Canine Mitral Complex: ULTRASTRUCTURE AND ELECTROMECHANICAL PROPERTIES

The mitral complex is a functional entity composed of the annulus, valve leaflets, chordae, and papillary muscles. The mechanical properties of the complex are dependent on the unique structural relations of the collagen in the leaflets and chordae. In the chordae the collagen is arranged in avascular columns. These columns interdigitate between muscle fibers in the papillary muscles, and the collagen is anchored to the myofiber membrane by microfibrils. In the leaflet the chordae are continuous with the dense fibrous tissue, forming a sheet of collagen which merges with the annulus. Within the leaflet there are cardiac muscle fibers in direct continuity with left atrial muscle. Contraction of isolated valve preparations can be initiated by electrical stimulation and is preceded by a propagated depolarization. Action potentials from cells in the middle third of the leaflet have a slow upstroke velocity, prominent plateau, and a characteristic positive afterpotential. Valve muscle electromechanical properties are markedly altered by 1 x 10 -7 M acetylcholine; this concentration has little effect on working left atrial muscle. In preparations containing portions of the left atrium and valve leaflet, the excitation wave spreads into the leaflet after electrical stimulation of the atrial muscle. This suggests that the accompanying contractile event may occur in situ before the initiation of systole.