Unitary Current Analysis of L-type Ca2+ Channels in Human Fetal Ventricular Myocytes

INTRODUCTION: L-type calcium channels were studied in cell-attached patches from ventricular cell membranes of human fetal heart. METHODS AND RESULTS: Experiments were performed in the presence of 70 mM Ba2+ as the charge carrier at 22 degrees C to 24 degrees C. Unitary current sweeps were evoked by 300-msec depolarizing pulses to 0 mV from a holding potential of -50 mV at 0.5 Hz. Recorded currents were blocked by nisoldipine (1 microM) and stimulated by (-)Bay K 8644 (1 microM). During control, channel activity was seen in 13.9%+/-4.2% of the total 200 sweeps. Ensemble average current amplitude was 0.03+/-0.01 pA (n = 6) and average conductance was 20.4+/-0.2 pS (n = 5). Analysis of single channel kinetics showed open time and closed time histograms were best fit by one and two exponentials, respectively. Mean open time was tau(o) = 0.99+/-0.05 msec (n = 6). Mean closed time fast (tau(cf)) and slow (tau(cs)) component values were tau(cf) = 0.85+/-0.09 msec and tau(cs) = 8.0+/-0.94 msec (n = 6), respectively. With intrapipette (-)Bay K 8644 (1 microM), mean open time was best fit by two exponentials, tau(of) = 0.9+/-0.2 msec (n = 10) and tau(os) = 13.4+/-2.6 msec (n = 10); mean close time values were tau(cf) = 0.6+/-0.1 msec (n = 10) and tau(cs) = 9.8+/-1.9 msec (n = 10), respectively. With (-)Bay K 8644, channel activity was 66.5%+/-7.4%, the ensemble average current was 0.52+/-0.04 pA (n = 10) and the conductance 20.7+/-0.5 pS (n = 5). CONCLUSION: (1) the data establishes the characteristics of L-type Ca channels of human fetal hearts and their modulation by dihydropyridines; (2) the open time kinetics differ from those of avian embryonic and rat fetal hearts; and (3) the findings provide new and relevant information for understanding the physiologic behavior of unitary Ca2+ channels in the developing human heart and the baseline comparison for diseases that implicate Ca2+ channels in their etiology, such as autoimmune-associated congenital heart block.