An investigation of the effects of haem on the activity of rat arterial large-conductance Ca2+-activated K+ (BKCa) channels

Local Ca2+ release in smooth muscle cells (SMCs) activates large-conductance Ca2+-activated K+ (BKCa) channels resulting in spontaneous transient outward currents (STOCs) which counteract vasoconstriction. Evidence shows that haem regulates a variety of ion channels, including BKCa channels. However, little is known about the mechanism of action of haem and its degradation products on BKCa channels. Earlier studies, mostly on inside-out patches, show that haem and CO (a haem degradation product) inhibit and stimulate BKCa channel activity respectively. Thus, it is hypothesised that the haem interaction with BKCa channels is involved in the vasospasm that occurs during haemolytic diseases. This study focuses primarily on the effects of haem and CO on whole-cell BKCa currents. SMCs were isolated from the mesenteric artery of male Wistar rats. Single and whole-cell BKCa channel currents were recorded using inside-out, outside-out, ruptured and perforated patch recordings as appropriate. As in earlier studies, intracellular haem or CO application to BKCa channels in inside-out patches inhibited and stimulated channel activity, respectively. However, extracellular haem enhanced whole-cell BKCa currents. Based on the hypothesis that the stimulatory effect of haem resulted from CO generated from HO-mediated haem degradation, it was surprising that extracellular haem enhanced whole-cell BKCa currents in cells pre-treated with the HO-inhibitor, zinc protoporphyrin-IX (ZnPP-IX). Extracellular haem also enhanced single BKCa channel activity in outside-out patches. This is the first study to report a haem mediated CO-independent stimulation of STOCs. The discrepancy between the effects of haem in inside-out patches and intact cells suggest that intracellular components play a key role in the haem-mediated stimulatory effect on STOCs. Nevertheless, the outside-out and ruptured patch results indicate that haem could enhance channel activity independent of intracellular components. In conclusion, this study shows that haem may act as a feedback regulator to counteract the vasospasm that occurs during haemolytic diseases.