The role of calcium in supersensitivity to the inotropic effects of norepinephrine

Experiments using electrically stimulated rabbit left atria have demonstrated that supersensitivity to the inotropic effects of norepinephrine can be induced by either chronic reserpine pretreatment or hypothermia (lowering the temperature of the bathing medium). These two experimental conditions for inducing supersensitivity were not additive implying that they shared a common mechanism of action. Norepinephrine had no significant effect on the amplitude of a potentiated contraction of the rabbit atrium when the temperature was reduced from 37 to 30°C or following pretreatment with reserpine (30 or 37°C). Under these same conditions the ED₅₀ of norepinephrine on the normal contraction was reduced. It is concluded that both reserpine pretreatment and hypothermia induced supersensitivity to the inotropic effects of norepinephrine by enhancing the cellular store of activator calcium while not affecting the ability of norepinephrine to release activator calcium.