M cells and transmural heterogeneity of action potential configuration in myocytes from the left ventricular wall of the pig heart

Objective: Heterogeneity of action potential configuration in the left vent ricle (LV), and the contribution of M cells to it, has been observed in the human heart and is important for arrhythmogenesis. Whether the pig heart h as similar properties remains a controversial but important issue as the pi g heart is currently under study for use in xenotransplantation. Methods: S ingle myocytes were enzymatically isolated from the epicardium (EPI, n(cell s)=29), midmyocardium (MID, n(cells)=38), and endocardium (ENDO, n(cells)=1 3) of the free LV wall (n(pigs)=26, 14-22 weeks old, 55-80 kg), and studied at different stimulation rates during whole-cell recording (normal Tyrode' s solution, K+-aspartate-based pipette solution, 50 mu M K(5)fluo-3 as [Ca2 +], indicator, 37 degrees C). Standard six-lend ECGs were recorded from ane sthetized pigs. Results: The action potential duration (APD) was not signif icantly different at 0.25 Hz vs. 2 Hz for the majority of cells in all thre e layers. However, a subpopulation of cells behaved like M cells and had a very steep frequency response (APD(90) at 0.25 Hz 538 +/- 30 ms, vs. 337 +/ - 9 ms at 2 Hz, P<0.05, n=22). These cells were found predominantly in the MID layer (34% of cells), but also (24%) in EPI. M cells had a more pronoun ced spike-and-dome configuration, with a significantly larger phase 1 magni tude and plateau voltage. The frequency response of these parameters was di fferent from the other cell types. [Ca2+], transients tended to be larger i n M cells. For the in vivo ECG of anesthetized pigs, the QT time was close to the APD(90) of M cells, and J waves were seen in 7/12 recordings. Conclu sions: In young adult pigs, M cells can be identified by a steep frequency response of the APD and by a spike-and-dome configuration. These cells are mostly, but not exclusively, found in the midmyocardium, and could contribu te to the ECG characteristics. Their properties may however be different fr om those of other species, including humans. (C) 2000 Elsevier Science B.V. All rights reserved.