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 α （TCα） and β （TCβ） accordingly consid
Background Few studies have explored the inward sodium current （INa） kinetics of transitional cardiomyocytes. This study aimed to explore the kinetics of transitional cardiomyocytes types α and β. Methods The whole-cell patch clamp technique was used to study the rapid INa of isolated transitional cardiomyocytes in the Koch triangle of rabbit hearts. Results Maximal amplitude and density of INa in type a and type β was （-1627±288） pA （α）, （-35.17±6.56） pA/pF （β） and （-3845±467） pA （α）, （-65.64±10.23） pA/pF （β） （P 〈0.05）. Steady state activation curves of INa, fitted to a Boltzmann distribution for both types, were sigmoid in shape. Half activation voltage and slope factors did not significantly differ between types at （-43.46±0.85） mV （α）, （-41.39±0.47） mV （β） or （9.04±0.66） mV （α）, （11.08±0.89） mV （β）. Steady state inactivation curves of INa, fitted to a Boltzmann distribution in both types were inverse ＂S＂ shape. Half inactivation voltage and slope factors were （-109.9±0.62） mV （α）, （-107.5±0.49） mV （β） and （11.78±0.36） mV （α）, （11.57±0.27） mV （β）, （P〉0.05）, but time constants of inactivation were significantly different at （1.10±0.19） mV （α） and （2.37±0.33） ms （β）, （P 〈0.05）. Time constants of recovery from inactivation of INa for both types were （122.16±27.43） mV （α） and （103.84±2.8.97） ms （β） （P 〈0.05）. Conclusions Transitional cardiomyocytes in rabbit hearts show a heterogeneous, voltage gated and time dependent fast inward sodium current. Types α and β show the features of INa similar to those in slow- and fast-response myocytes, with probably better automaticity and conductivity, respectively.
