LONG-TERM REGULATION OF NUCLEOSIDE TRANSPORT BY THYROID-HORMONE (T3) IN CULTURED CHROMAFFIN CELLS

The adenosine transport in cultured chromaffin cells was increased by the presence of triiodo-L-thyronine (T3) throughout the prolonged period studied. The V(max) values of this transport obtained in absence and presence of 1 muM T3 were 36.21 +/- 2.1 and 44.17 +/- 3.5 (means +/- SD) pmol/10(6) cells/min respectively for 26 hours incubation-time with the hormone. The K(m) values were not significantly modified. The number of adenosine transporters in cultured chromaffin cells, measured by [H-3]nitrobenzylthioinosine (NBTI) binding, was increased by 1 muM T3 for 26 hours incubation-time. The values of binding sites per cell were 33,500 +/- 3,000 and 40,153 +/- 3,700 in absence and presence of T3 respectively, without changing the K(d) constant. When the transport studies were carried out in presence of cycloheximide, an inhibitor of protein synthesis, the adenosine transport capacity decreased with a half-life values of 23.9 +/- 2.8 and 24.3 +/- 2.1 hours both in the presence or absence of T3 respectively. When cells were incubated in the presence of both T3 and cycloheximide, not only the activatory effect of T3 was completely abolished but also adenosine transport was decreased to the same extent as with cycloheximide alone. These results indicated that T3 activation of adenosine transport in chromaffin cells required the protein-synthesizing mechanism.