Positive Inotropic Responses and Catecholamine Content of Iqolated Rabbit Atria Exposed to Tyramine

The noradrenaline (NA) content of isolated rabbit atria was essentially unchanged following perfusion with tyramine (100 μg/ml) for 30 min. Over the same period the positive inotropic effect of tyramine was still 75 per cent of the peak response. After prolonged exposure (2–3 hours) the NA content decreased about 25 per cent and the inotropic effect about 50 per cent. In general, there was a correlation between the reduction in the response to c and the NA content. However, the NA content was still 40–50 per cent of the control value when the tissue responded no longer to tyramine. At that time the tissue was fully responsive to administered NA. The reduction in the NA content of the tissue could not supply more than a total of 0.1 to 0.2 μgNA over 2–3 hours. This suggests a considerable synthesis of NAin vitroif the effect of tyramine is indeed mediated through NA release. Direct measurement showed that 30 min after the end of a prolonged tyramine infusion the NA tissue content was restored by an amount corresponding to about 20 per cent of the control value. This however, did not restore the response to tyramine.