 ± 0.26), was lower in 42- to 45-day-old rats (1.44 ± 0.19) and 4-mo-old rats (0.78 ± 0.15), and was lowest in 14-day-old rats (0.50 ± 0.16) (P = 0.0009, n = 3). The Michaelis constants (mM P(i)) were not significantly different in the four age groups. Northern blot analysis revealed that the abundance of Na⁺- P(i) transporter mRNA was similar in all four age groups (n = 5). Western blot analysis demonstrated the highest immunoreactive protein signal in the 21-day-old rat (Na⁺-P(i)/β-actin = 4.15 ± 1.16), followed by decreasing protein levels in 42-day-old rats (2.13 ± 0.22), 4-mo-old rats (0.85 ± 0.25), and 14-day-old rats (0.75 ± 0.37) (P = 0.022, n = 5). Immunohistochemical analysis of kidney cortex in the four age groups showed specific staining of only apical membranes in all samples. We conclude that posttranscriptional mechanisms play a role in regulating this transporter during rat ontogeny.