Kevin Lee

Hydrogen Peroxide Induces Intracellular Calcium Overload by Activation of a Non-selective Cation Channel in an Insulin-secreting Cell Line

Fura-2 fluorescence was used to investigate the effects of H2O2 on [Ca2+] i in the insulin-secreting cell line CRI-G1. H2O2 (1–10 mm) caused a biphasic increase in free [Ca2+] i , an initial rise observed within 3 min and a second, much larger rise following a 30-min exposure. Extracellular calcium removal blocked the late, but not the initial, rise in [Ca2+] i . Thapsigargin did not affect either response to H2O2, but activated capacitive calcium entry, an action abolished by 10 μm La3+. Simultaneous recordings of membrane potential and [Ca2+] i demonstrated the same biphasic [Ca2+] i response to H2O2 and showed that the late increase in [Ca2+] i coincided temporally with cell membrane potential collapse. Buffering Ca2+ i to low nanomolar levels prevented both phases of increased [Ca2+] i and the H2O2-induced depolarization. The H2O2-induced late rise in [Ca2+] i was prevented by extracellular application of 100 μm La3+. La3+(100 μm) inhibited the H2O2-induced cation current and NAD-activated cation (NSNAD) channel activity in these cells. H2O2 increased the NAD/NADH ratio in intact CRI-G1 cells, consistent with increased cellular [NAD]. These data suggest that H2O2 increases [NAD], which, coupled with increased [Ca2+] i , activates NSNAD channels, causing unregulated Ca2+ entry and consequent cell death.

The effects of neuroleptic and tricyclic compounds on BKCa channel activity in rat isolated cortical neurones

1 The actions of several neuroleptic and tricyclic compounds were examined on the large conductance Ca2+‐activated K+ (BKCa) channel present in neurones isolated from the rat motor cortex. 2 Classical neuroleptic compounds including chlorpromazine and haloperidol applied to the intracellular surface of inside‐out patches produced a concentration‐dependent reduction in BKCa channel activity. Similar effects were observed when these compounds were applied to the extracellular surface of outside‐out patches. 3 In contrast, the atypical neuroleptic compounds clozapine and sulpiride did not affect BKCa channel activity (100u2003nM–1u2003mM) in either inside‐out or outside‐out patches, while 10u2003μM pimozide produced 73% of the inhibition produced by 10u2003μM chlorpromazine. 4 BKCa channel activity was also unaffected by application of structurally related tricyclic compounds including the anti‐cholinesterase tacrine and the anti‐epileptic carbamazepine. The tricyclic antidepressant drug amitriptyline was found to inhibit BKCa channel activity but was much less effective than the classical neuroleptic compounds. 5 It is concluded that compounds belonging to the classical neuroleptic group of drugs inhibit BKCa channel activity in the rat motor cortex in a structurally‐specific manner. This observation may be of clinical significance as it may contribute to some of the side effects associated with classical neuroleptic drug therapy.