On the mechanism of tachyphylaxis to tyramine in the isolated rat heart.

Tyramine was shown to release [(3)H]-catecholamines from an isolated rat heart previously perfused with [(3)H]-noradrenaline. With successive injections of tyramine the amount of [(3)H]-catecholamine released fell progressively and there was a parallel decrease in the increment of amplitude and rate of contraction of the heart. Reserpinized hearts were shown to take up less [(3)H]-noradrenaline than normal hearts. Release of radioactivity and loss of responsiveness to tyramine occurred more rapidly in the reserpinized heart. In the same preparation the uptake of [(14)C]-tyramine exceeded the quantity of the noradrenaline released.