Biochemical and Ionic signaling mechanisms for ACTH-stimulated cortisol production., 2005
Adrenocorticotropic hormone (ACTH)-stimulated cortisol production by adrenal zona fasciculata cells requires coordinated biochemical and ionic signaling mechanisms that employ adenosine 3', 5'-cyclic monophosphate (cAMP) and Ca(2+) as intracellular messengers. As the primary messenger generated in response to ACTH receptor activation, cAMP acts at multiple sites to produce the full steroidogenic response that includes both rapid and delayed components. Biochemically, cAMP activates and induces the expression of multiple proteins that function in converting cholesterol to cortisol. These include the steroid acute regulatory (StAR) protein as well as steroidogenic enzymes. cAMP also inhibits a background K(+) channel (bTREK-1), which sets the resting potential of adrenal zona fasciculata (AZF) cells, thereby triggering membrane depolarization and Ca(2+) entry through voltage-gated Ca(2+) channels. Ca(2+) also accelerates the production of cortisol from cholesterol by activating or inducing the synthesis of steroidogenic proteins. In this scheme, background K(+) channels act pivotally by transducing a hormonal signal at the cell membrane to an ionic signal, leading to depolarization-dependent Ca(2+) entry. In this way, ACTH receptor activation increases cAMP and Ca(2+) in the AZF cell, yielding the full steroidogenic response. In addition to acutely regulating the activity of AZF cell ion channels, ACTH and cAMP also regulate the expression of genes coding for these ion channels. The tonic control of the expression of AZF cell ion channels through the hypothalamic-pituitary-adrenal axis suggests that prolonged stimulation of the AZF cell by ACTH may alter the electrical properties of these cells in a manner which matches the organism's requirement for cortisol.
Ca2+ and K+ channels of normal human adrenal zona fasciculata cells: properties and modulation by ACTH and AngII. 2013
In whole cell patch clamp recordings, we found that normal human adrenal zona fasciculata (AZF) cells express voltage-gated, rapidly inactivating Ca(2+) and K(+) currents and a noninactivating, leak-type K(+) current. Characterization of these currents with respect to voltage-dependent gating and kinetic properties, pharmacology, and modulation by the peptide hormones adrenocorticotropic hormone (ACTH) and AngII, in conjunction with Northern blot analysis, identified these channels as Cav3.2 (encoded by CACNA1H), Kv1.4 (KCNA4), and TREK-1 (KCNK2). In particular, the low voltage-activated, rapidly inactivating and slowly deactivating Ca(2+) current (Cav3.2) was potently blocked by Ni(2+) with an IC50 of 3 µM. The voltage-gated, rapidly inactivating K(+) current (Kv1.4) was robustly expressed in nearly every cell, with a current density of 95.0 ± 7.2 pA/pF (n = 64). The noninactivating, outwardly rectifying K(+) current (TREK-1) grew to a stable maximum over a period of minutes when recording at a holding potential of
80 mV. This noninactivating K(+) current was markedly activated by cinnamyl 1-3,4-dihydroxy-α-cyanocinnamate (CDC) and arachidonic acid (AA) and inhibited almost completely by forskolin, properties which are specific to TREK-1 among the K2P family of K(+) channels. The activation of TREK-1 by AA and inhibition by forskolin were closely linked to membrane hyperpolarization and depolarization, respectively. ACTH and AngII selectively inhibited the noninactivating K(+) current in human AZF cells at concentrations that stimulated cortisol secretion. Accordingly, mibefradil and CDC at concentrations that, respectively, blocked Cav3.2 and activated TREK-1, each inhibited both ACTH and AngII-stimulated cortisol secretion. These results characterize the major Ca(2+) and K(+) channels expressed by normal human AZF cells and identify TREK-1 as the primary leak-type channel involved in establishing the membrane potential. These findings also suggest a model for cortisol secretion in human AZF cells wherein ACTH and AngII receptor activation is coupled to membrane depolarization and the activation of Cav3.2 channels through inhibition of hTREK-1.