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 ac tivity can be recorded in excised inside-out and cell-attached membrane pat ches from human atrial myocytes. In control conditions, with Ba2+ or Ca2+ a s charge carrier, single-channel activity spontaneously appeared in irregul ar bursts separated by quiescent periods of 2-17 min, in nearly 25% of test ed patches. Channel activity was recorded at steady-state applied membrane potentials including the entire range of physiological values, and displaye d no "rundown" in excised patches. During activity a variety of kinetic beh aviors could be observed with more or less complex gating patterns. This ty pe of channel activity was triggered or markedly increased when chlorpromaz ine (CPZ 20 or 50 mu M) was applied to internal face of inside-out patches, with a proportion of active patches of approximate to 25%. CPZ-activated c hannels were potential-independent in the physiological range of membrane p otential. In 96 mM Ba2+ solution, three conductance levels: 23, 42 and 85pS were routinely observed in the same excised membrane patch, sometimes comb ining 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 atri al myocytes. Application of H2O2 (from 0.1-10mM) in cell-attached mode indu ced an activation of Ba2+ permeable channel activity in a dose-dependent ma nner, with an estimated EC50 of 9.7mM. In the same type of experiments, 10m M deoxyglucose also induced similar Ba2+ permeable channel activity. When 5 00 mu M CPZ were applied to myocytes studied in the whole-cell configuratio n: and maintained at a holding potential of -80mV in the presence of 5 mM e xternal Ca2+, a noticeable inward current could be observed. The mean CPZ-a ctivated current density, determined from seven myocytes was 0.63 pA/pF. (C ) 1998 Academic Press.