B-type Ca2+ channels activated by chlorpromazine and free radicals in membrane of human atrial myocytes

The present study demonstrates that background or B-type calcium channel activity can be recorded in excised inside-out and cell-attached membrane patches from human atrial myocytes. In control conditions, with Ba2+ or Ca2+ as charge carrier, single-channel activity spontaneously appeared in irregular bursts separated by quiescent periods of 2-17 min, in nearly 25% of tested patches. Channel activity was recorded at steady-state applied membrane potentials including the entire range of physiological values, and displayed no "rundown" in excised patches. During activity, a variety of kinetic behaviors could be observed with more or less complex gating patterns. This type of channel activity was triggered or markedly increased when chlorpromazine (CPZ 20 or 50 microM) was applied to internal face of inside-out patches, with a proportion of active patches of approximately 25%. CPZ-activated channels were potential-independent in the physiological range of membrane potential. In 96 mM Ba2+ solution, three conductance levels: 23, 42 and 85 pS were routinely observed in the same excised membrane patch, sometimes combining to give a larger level. As previously observed by Wang et al. (1995) in membrane of rat ventricular myocytes, increasing free-radicals level and metabolic poisoning readily enhanced B-type channel activity in human atrial myocytes. Application of H2O2 (from 0.1-10 mM) in cell-attached mode induced an activation of Ba2+ permeable channel activity in a dose-dependent manner, with an estimated EC50 of 9.7 mM. In the same type of experiments, 10 mM deoxyglucose also induced similar Ba2+ permeable channel activity. When 500 microM CPZ were applied to myocytes studied in the whole-cell configuration and maintained at a holding potential of -80 mV in the presence of 5 mM external Ca2+, a noticeable inward current could be observed. The mean CPZ-activated current density determined from seven myocytes was 0.63 pA/pF.