Abstract

The uptake and efflux of methyl-C¹⁴-labeled nicotine in frog sartorius muscle are measured at pH 8.40 and pH 7.40. At both pH levels nicotine-C¹⁴ uptake is about 90% of maximum in 10 min. The total tissue space occupied after 1-hr incubation is 2.722 ± 0.155 times the extracellular concentration of nicotine at pH 8.40 and 1.003 ± 0.089 times at pH 7.40. Lowering the extracellular pH initiates a sustained increase in the efflux rate of nicotine-C¹⁴, whereas raising the pH decreases efflux. Addition of 2.50 mM nonradioactive nicotine induces a transient increase in the rate of loss of nicotine-C¹⁴ which is larger at pH 8.40 than at pH 7.40. Nornicotine (10.0 mM) initiates a similar but much smaller transient increase in nicotine-C¹⁴ efflux and also a sustained increased efflux, whereas cotinine, a nicotine metabolite, has no effect on nicotine-C¹⁴ uptake or efflux. Intracellular pH estimations derived from C¹⁴-labeled 5,5-dimethyl-2,4-oxazolidinedione tissue spaces indicate that nicotine is distributed across the cell membrane in accordance with the pH gradient. The transient increase in nicotine release may represent a displacement of bound nicotine from sites in the cell membrane, whereas alterations in extracellular pH are rapidly followed by shifts in intracellular nicotine-C¹⁴ that appear to be due to an altered backflux of nicotine-C¹⁴ into the muscle cell.