Transport of purine nucleotides and nucleosides by in vitro rabbit ileum

The kinetics of [14C]ATP and [14C]adenosine entry into rabbit ileum were measured using 1-min mucosal exposures. ATP influx was the sum of a linear (P = 1.02 X 10(-3) cm/min) and nonlinear component (Jm = 4.07 nmol/min cm2, Km = 0.31 mM). Adenosine yielded quantitatively similar results. The kinetics of [gamma-32P]ATP entry were only linear (P = 0.55 X 10(-3) cm/min). These data indicate that at least 85% of the terminal phosphate of ATP is hydrolyzed during transport. Inhibitory measurements indicate that ring-labeled ATP and adenosine compete for a common entry path; however, adenosine does not inhibit the saturable component of [14C]ATP to the degree expected of a simple carrier-mediated system. Adenosine (5 mM) produced no inhibition of [gamma-32P]ATP (0.1-1.0 mM) entry. Measurement of hydrolysis during incubation indicates a negligible fraction of uptake results from labeled adenosine released into the bulk solution. Hydrolysis of ATP followed by a preferred uptake of the adenosine released at the membrane surface probably accounts for most of the observed transport. [14C]ATP entry depended neither on [Na+], [Mg2+], nor [Ca2+] of the bulk solution. Structural requirements for the saturable pathway were also investigated. Autoradiography of [3H]ATP confirmed that the labeled material entered the epithelial cells and that uptake was nonuniform among cells.