EFFECT OF FORSKOLIN AND CYCLIC-AMP ANALOG ON ADENOSINE TRANSPORT IN CULTURED CHROMAFFIN CELLS

The adenosine transport in cultured chromaffin cells was inhibited by the presence of the adenylate cyclase activator, forskolin, and a cAMP analog. The Vmax values of this transport obtained for control and in the presence of 8-(-4-chlorophenylthio)adenosine-3′:5′-monophosphate cyclic (ClPhcAMP, 100 μM) or forskolin (0.5 μM) were 85 ± 5; 45 ± 1.5 and 38 ± 3 pmol/106 cells/min, respectively. The Km values were not significantly modified. The number of adenosine transporters in cultured chromaffin cells, measured by nitrobenzylthioinosine (NBTI) binding, were decreased by the above mentioned effectors. The values of binding sites per cell were 30,000 ± 3200; 12,000 ± 1000 and 21,300 ± 2000 for control, ClPhcAMP and forskolin, respectively; without changing the dissociation constant. When the binding studies were conducted with cellular homogenates, a significant decrease in the maximal binding capacity for nitrobenzylthioinosine was obtained. The values were as follows: 0.087 ± 0.01 pmol/mg protein for control, 0.044 ± 0.02 pmol/mg protein for ClPhcAMP; and 0.032 ± 0.01 pmol/mg protein for forskolin. In this neural tissue, the adenosine transport system seems to be inhibited by stimulation of the adenylate cyclase or by the cyclic AMP analogue that enters the cells. These results suggest that this inhibition could be mediated by a molecular modification of adenosine transporters, the binding with NBTI is therefore a possible parameter of this modification. © 1990.