Negative Inotropic Effect Of Acetylcholine Research Articles

Changes in membrane currents in bullfrog atrium produced by acetylcholine.

1. A double sucrose-gap voltage-clamp technique has been used to study the effects of acetylcholine on the membrane currents in atrial trabeculae of the bullfrog, Rana catesbeiana. 2. The second, or slow inward (Ca2+/Na+) current, was found to be markedly reduced by concentrations of acetylcholine greater than approximately 2-0 X 10(-8)M. The resulting decrease in net calcium entry provides a straightforward explanation for the negative inotropic action of acetylcholine in atrial muscle. 3. Measurements of membrane resistance near the resting potential showed that relatively high doses of acetylcholine (approximately 10(-7) M) decrease membrane resistance by about twofold. This effect is shown to be the result of an increase in a time-independent background current which appears to be carried mainly by potassium ions. 4. Using appropriate pharmacological techniques, it has been possible to demonstrate: (i) that the peak slow inward current is reduced to about half its initial value before any significant increase in background current occurs; (ii) that even when a sufficient dose of acetylcholine to produce an increase in background current is used, the background current shows inward-going rectification and cannot account for the observed reduction in the slow inward current. 5. No consistent change was observed in the degree of activation of the time-dependent outward membrane currents after application of concentrations of acetylcholine which produced large decreases in the peak slow inward current. 6. These results are discussed in relation to previous electro-physiological and radioisotope studies of the mechanism of the negative inotropic effect of acetylcholine in cardiac muscle.

On the mechanism of the negative inotropic effect of acetylcholine.

The negative inotropic action of ACh was investigated by voltage clamping mammalian atrial myocardium with the single sucrose gap method. Acetylcholine (ACh) affected the outward current, slow inward current and clamp tension in a concentration dependent way. 1. Concentrations of ACh which reduced action potential twitch tensions by up to 30 or 40% (ED-30-ED-40) increased steady state outward currents but had no effect on the time dependent outward current, the slow inward current or voltage clamp tension. This indicates that in this dose range the negative inotropy during normal activity can be completely explained by an "indirect" effect on the slow inward current, i.e. increased outward current shortens the action potential and prevents the slow inward current from running its normal time course. 2. Higher concentration of ACh (ED-70-ED-90) greatly increased the steady state outward currents and abolished anomalous rectification without affecting delayed rectification. The slow inward current and voltage clamp tension were reduced indicating that in this concentration range a 'direct' effect of ACh on the slow inward current and tension may be expected to add to the 'indirect' effect mentioned above.

The inhibition of frog tissue cholinesterases and the influence of eserine and prostigmine on the action of acetylcholine on the frog heart

The activity and type of cholinesterases in various tissues of the frog ( Rana esculenta) and possible seasonal differences in the activity were investigated. The activity is highest in the sympathetic chain, the brain and spinal ganglia, somewhat lower in Bidder's ganglia of the heart, and low in cardiac muscle, skeletal muscle, blood and liver. Cholinesterases in frog tissues are acetylcholinesterases; their sensitivity to eserine is lower than that of mammalian cholinesterases, which might be responsible for the failure of eserine to increase the negative inotropic effect of acetylcholine in the frog heart.

Modification by ouabain of the electrical and mechanical effects of acetylcholine in isolated rabbit atria.

1. Left atrial preparations isolated from rabbits were stimulated electrically at frequencies between 6 and 240/min. Tension-frequency curves were obtained from control preparations and preparations treated with ouabain and acetylcholine. Transmembrane potentials were recorded from single cells of the left atrium stimulated at different frequencies.2. The tension-frequency curve was moved downwards by acetylcholine (10(-6) g/ml). Ouabain (10(-6) g/ml) caused characteristic alterations in the tension-frequency relationship, enhancing the contractile tension at low but not high frequencies. The negative inotropic effect of acetylcholine was reduced by treatment with ouabain.3. Action potential durations were significantly influenced by alterations in frequency of contraction. The 10% duration increased with frequency within the range between 6 and 60/min but decreased at frequencies higher than 120/min. The 50% duration increased with frequency between 6 and 120/min but decreased at frequencies higher than 180/min. The dependence of the 50% duration upon frequency paralleled that of contractile tension. The 90% duration, the overshoot and the resting potential were not affected by frequency of contraction.4. Acetylcholine (10(-6) g/ml) shifted the 10%, 50% and 90% duration-frequency curves downwards, but did not significantly alter the overshoot and the resting potential. Ouabain (10(-6) g/ml) shifted the duration-frequency curves downwards and also reduced the size of the overshoot and the resting potential. Treatment of atrial preparations with 10(-6) g/ml ouabain potentiated the membrane effects of acetylcholine.5. The inhibition by ouabain of the negative inotropic effect of acetylcholine did not appear to be due to antagonism at the receptor level, but to interference with the mechanisms responsible for the mechanical events.

�ber die Verteilung von Diisopropyl-phosphorofluoridat (DFP) zwischen isoliertem Vorhofgewebe und zirkulierendem oxygenierten Vollblut von Meerschweinchen

The distribution of DFP has been investigated by several authors in intact animals and human beings. Since investigations of this kind depend on a rather large number of variables, in the present paper the distribution of3H-labelled DFP was studied in a simplified model using isolated guinea-pig atria incubated in oxygenated whole blood. The degree of cholinesterase-inhibition and the corresponding negative inotropic effect of acetylcholine has been simultaneously determined. 1. Taking the binding of DFP by the plasma proteins into account, the uptake of DFP by atrial tissue and erythrocytes was still rather low, since the tissue-medium ratio did not exceed 0.5. In addition to the uptake by erythrocytes there is an appreciable but easily reversible binding to the membrane. 2. The uptake processes approached equilibrium values after about 4 hours of incubation and revealed two exponential functions with different half life times. 3. The inhibition of the acetylcholinesterase of the red cells was proportional to the amount of DFP taken up. At the lowest DFP concentration (1.1×10−6M) the more sensitive butyrylcholinesterase of plasma and atrial tissue was already completely inhibited. 4. In the presence of DFP the acetylcholine-sensitivity of the electrically driven atria was increased as indicated by a parallel shift of the dose-response curves towards lower concentrations of ACh.