1. The effect of
adrenaline on
contracture and twitch tension in frog's ventricle has been examined, using the superfused preparation.2. In 1 mM-Ca Ringer,
contractures induced with excess KCl concentrations from 50 to 200 mM, are reduced by 1 x 10(-6) g/ml.
adrenaline to an average of 0.62 of control values, in marked contrast to the well known positive inotropic effect of
adrenaline on the heart twitch. This effect of
adrenaline is directly dose dependent. Increasing [Ca](o) diminishes the effect of
adrenaline on
contracture tension, and on the twitch tension.3.
Adrenaline has a significantly greater effect on the KCl
contracture tension than
noradrenaline or
isoprenaline.4. In 1 mM-Ca Ringer, Na-free
contractures are reduced to 0.72 of controls by 1 x 10(-6) g/ml.
adrenaline.
Adrenaline also significantly reduces tension in
contractures induced by 50 c/s alternating current.5. The action of
adrenaline on
contracture tension is largely complete in 1-2 min at various rates of stimulation and
calcium concentrations. A similar time course has been found for the effect of
adrenaline on membrane potential.6.
Pronethalol blocks the action of
adrenaline on both twitch and
contracture. The action on the
contracture can also be blocked by
ouabain (1 x 10(-5)M), and exposure of the tissue to K-free or Na-free
Ringer solution.7.
Adrenaline hyperpolarizes the membrane potential with a range of [K](o) from 0 to 200 mM. This effect is blocked by
pronethalol and
ouabain. After exposure to
ouabain,
adrenaline causes a significant decrease in the membrane potential. This may be due to an increase in the
sodium permeability.8. At low values of the [Ca]/[Na](2) ratio,
adrenaline takes a relatively constant number of beats for full action, but at high values of the ratio the development of full effect is largely time dependent.9. The time course of the effect on the twitch of changing from 0.5 to 2 mM-Ca Ringer has been studied at various rates of stimulation. The equilibration time has been found to depend on the heart rate.10. The effect on the
contracture suggests that
adrenaline decreases the
calcium permeability. It is further suggested that the development of twitch tension is not due to direct Ca entry but is due to the release of Ca from a local store within or between the cells. The inotropic action of
adrenaline is explained in terms of this store.