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

The uptake of α-aminoisobutyric (AIB) by guinea pig taenia coli was linear with time over a 5-h period. There was a saturable component with D(m) 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⁺]o) was approximately linear over the range 0-140 mM. With [Na⁺]o 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 nM) prevented the uptake of Na⁺ seen in K⁺-free medium 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 AIB in this tissue are Na⁺ dependent, and that the accumulation of AIB relies on the transmembrane Na⁺ gradient.