Adenosine uptake and [³H]nitrobenzylthioinosine binding in developing rat brain

The ontogenesis of adenosine tranport sites as labelled with [³H]nitrobenzylthioinosine ([³H]NBI) was examined using radioligand binding and membrane preparations from whole brain and 4 brain regions of rats between the postnatal ages of one day through to adulthood. In whole brain, cerebral cortex and cerebellum, [³H]NBI binding was two-fold higher in 6-day-old than in 50-day-old rats. In contrast, [³H]NBI binding was higher in adults than in one-day-old rats by 4-fold in hypothalamus and 8-fold in superior colliculus. In cortex and hypothalamus, the levels of [³H]NBI binding in newborn and adult rats were reflected by changes in B_max and not K_d values. As a measure of the utility of [³H]NBI as a probe for identifying functional adenosine transport sites, we examined [³H]NBI binding to and [³Hadenosine accumulation by intact brain cells prepared from adult and newborn rats. For [³H]NBI binding to brain cells from adult rats, the values of K_nd were 0.092 nM and of K_max were 274 fmol/mg protein. For newborns, slightly higher K_d and B_max values were observed: 0.2 nM and 395 fmol/mg protein, respectively. [³H]Adenosine accumulation was higher in brain cells from one-day-old from adult rat brains. Kinetically this uptake was best described by a two-component model: the V_max values for the high low-affinity uptake, and the K_m value for the high-affinity component in one-day-old rats were greater than in adults. These results are consistnt with the increased demand for purines in rapidly dividing cells and the concept that [³H]NBI, at least in some tissues and for some purposes,is a useful marker substance for adenosine uptake sites.