Glass Microelectrode Studies on Intramural Papillary Muscle Cells

Although the electrophysiological properties of intramural ventricular myocardial cells are important to an understanding of cardiac excitation and conduction, they have not been well defined. The paucity of information stems from limitations on the depth of penetration by glass microelectrodes and from difficulties in perfusing the deep layers. Therefore, a tissue slicing technique that satisfactorily exposes all the layers of a papillary muscle specimen from the endocardium to the epicardium was developed for electrophysiological examination. Glass microelectrodes were then used to explore these slice preparations to define the electrophysiological characteristics of intramural cells in the normal dog. Transmem-brane potentials recorded from subendocardial and deep myocardial cells in the papillary muscle slices were comparable to those recorded from standard preparations. Similarities included action potential duration and configuration, relationships among resting potential, action potential configuration, and extracellular potassium concentration, and dependency of action potential duration on cycle length. However, the average magnitudes of measured electrophysiological characteristics were consistently greater in the subsurface cells tested in papillary muscle slices than they were in the surface cells tested in the standard preparations, i.e., resting potential was 2-4 my more negative, action potential amplitude was 7-9 my larger, and maximum rate of voltage change (maximum dV/dt) was 40-140 v/sec larger. Deep myocardial tissues also exhibited enhanced responsiveness (the curve relating activation potential to maximum dV/dt of the response shifted up and to the left), cell populations with large maximum dV/dt, and estimated conduction velocities in excess of three times those in the surface cell layers. These findings provide a reasonable explanation for (1) discrepancies between previously reported values for ventricular conduction velocity, (2) rapid impulse spread to the papillary muscle tip, and (3) bidirectional activation of the papillary muscle and large trabeculae. They also suggest the possibility of functional pathways that facilitate rapid activation of the deep myocardial lavers.