Reentrant tachyarrhythmias in right atria of cardiomyopathic versus healthy Syrian hamster

We studied the role of acetylcholine (ACh) and calcium overload in the indu ction of atrial flutter or atrial fibrillation (AF) in right atria from 34 normal male Syrian hamsters (F1B) and 33 cardiomyopathic Syrian hamsters (B IO 14.6) associated with focal myocardial necrosis. Action potential (AP) w as recorded with conventional microelectrode techniques and twitch force by a transducer. ACh (0.1, 1 and 10 mu M) induced high-frequency AF (around 3 3 Hz) along with tension oscillations and contracture in 7 of 12 normal ham ster atria, These effects of ACh were abolished by tetrodotoxin or quinidin e as well as by atropine, In contrast, ACh induced AF only in 1 of 12 myopa thic atria, In both normal and myopathic atria, ACh induced similar changes in AP duration, spontaneous rate and force. The effects of calcium overloa d were tested by means of a high [Ca2+](o) (8.1 mM) low [K+](o) (1 mM) solu tion in another series of experiments. This solution also induced incidence of AF higher in normal (10/12) than in myopathic atria (4/12). The calcium load was also increased by high-frequency pacing (32 Hz for 3 or 30 s): AF occurred in normal atria (5/8), but not in myopathic atria (0/8). Measurem ent of the refractory period revealed a longer refractory period in myopath ic than in control atria. We concluded that the lower incidence of AF in my opathic atria was probably due to their longer refractory period and the as sociated focal myocardial necrosis which then hindered the establishment of such a reentrant rhythm.