EVIDENCE FOR THE PRESENCE OF M-CELL IN THE GUINEA-PIG VENTRICLE

M Cells in the Guinea Pig, Introduction: Recent studies have described the presence of M cells in the deep layers of the canine and human ve ntricle displaying electrophysiologic and pharmacologic features diffe rent from those of epicardial (EPI) and endocardial (ENDO) cells. The M cell is distinguished electrophysiologically by the ability of its a ction potential to prolong disproportionately to that of other myocard ial cells with slowing of the stimulation rate and pharmacologically b y its unique sensitivity to Class III antiarrhythmic agents. The prese nt study was designed to test the hypothesis that similar cells are pr esent in the guinea pig ventricle. Methods and Results: We used a derm atome to obtain thin strips of left ventricular free wall from the hea rts of guinea pigs (8 to 14 weeks old) and standard microelectrode tec hniques to record transmembrane activity. Action potential duration me asured at 90% repolarization (APD(90)) was significantly longer in mid -myocardial (MID) cells than in surface EPI or ENDO cells at ail basic cycle lengths (BCLs) tested. At a BCL of 300 msec, APD(90) was 102 +/ - 21, 136 +/- 9, and 95 +/- 15 msec in EPI, MID, and ENDO cells (mean +/- SD; n = 12). At a BCL of 5000 msec, APD(90) was 133 +/- 14, 185 +/ - 24, and 135 +/- 13 msec in EPI, MID, and ENDO cells ([K+](0) = 4 mM) . Thus, APD-rate relations were more pronounced in the MID cells. MID cells were also more sensitive to agents with Class III actions (e.g., d,l-Sotalol: 10 to 100 mu M), exhibiting a greater APD prolongation t han EPI or ENDO. d,l-Sotalol also induced early afterdepolarizations i n MID cells but not in EPI or ENDO cells. The rate of rise of the acti on potential upstroke (V-max) was significantly greater in MID cells: 129 +/- 13, 240 +/- 42, and 192 +/- 28 V/sec in EPI, MID, and ENDO cel ls (n = 10 to 18). Conclusion: Our results demonstrate the existence o f important transmural electrical heterogeneity in guinea pig ventricu lar myocardium. The study provides data in support of the existence of M cells in the mid-myocardial layers of the guinea pig ventricle exhi biting longer APDs and a greater sensitivity to agents with Class III antiarrhythmic action.