TREK-1 K+ channels couple angiotensin II receptors to membrane depolarization and aldosterone secretion in bovine adrenal glomerulosa cells

Bovine adrenal glomerulosa (AZG) cells were shown to express bTREK-1 background K+ channels that set the resting membrane potential and couple angiotensin II (ANG II) receptor activation to membrane depolarization and aldosterone secretion. Northern blot and in situ hybridization studies demonstrated that bTREK-1 mRNA is uniformly distributed in the bovine adrenal cortex, including zona fasciculata and zona glomerulosa, but is absent from the medulla. TASK-3 mRNA, which codes for the predominant background K+ channel in rat AZG cells, is undetectable in the bovine adrenal cortex. In whole cell voltage clamp recordings, bovine AZG cells express a rapidly inactivating voltage-gated K+ current and a noninactivating background K+ current with properties that collectively identify it as bTREK-1. The outwardly rectifying K+ current was activated by intracellular acidification, ATP, and superfusion of bTREK-1 openers, including arachidonic acid (AA) and cinnamyl 1-3,4-dihydroxy-alpha-cyanocinnamate (CDC). Bovine chromaffin cells did not express this current. In voltage and current clamp recordings, ANG II (10 nM) selectively inhibited the noninactivating K+ current by 82.1 +/- 6.1% and depolarized AZG cells by 31.6 +/- 2.3 mV. CDC and AA overwhelmed ANG II-mediated inhibition of bTREK-1 and restored the resting membrane potential to its control value even in the continued presence of ANG II. Vasopressin (50 nM), which also physiologically stimulates aldosterone secretion, inhibited the background K+ current by 73.8 +/- 9.4%. In contrast to its potent inhibition of bTREK-1, ANG II failed to alter the T-type Ca2+ current measured over a wide range of test potentials by using pipette solutions of identical nucleotide and Ca2+-buffering compositions. ANG II also failed to alter the voltage dependence of T channel activation under these same conditions. Overall, these results identify bTREK-1 K+ channels as a pivotal control point where ANG II receptor activation is transduced to depolarization-dependent Ca2+ entry and aldosterone secretion.