Kinetic analysis of inorganic phosphate transport in oocytes expressing NPT2a or co-expressing NPT2a and CFTR.

<p>Phosphate-induced currents (IPi) were measured (Vc = −50 mV) in NPT2a-oocytes (empty circles) and in NPT2a+CFTR-oocytes (filled circles). The dashed lines through the points (means ± SEM from n = 6 to 14, N = 5) were calculated using SigmaPlot software (Systat software Inc., San Jose, CA). <u>A</u>: IPi was induced by increasing Pi concentration in ND 96, pH 7.5, as indicated in abscissa and normalized against extrapolated maximal current (IPi_max). Data were fitted to Michaelis-Menten equation. Results, as means ± SD, were as follows: the apparent concentration of Pi substrate (K_m) giving the half IPi_max was not changed by CFTR expression (K_m = 0.07± 0.02 <i>vs</i> 0.06 ± 0.02 mM in NPT2a- and NPT2a+CFTR-oocytes, respectively), but IPi_max was decreased by 40±3% (P <0.05). <u>B</u>: IPi was induced by 1 mM Pi at increasing Na⁺ concentrations (equimolary substituted by choline⁺, pH 7.5) and normalized against IPi_max. Data were fitted to the modified Hill equation. Results, as means ± SD, were as follows: the apparent K_m for Na⁺ substrate was 40.7 ± 14.6 in NPT2a-oocytes, not different from 41.8 ± 13.4 mM in NPT2a+CFTR-oocytes; IPi_max was decreased by 39±7% in NPT2a+CFTR-oocytes compared to NPT2a-oocytes (P < 0.05). Inset: Effect of varying extracellular pH from 7.0 to 8.0 on the current induced by 1 mM Pi in NPT2a-oocytes (white column) and in NPT2a+CFTR-oocytes (black column). IPi was normalized against the Pi-induced current measured at pH 7.5 in NPT2a-oocytes from the same batch of oocytes. Results are shown as means± SEM, n = 5 oocytes of each type.