Factors affecting action of acetylcholine on transmembrane flux of K in isolated rabbit atria

¹ Department of Pharmacology, Vanderbilt Medical School, Nashville, Tennessee

The effects of temperature, cocaine, anoxia, acetylcholine (ACh), K, Na and Ca on influx and efflux of K in isolated rabbit atria have been studied. Increasing the K concentration in the medium (K₀) increases both efflux and influx of K; having a much greater effect on the latter. Both fluxes were estimated to be equal at a K₀ of 6.0–6.5 mm. ACh increases both influx and efflux. An increase of K₀ hardly modifies the effects of ACh in efflux, but greatly enhances its effect on influx. In the presence of ACh both fluxes were estimated to be equal at a K₀ of 4.0–4.5 mm. Studies on the influence of temperature, cocaine and anoxia suggest that at K₀'s less than 5.4 the two fluxes are passive in nature. At K₀'s above this, influx appears to have an active component. Evidence is presented which suggest that at low K₀'s ACh affects only passive movements of K, whereas at higher K₀'s the agent stimulates active transport. A decrease of Na in the medium to one-fifth depresses efflux and influx about 50% when the Ca concentration is maintained at a normal level. A decrease of Ca blocks the effect of low Na. These findings are interpreted to mean that Ca either increases passive movements of K or determines the degree of coupling between Na and K fluxes.