Actions of epoxyeicosatrienoic acid on large-conductance Ca2+-activated K+ channels in pituitary GH3 cells

Epoxyeicosatrienoic acids (EETs), a family of cytochrome P450 epoxygenase metabolites of arachidonic acid, are believed to have an autocrine or paracrine role in the regulation of neurons or neuroendocrine cells. The effects of 14,15-EET on ionic currents were investigated in rat pituitary GH(3) cells. In the whole-cell configuration, 14,15-EET (3 mu M) reversibly increased the amplitude of the Ca2+-activated K+ current (I-K(Ca)). The 14,15-EET-induced increase in I-K(Ca) was unaffected in the presence of 10 mu M thyrotropin-releasing hormone externally or 10 mu M inositol trisphosphate in the recording pipette. In cells preincubated with pertussis toxin or herbimycin A, the 14,15-EET-induced increase in I-K(Ca) was also not changed. In the inside-out configuration, 14,15-EET applied intracellularly did not change single-channel conductance, but did increase the opening probability of large-conductance Ca2+-activated K+ (BKCa) channels. 14,15-EET (3 mu M) shifted the activation curve of BK,, channels to less positive membrane potential by approximately 15 mV. The change in the kinetic behavior of BKCa channels caused by 14,15-EET is explained by a lengthening of open and a shortening of closed times. 14,15-EET increased the activity of BKCa channels in a concentration-dependent manner with an EC50 value of 1 mu M. However, 14,15-EET did not affect the Ca2+ sensitivity of BKCa channels. The present study indicates that 14,15-EET is an opener of BKCa channels in GH(3) cells and that the stimulatory effect of 14,15-EET on these channels may, at least in part, contribute to the underlying cellular mechanisms by which EETs affect neuronal or neuroendocrine function. (C) 2000 Elsevier Science Inc.