Canine Mitral Complex: ULTRASTRUCTURE AND ELECTROMECHANICAL PROPERTIES

Abstract

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 10~7M 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.KEY WORDS acetylcholine mitral valve leaflet action potential comparative pathology papillary muscle length-force relation electrophysiology chordae tendineae left atrial muscle mitral excitation• The anatomical structures regulating the flow of blood at the mitral orifice have been termed the mitral complex (1). This is composed of the mitral annulus, valve leaflets, chordae tendineae, and papillary muscles. Mitral valvular fibrosis in man and dog is well known clinically and pathologically. In the dog, the abnormal physiological function resulting in mitral insufficiency may be related to structural alterations in one or more regions

Active stiffening of mitral valve leaflets in the beating heart

“…3, A and D, this muscle is present at this location in the ovine anterior leaflet. The ultrastructure of this cardiac muscle is similar to that of atrial muscle (12), and, similar to LA muscle, such cells contract with electrical stim- …”

“…MV leaflets have extensive passive collagen fiber networks (5,11,12,15,18), and the stress-strain relationship of isolated leaflets (3,14,22,27) is consistent with the material properties of passive collagen fibers. Our recent study (21), however, suggested that leaflet stiffness in the beating heart (slope of the leaflet stress-strain curve) was much greater than that of passive tissue, and studies of freshly isolated, oxygenated, stimulated leaflets in myographs (39) and in open hearts on cardiopulmonary bypass (CBP) (7,32) have suggested that the mechanical properties of MV leaflets may be altered actively.…”

“…The atrialis layer of the anterior MV leaflet contains cardiac muscle cells and well-defined cell bundles, several myofibrils thick, arranged end-to-end and side-to-side, in continuity with LA muscle, surrounded by connective tissue, accompanied by blood vessels and efferent nerves, and tapering in thickness, number, and orientational regularity from annular base to central leaflet, with only slips of muscle near the occlusive margin (2,6,11,12,18,30,39,47,48). As shown in Fig.…”

“…Ecirc ( ulation (12,39) via propagated depolarization [35-ms stimulus latency, 210-ms refractory period, 6% shortening (39)] and exhibit a positive inotropic response to norepinephrine and tyramine and a negative inotropic response to stimulus frequency (25,39) and acetylcholine (reversible with atropine) (12,39).…”

“…Two important hypotheses have been advanced regarding this activity (46). 1) Contractile element force development within the leaflets, in response to leaflet excitation (7,12,32,33,48), provides modulation of leaflet tone to adapt to changes in left ventricular (LV) pressure (LVP) load on the closed valve (33,49). A key prediction of this hypothesis is that excitation can vary leaflet stiffness in the closed valve.…”