Abstract

The results reported have shown that mass fiagmentography allows measurement of the DM pool in noradrenergic axons. By combining mass fragmentography and radiochemistry, it is possible to measure turnover rate of NE in heart ventricles from the precursor product relationship of DM and NE stored in the noradrenergic nerves. The turnover rate value obtained is compatible with those obtained by other approaches. Since the DM pool is the obligatory precursor for the NE, these data fail to give support to the view that there are several pools of NE in noradrenergic nerves which can be distinguished by virtue of their kinetic parameters.

A compartmentation can be distinguished concerning tyrosine in rat heart ventricles. From the relationship between tyrosine and DM we have calculated the size of the kinetic pool of tyrosine which serves as a precursor for DM and found that it is 12% of the total tyrosine in rat heart ventricles. Moreover, the specific radioactivity of this pool could be calculated to be 18-fold that of the rest of the tyrosine pool. These calculations of the tyrosine compartmentation rest on a number of assumptions which include the postulate that the DOPA pool is very small and that the transformation from DOPA to DM is practially instantaneous. The conversion by a soluble ensyme of DOPA into DM occurring in the cytoplasm supports this possibility.

The formation of DM, from a tyrosine pool, kept in a compartment which we have defined only kinetically yields a k₁''' (model b) of 2.8 hr^-1 indicating that 0.78 nanomoles/g/hr of DM turnover in 1 hr. By measuring the precursor product relationship between NE and DM we have obtained a turnover of NE of 0.69 nanomoles/g/hr. It is concluded that this value represents a reliable approximation of the absolute rate of NE synthesis in heart ventricles of rats