The uptake and efflux of α-aminoisobutyric acid by the smooth muscle of the guinea pig's taenia coli

The uptake of α-aminoisobutyric acid (AIB) by guinea pig taenia coli was linear with time over a 5-h period. There was a saturable component with Km of 0.7 mM and a maximal value of 4.75 mmol/kg tissue per hour. A second nonsaturable component of uptake became important at high concentrations of AIB (> 5 mM). Preincubation in K+-free or Na+-free (high-K+) media greatly reduced uptake. The dependence of AIB uptake on outside [Na+] ([Na+]0) was approximately linear over the range 0–140 mM. With [Na+]0 equal to 25 mM the uptakes with various substitutes were in the order sucrose > Mg2+ > choline+ > Li+ > Cs+ > Rb+ = K+. Addition of ouabain or removal of K+ during efflux of AIB led to marked increases in the efflux rate, ouabain being the more effective treatment. Changes in tissue Na+ and K+ levels were found to be slightly greater with ouabain than with K+-free medium. Lanthanum (5 mM) prevented the uptake of Na+ seen in K+-free medium and also abolished the increase in the rate of loss of AIB. It also reduced the rise in tissue Na+ produced by ouabain; there was a corresponding reduction in the ouabain-induced increase in the rate of loss of AIB. It is concluded that both the influx and efflux of AIB in this tissue are Na+ dependent, and that the accumulation of AIB relies on the transmembrane Na+ gradient.