Concentration-dependent adenosine uptake

An example of this analysis method is shown in Figure 14.2B for [3H]adenosine, which is taken up into TR-iBRB cells. [3H]Adenosine uptake increases linearly for at least 5 min in the presence or absence of Na+. Concentration-dependent adenosine uptake is known to take place with... 

Absorption of purines and pyrimidines by cestodes occurs by a combination of passive diffusion and mediated transport. In H. diminuta, purine and pyrimidine uptake is very complex and seems to involve at least three carrier systems (Table 6.9), two of which appear to bind several substrate molecules simultaneously (631). Pyrimidine transport was thought to involve allosteric regulation because the relation between initial uptake and substrate concentration was sigmoidal. However, more recent work (890) has indicated that the sigmoidal kinetics of pyrimidine transport in H. diminuta is an isotope effect, obtained only when 2-14C-labelled pyrimidines were used absorption kinetics of methyl-l4C- and 3H-labelled pyrimidines were hyperbolic. Nucleosides (thymidine, uridine, adenosine and guanosine) are absorbed by H. diminuta, H. citelli and H. microstoma via a specific sodium-dependent, mediated system involving at least two carriers (347). Interestingly, the mechanism displays a diurnal periodicity in H. diminuta (616, 617). 

Yessotoxin also potentiated the calcium uptake induced in lymphocytes exposed to maitotoxin, although in this case, the effect was insensitive to SKF 96365 [48]. In the presence of extracellular calcium, but not in its absence, yessotoxin decreased cellular levels of adenosine 3, 5 -cyclic monophosphate, owing to activation of phosphodiesterases [38]. hi isolated mitochondria, yessotoxin opened the permeability transition pore, a voltage-dependent calcium channel, at concentrations between 10 and 10 M. Again, this effect required the presence of calcium in the incubation medium [49]. In the presence of the chemotactic tripeptide A-formyl-Meth-Leu-Phe, yessotoxin increased the motihty of mussel immunocytes, an effect that was again inhibited by verapamil [50]. 

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