Phosphate transport inhibition by KW-3902, an adenosine A 1 receptor antagonist, is mediated by cyclic adenosine monophosphate

Abstract

We have previously demonstrated that 1,3-dipropyl-8-(3-noradamantyl) xanthine (KW-3902) has an inhibitory effect on phosphate (Pi) transport with no effect on glucose transport in the rat renal proximal tubular cell, similar to that of parathyroid hormone (PTH). In the current studies we investigated the effect of KW-3902, rat PTH (1–34), and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), another selective adenosine A 1 receptor antagonist, on Pi transport and the production of cyclic adenosine monophosphate (cAMP). We then compared these effects of KW-3902 with those of rat PTH in rat renal proximal tubule cells. The results showed that both KW-3902 (30 μmol/L) and rat PTH (1–34, 5 μmol/L) significantly inhibited Pi uptake in proximal cells from a control level of 61 ± 3 to 19 ± 3 (a reduction of 69%) and 46 ± 4 picomoles phosphate/mg protein/min (a reduction of 25%), respectively ( P < 0.01). The inhibitory effect of 30 μmol/L KW-3902 alone on Pi transport was more than twice that of 5 μmol/L rat PTH (1–34) alone ( P < 0.01). KW-3902 stimulated the production of cAMP in a dose-dependent manner ( r = 0.997, P < 0.01). Rat PTH (1–34; 5 μmol/L) also stimulated cAMP production, which was greater than that induced by 30 μmol/L KW-3902 alone. A significant increase in cAMP production by 30 μmol/L DPCPX was also observed. When 30 μmol/L KW-3902 and 5 μmol/L rat PTH (1–34) were added to the medium together, their combined effect on Pi transport and cAMP production was additive and significantly greater than that of either agent alone ( P < 0.01). DPCPX (30 μmol/L) also significantly inhibited Pi uptake by proximal cells. However, when 30 μmol/L KW-3902 and 30 μmol/L DPCPX were combined, no additional inhibitory effect on Pi transport was observed. In additional experiments, the addition of adenosine deaminase (10 units/mL) to the transport medium, resulting in the inhibition of adenosine binding to its receptors, also decreased phosphate uptake (by 42.6%, P < 0.01) in renal proximal cells. These data suggest that the inhibitory effect of both KW-3902 and PTH on Pi transport is mediated through the stimulation of cAMP. The effect of KW-3902 and PTH on cAMP production was additive, suggesting that their effects on Pi transport are mediated by different mechanisms.