Background: The morphological and electrophysiological characteristics of cardiac cells in Koch triangle are still disputed. We studied the appearance and electrical properties of these diverse myocytes to elucidate their complex electrophysiological phenomena.
Methods: Experiments were conducted using cooled charge coupling device (CCD) system and whole cell, patch clamp technique to determine the morphology, action potential and sodium current density of single viable myocytes enzymatically isolated from the Koch triangle of rabbit hearts.
Results: Morphologically, cardiac cells in shape of spider, tiny spindle, slender spindle, rod and strip were observed in percentage of 3.0 +/- 0.3, 35.0 +/- 5.0, 15.0 +/- 2.0, 40.0 +/- 5.0 and 6.0 +/- 0.7 respectively. The cellular dimensions and capacitance gradually increased in the above order (all P < 0.05). Electrophysiologically, action potential configurations recorded from them were similar respectively to nodal (N), atrial nodal (AN), nodal Hisian (NH), atrial (A) and Hisian like potentials obtained from the intact atrioventricular nodal preparations. Diastolic depolarization appeared in all myocytes except for rod cells. Sodium current density increased in the order of tiny spindle, strip, rod, slender spindle cell (all P < 0.05), but could not be detected in spider-shaped cells. Linear regression analysis revealed that membrane capacitance was correlated negatively to the rate of diastolic depolarization r = -0.70, P < 0.001, but positively to maximum depolarization potential, amplitude of action potential, upstroke velocity and maximum peak value of sodium current density r = 0.84, 0.80, 0.87 and 0.75, respectively; all P < 0.001.
Conclusions: The results demonstrated that spider-shaped, spindle, rod and strip cells in Koch triangle might correspond to pacemaking, transitional, atrial and Purkinje like cells, respectively. Furthermore, tiny spindle and slender spindle cells were referred to transitional cell alpha (TCalpha) and beta (TCbeta) accordingly considering their distinctive electrical properties. Different myocytes with diverse electrical properties constituted the infrastructure of sophisticated electrophysiological phenomena in Koch triangle. In view of the prominent percentage and electrical properties, tiny spindle and slender spindle cells were presumed to play important roles.