Role of the D-2 dopamine receptor in molecular adaptation to chronic hypoxia in PC12 cells

We have previously shown that pheochromocytoma (PC12) cells rapidly depolar ize and undergo Ca2+ influx through voltage-dependent Ca2+ channels in resp onse to moderate hypoxia and that intracellular free Ca2+ is modulated by a ctivation of dopamine D-2 receptors in this cell type. The present study sh ows that D-2 (quinpirole-mediated) inhibition of a voltage-dependent Ca2+ c urrent (I-Ca) in PC12 cells is dramatically attenuated after chronic exposu re to moderate hypoxia (24 h at 10% O-2). Pretreatment of cells with pertus sis toxin abolished D-2-mediated inhibition of I-Ca. The D-2-induced inhibi tion-of I-Ca did not depend on protein kinase A (PKA), as it persisted both in the presence of a specific PKA inhibitor (PKI) and in PKA-deficient PC1 2 cells. Prolonged exposure to hypoxia (24 h) significantly reduced the lev el of G(i/o)alpha immunoreactivity, but did not alter G beta levels. Furthe rmore, dialysis of recombinant G(o)alpha protein through the patch pipette restored the inhibitory effect of quinpirole in cells chronically exposed t o hypoxia. We conclude that the attenuation of the D-2-mediated inhibition of I-Ca by chronic hypoxia is caused by impaired receptor-G protein couplin g, due to reduced levels of G(o)alpha protein. This attenuated feedback mod ulation of I-Ca by dopamine may allow for a more sustained Ca2+ influx and enhanced cellular excitation during prolonged hypoxia.