Transitional Cardiac Cells of the Conductive System of the Dog Heart

Abstract

Cardiac cells with distinctive electrophysiological and morphological features were found at the junctional region between Purkinje and ventricular cells of the dog heart . The electrophysiological exploration of these "transitional" cells revealed action potentials markedly different in configuration from those generated by Purkinje or by ventricular cells . The impaled cardiac cells which generated transitional action potentials were identified in serial sections and studied with the light and the electron microscopes . The transitional cells were found to be characterized cytologically by : (a) their subendocardial location, (b) their small diameter, (c) the absence of T system and sarcoplasmic reticulum, and (d) the lack of intercalated discs under the light microscope and the sparsity of specialized intercellular junctions under the electron microscope . Purkinje, transitional, and ventricular cells were found to be joined by gap junctions permeable to lanthanum . A quantitative difference in the extent and distribution of specialized intercellular junctions may be one of the factors responsible for the slow velocity of conduction characteristic of the Purkinje-ventricular junctional region .Electrophysiological studies on the specialized conductive system of the mammalian heart have demonstrated the presence of regions in which the impulses propagate under critical conditions, as evidenced by the conduction delays which occur under normal circumstances and the blockages of propagation induced by various experimental conditions (1-3) . These regions are located at the junctions between different myocardial tissues, e .g ., auricle and atrioventricular (A . V .) node ; A. V . node and bundle of His ; and at the union between Purkinje fibers and ventricular cells (P-V junction) . Intracellular recordings from the junctional regions have shown cells which generate "transitional" action potentials distinctive from those of neighboring fibers. These cells are likely to be responsible . for some physiological properties of the junctional areas (3, 4, 7) .Morphological examination of the conductive system of the heart has suggested that terminal Purkinje fibers are followed by a transitional cell type which makes contact with the adjacent ventricular fibers (35) . The identification of transitional cells is not readily achieved by means of light microscopy alone, since the morphological differences reported with this technique are based on minor variations of shape and cell diameter . The present study was undertaken with the aim of identifying at the ultrastructural level the transitional cells located at the P-V junctional region. The fine structural features of those cardiac cells which generate a transitional type of action potential were found to be distinctive from those of adjacent Purkinje and ventricular cells, particularly in regard to the extent and distribution of specialized intercellular junctions .

Impulse Propagation Through the Cardiac Junctional Regions of the Axolotl and the Turtle

“…The present results give additional support to the interpretations advanced by several authors (HOFFMAN and CRANEFIELD, 1960;SANO et al, 1964;MATSUDA et al, 1967;BENITEZ, 1967, 1970;MARTINEZ-PALOMO et al, 1970) in regard of the mechanisms responsible for the delays that appear in certain regions of the mammalian heart and in the cold blooded species (SHINOZAKI, 1942;INOUE, 1959 ;KANNO, 1963 ;TSUJI, 1963 ;IRISAWA et al, 1965). We have shown that in the axolotl and turtle hearts there are limited regions in which the propagation of impulses is delayed.…”

“…junctional region differ in fine structural features from adjacent fibers, particularly as to diameter, the quantity of myofibrils and the distribution and number of specialized intercellular junctions. Moreover, in an earlier paper (MARTINEZ-PALOMO et al, 1970) it was reported that in the dog heart there are cells located between the Purkinje and ventricular fibers which generate transitional potentials markedly different in configuration from those originated in the Purkinje or ventricular cells. These junctional cells have some distinguishing cytological characteristics that, according to the electrical theory, may determine the slow rate of conduction.…”

“…In previous papers BENITEZ, 1964a, b, 1970;BENITEZ and ALANIS, 1970 ;MARTINEZ-PALOMO et al, 1970) we have analyzed some aspects of the propagation of impulses through different cardiac tissues of the mammalian heart. Among other factors, the complex histological features of the regions in which propagation becomes critical make it difficult to carry out a detailed experimental analysis of the mechanisms involved in the propagation of impulses.…”

Selective Deposition of Lanthanum in Mammalian Cardiac Cell Membranes

“…The localization of lanthanum in the outer leaflets of plasma membranes of frog nerve fibers was first reported by Doggenweiler and Frenk (4) . In addition, lanthanum used as colloidal suspensions (19) or in ionic form (7,15,16) acts as an electron-opaque tracer of the extracellular space . Revel and Karnovsky (19) used colloidal lanthanum to identify a previously undetected 20 A intercellular space at the gap junctions of mouse heart and liver ; we later obtained similar findings in a variety of epithelial and myocardial tissues (13,15,16) .…”

“…In addition, lanthanum used as colloidal suspensions (19) or in ionic form (7,15,16) acts as an electron-opaque tracer of the extracellular space . Revel and Karnovsky (19) used colloidal lanthanum to identify a previously undetected 20 A intercellular space at the gap junctions of mouse heart and liver ; we later obtained similar findings in a variety of epithelial and myocardial tissues (13,15,16) . Colloidal lanthanum has also been used to stain certain components of the surface of embryonic (9,10,14,21) and differentiated cells (22) .…”