The Nature of the M Band Enzyme in Rat Ventricular Muscle

The types of cardiac cholinesterases (1) and the responses of the myocardium to acetylcholine (2, 3) differ in various species and at various sites in the heart. It has not been possible to explain the differing physiological responses in terms of the types of cholinesterases present (1-3). A comparative study of the histochemical localization at the ultrastructural level of cholinesterase activity in cardiac muscle, correlated with biochemical and physiological findings, might shed some light on the B R I E F N O T E S ~5functions of the cholinesterases of cardiac muscle. Barrnett and Palade (4) applied the thiolacetic acid histochemical method to rat ventricular muscle. Non-specific esterases (5) and specific (acetyl) cholinesterase (6) are known to hydrolyse thiolacetic acid, and the latter may be distinguished by the use of specific inhibitors. Hydrolysis of thiolacetic acid, CH~COSH, yields acetic acid and hydrogen sulfide: the latter is precipitated as lead sulfide, giving, at the sites of enzyme activity, an electron-opaque deposit. In rat ventricular muscle, Barrnett and Palade (4) found the most striking activity in the M band. This activity was reported to be inhibited partially by 10-6 M eserine and almost completely by 10-s M tetraethylpyrophosphate and by 10-4 M diisopropylfluorophosphate. They suggested that this enzyme was possibly a cholinesterase. No other site of cholinesterase activity in ventricular muscle was observed. We have not been able to confirm these observations on the effects of inhibitors using a modification of the same technique. On the basis of our findings, we conclude that the M band enzyme is not a cholinesterase, but may be rather an organophosphate-resistant non-specific esterase.