Choline acetyltransferase in the heart of adult rats

The distribution of choline acetyltransferase (ChAT, EC 2.3.1.6.) in the heart of adult rats has been reinvestigated in view of recent discoveries that acetylcholine (ACh) can be synthesized not only by ChAT, but also by carnitine acetyltransferase (CarAT, EC 2.3.1.7) and that it is possible to distinquish between the ACh-synthesizing activity of ChAT in intramuscular nerves and the CarAT-mediated extraneural synthesis of ACh by means of bromoacetylcholine (BrACh), a specific inhibitor of ChAT. BrACh (0.002 mmol/l) has been found to inhibit the synthesis of ACh in the atria by 66-85% and in the ventricles by only 19-29%. Bromoacetylcarnitine (BrACar, 0.02 mmol/l), and inhibitor of CarAT, inhibited the synthesis of ACh in the atria by 34% and in the ventricles by 74-80%. These findings indicate that ChAT is responsible for most of the synthesis of ACh observed in the homogenates of the atria; in the ventricles, it catalyses only a minor portion of the total ACh synthesis observed. In the investigation of the regional distribution of ChAT in the heart, the BrACh-sensitive part of ACh synthesis was taken as the measure of ChAT activity. The highest activity of ChAT (nmol ACh synthesized g-l.h-l) was found in the region of the sinoatrial node (1775); it decreased in the order: interatrial septum (781) greater than rest of the right atrium (712) greater than left atrium (416) greater than basal part of the right ventricle (366) greater than apical part of the right ventricle (250) greater than inter-ventricular septum (239) greater than basal and apical part of the left ventricle (208 and 205). The results indicate that earlier investigations of the distribution of ChAT in the heart provided a basically correct picture although the contribution of CarAT to the synthesis of ACh measured had not been excluded, and confirm that ChAT is present throughout the heart, including the apical parts of the ventricles, However, the sino-atrio-ventricular gradient of ChAT distribution is steeper when the contribution of CarAT to the synthesis of ACh is excluded.