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Given an abstract of a paper, generate a title for this paper such that conveys the key focus of the paper.

Abstract: Proximal straight tubules (PST) has been shown to be an important nephron segment of renal ammonia production. To clarify the nature of glutamine (substrate of ammonia production) transport in PST, both luminal resorption and peritubular uptake of glutamine were measured in isolated rabbit PST. Luminal glutamine resorption (Jgln) was measured at various perfusate glutamine concentration (0.05 to 20 mM) at 38 degrees C and 12 degrees C. Jgln measured at 12 degrees C were proportional to mean luminal glutamine concentrations. This flux was thought to be a passive glutamine flux. The flux, which was obtained by subtracting passive glutamine flux from Jgln obtained at 38 degrees C, was thought to be active luminal resorption. This flux exhibited saturation kinetics (Vmax 20.9 pmol min-1 X mm-1, km 5.2 mM). When bath pH (HCO3) was lowered from 7.4 to 6.8, Jgln showed no change or a small decrease (12%) at perfusate glutamine concentrations of 0.05 or 5 mM, respectively. When perfusate pH (HCO3) was lowered from 7.4 to 6.8 Jgln showed a small decrease (10%) at 5 mM perfusate glutamine concentration. Peritubular glutamine uptake was determined in isolated nonperfused PST incubated for 5 to 50 min in [14C]-glutamine containing solution. When incubated in pH 7.4 HCO3 buffer solution, cell to medium 14C concentration ratio was higher than unity (3.83 +/- 0.34, P less than 0.001) at 5 min, and reached a maximum (11.37 +/- 1.13) at 30 min.(ABSTRACT TRUNCATED AT 250 WORDS)
Output:
Glutamine transport in the rabbit proximal straight tubule: effect of acute acid pH.