Polymyxin B, a novel inhibitor of red cell Ca2+‐activated K+ channel

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Abstract
Polymyxin B (PXB), a cyclic peptide antibiotic, in concentrations 0.1–3.0 mg/ml (0.08–4.0 mmol/l), inhibited the K+ efflux induced by opening of the Ca2+‐activated K+ channel (the Gárdos effect) in intact human red blood cells. The inhibition was observed when the Gárdos effect was elicited by Ca2+ in the presence of vandate, or propranolol, in ATP‐depleted cells, and in A23187‐treated cells. The inhibition of the Gárdos effect is caused neither by the inhibition of the anion channel by PXB nor by the inhibition of Ca2+ entry. It can be ascribed to the inhibition of the Ca2+‐activated K+ channel. The mechanism of the inhibition remains to be elucidated.