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

Abstract

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.