A delayed rectifier potassium channel cloned from bovine adrenal medulla. Functional analysis after expression in Xenopus oocytes and in a neuroblastoma cell line

Caffeic Acid Esters Activate TREK-1 Potassium Channels and Inhibit Depolarization-Dependent Secretion

In whole-cell and single-channel patch-clamp recordings from bovine adrenal fasciculata cells, it was discovered that selected caffeic acid derivatives dramatically enhanced the activity of background TREK-1 K+ channels. Cinnamyl 1-3,4-dihydroxy-alpha-cyanocinnamate (CDC), activated TREK-1 when this agent was applied externally to cells or outside-out patches at concentrations of 5 to 10 microM. Structure/activity studies showed that native bTREK-1 channels were also activated by other caffeic acid esters, including caffeic acid phenethyl ester (CAPE), which contain a benzene or furan ring in the ester side chain. The activation of bTREK-1 by caffeic acid derivatives did not occur through inhibition of lipoxygenases because other potent lipoxygenase inhibitors failed to activate bTREK-1. In bovine adrenal zona fasciculata (AZF) cells, bTREK-1 K+ channels set the resting membrane potential. Inhibition of these channels by corticotropin leads to depolarization-dependent Ca2+ entry and cortisol secretion. CDC, which activates up to thousands of dormant bTREK-1 channels in AZF cells, was found to overwhelm the inhibition of bTREK-1 by corticotropin, reverse the membrane depolarization, and inhibit corticotropin-stimulated cortisol secretion. These results identify selected caffeic acid derivatives as novel K+ channel openers that activate TREK-1 background K+ channels. Because of their ability to stabilize the resting membrane potential and oppose electrical activity and depolarization-dependent Ca2+ entry, these compounds may have therapeutic potential as neuroprotective or cardioprotective agents.

Differential expression of K(V) channel alpha- and beta-subunits in the bovine pulmonary arterial circulation

Resistance pulmonary arteries constrict in response to hypoxia, whereas conduit pulmonary arteries typically do not respond or dilate slightly. One proposed mechanism for this differential response is the variable expression of pulmonary arterial smooth muscle cell voltage-gated K(+) (K(V)) channel subunits (Kv1.2, Kv2.1, Kv1.5, and Kv3.1b) shown to be O(2) sensitive in heterologous expression systems. In this study, immunoblotting and immunohistochemistry were used to examine the expression of K(V) channel alpha- and beta-subunits in the bovine pulmonary arterial circulation to determine whether differential K(V) channel subunit distribution is responsible for the distinct sensitivities of pulmonary arteries to hypoxia. Surprisingly, there was little difference in the expression levels of Kv1.2, Kv1.5, and Kv2.1 between conduit and resistance pulmonary arteries. In contrast, expression of the Kv3.1b alpha-subunit and Kv beta.1, Kv beta 1.2, and Kv beta 1.3 accessory subunits dramatically increased along the pulmonary arterial tree. The differential expression of all the beta-subunits but of only one of the putative O(2)-sensitive alpha-subunits suggests that the alpha-subunits alone are not the O(2) sensors but further implicates the auxiliary beta-subunits in pulmonary arterial O(2) sensing.

A bovine adrenocortical Kv1.4 K(+) channel whose expression is potently inhibited by ACTH

We have cloned a bovine adrenal cortical (bKv1.4) K(+) channel cDNA whose expression is rapidly inhibited by adrenocorticotropic hormone (ACTH). The 4386-nucleotide cDNA is homologous to other voltage-gated, rapidly inactivating Kv1.4 channels, and includes a 1986-nucleotide coding region and large 5'- and 3'-untranslated regions. Bovine Kv1.4-specific mRNA from adrenal zona fasciculata (AZF) cells was rapidly and potently reduced by ACTH, with a t(12) of approximately 1 h and an IC(50) of 1.2 pm. The membrane-permeable cAMP analog 8-pcpt-cAMP also reduced bKv1.4 mRNA expression with kinetics similar to that observed with ACTH. Reduction of bKv1.4 mRNA expression by ACTH and 8-pcpt-cAMP was only partially inhibited by the selective protein kinase A antagonist H-89. Consistent with their effect on bKv1.4 mRNA, ACTH and 8-pcpt-cAMP both dramatically reduced the expression of bKv1.4-associated A-type current measured over 72 h. These results demonstrate that bovine AZF cells synthesize a Kv1.4-type channel whose expression is inhibited at the pretranslational level by ACTH and 8-pcpt-cAMP by a mechanism that is partially dependent on the activation of protein kinase A. The rapid, potent reduction of bKv1.4 mRNA produced by ACTH and 8-pcpt-cAMP indicates that the expression of this K(+) channel is under tonic inhibitory control of the hypothalamic-pituitary-adrenal axis. The basic electrical properties of AZF cells might be tightly regulated at the transcriptional level by the normal diurnal pattern of ACTH secretion, and altered during bouts of stress by the enhanced release of this pituitary peptide. Under conditions of prolonged stress or adrenal insufficiency, persistent ACTH-induced changes in the electrical properties of AZF cells could be coupled to parallel changes in cortisol secretion.

Molecular determinants of stereoselective bupivacaine block of hKv1.5 channels

Enantiomers of local anesthetics are useful probes of ion channel structure that can reveal three-dimensional relations for drug binding in the channel pore and may have important clinical consequences. Bupivacaine block of open hKv1.5 channels is stereoselective, with the R(+)-enantiomer being 7-fold more potent than the S(-)-enantiomer (Kd = 4.1 mumol/L versus 27.3 mumol/L). Using whole-cell voltage clamp of hKv1.5 channels and site-directed mutants stably expressed in Ltk- cells, we have identified a set of amino acids that determine the stereoselectivity of bupivacaine block. Replacement of threonine 505 by hydrophobic amino acids (isoleucine, valine, or alanine) abolished stereoselective block, whereas a serine substitution preserved it [Kd = 60 mumol/L and 7.4 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. A similar substitution at the internal tetraethylammonium binding site (T477S) reduced the affinity for both enantiomers similarly, thus preserving the stereoselectivity [Kd = 45.5 mumol/L and 7.8 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. Replacement of L508 or V512 by a methionine (L508M and V512M) abolished stereoselective block, whereas substitution of V512 by an alanine (V512A) preserved it. Block of Kv2.1 channels, which carry valine, leucine, and isoleucine residues at T505, L508, and V512 equivalent sites, respectively, was not stereoselective [Kd = 8.3 mumol/L and 13 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. These results suggest that (1) the bupivacaine binding site is located in the inner mouth of the pore, (2) stereoselective block displays subfamily selectivity, and (3) a polar interaction with T505 combined with hydrophobic interactions with L508 and V512 are required for stereoselective block.