Effect of uranyl ions on isometric contractions in rabbit atria

The effects of uranyl ions on isometric contractions of atrial strips of rabbit heart were studied. It was observed that uranyl ions, in a concentration of 1.1 X 10(-5) M, do not alter isometric contraction; 2.2 X 10(-5) M concentration produced a small positive inotropic effect that was magnified at a 4.5 X 10(-5) M concentration; concentrations greater than 1.8 X 10(-5) M produced a negative inotropic effect. There is evidence that the positive inotropic effect is mediated by catecholamine release, in a similar manner to the action of uranyl ions on intestinal smooth muscle preparations. This response was blocked by propranolol and also after tyramine treatment of the preparation. The negative inotropic effect seems to be mediated by a blocking effect on calcium entry through the sarcolemma during activation. Indirect evidence supporting this hypothesis was the observation of a decrease of action of uranyl ions in the presence of isoproterenol and of increased extracellular calcium concentration.