Mechanisms Underlying the Activation of Large Conductance Ca²⁺-Activated K⁺ Channels by Nordihydroguaiaretic Acid

Abstract

The mechanisms underlying the activation of large conductance Ca²⁺-activated K⁺ (BK) channel by nordihydroguaiaretic acid (NDGA) were examined in human embryonic kidney (HEK293) cells, where BK channel α (BKα) or α plus β1 subunit (BKαβ1) was heterologously expressed, and also in freshly isolated porcine coronary arterial smooth muscle cells (PCASMCs). The activity of both BKα and BK αβ1 channels was increased by 10 μM NDGA in similar manners, indicating the selective action on the α subunit to increase Ca²⁺ sensitivity. The application of NDGA to PCASMCs induced outward current and hyperpolarization under voltage and current clamp, respectively, in a concentration-dependent manner (≥3 μM). These effects were blocked by 100 nM iberiotoxin. Electrical events induced by NDGA (≥10 μM) were, unexpectedly, associated with the increase in [Ca²⁺]_i. After the treatment with caffeine and ryanodine, the [Ca²⁺]_i increase by NDGA was markedly reduced and the hyperpolarization by NDGA was attenuated. The Ca²⁺ release by 10 μM NDGA was preceded by membrane depolarization of mitochondria. These results indicate that BK channel opening by NDGA in PCASMCs is due to the direct action on α subunit and also to Ca²⁺ release from sarcoplasmic reticulum, presumably via, at least in part, the inhibition of mitochondria respiration.