Abstract
The effects of acetylchoiine and norepinephrine at 10-8 to 10-4 M were obtained for isolated rabbit atria. The minimum negative chronotropic response occurred at 10-7 M acetylchohine whereas 10-4 M produced a maximum decrease in rate. The maximum heart rate effect of norepinephrine occurred with 10-4 M. The effects of fixed concentrations of norepinephrine on the chronotropic effect of acetyhcholine and vice versa were determined. Acetylcholine (10-7, 10-6 and 10-5 M) shifted the norepinephrine curve to the right. Atropine (10-6 M) abolished the influence of acetylcholine. Norepinephrine (10-7, 10-6 and 10-5 M) increased heart rate above control when 10-8 M acetylcholine was present. Propranolol (10-5 M) prevented the influence of norepinephrine. As the concentration of acetylcholine was increased, norepinephrine became less effective in altering acetylchohine's chronotropic response. When both neurotransmitters were present in equimolar concentrations, a pure cholinergic effect was seen. For any norepinephrine/acetylchohine ratio, the effect could not be expressed as the algebraic sum of the two separate effects. Kinetic constants were estimated for norepinephrine alone and in the presence of 10-7 and 10-6 M acetyhchohine. Vmax. values were 64.5, 65.7 and 60.9% whereas the Km values were 1.6 x 10-6, 4.1 x 10-6 and 4.5 x 10-5 M, respectively. The results of the present study suggest that during vagal and sympathetic stimulation, the chohinergic system has the greater influence on the chronotropic response of rabbit atria.
Footnotes
- Received March 30, 1971.
- Accepted September 28, 1971.
- © 1972, by The Williams & Wilkins Company
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