Current Research Description
During the last 6 years, we have been studying ion channels of cortisol-secreting bovine adrenal zona fasciculata (AZF) cells. Initially, we identified and characterized each of the voltage-gated ion channels present in these cells with respect to biophysical properties and pharmacology. These channels included rapidly inactivating A-type K+ channels and low voltage activated T-type Ca2+ channels. Most importantly, it was discovered that these cells express a novel non-inactivating K+ channel (IAC) that is inhibited by subnanomolar concentrations of ACTH and AII, the two peptide hormones that physiologically regulate corticosteroid hormone secretion. IAC may be the first example of a K+ channel that is directly inhibited by cAMP. The results of our combined patch clamp and secretion studies suggest a model for ACTH-stimulated cortisol secretion wherein cAMP functions in a dual capacity as a second messenger to activate parallel signalling pathways (see figure). Along one path, cAMP directly combines with IAC K+ channels leading to their inhibition. IAC inhibition triggers the sequence of membrane depolarization, T-type Ca2+ channel opening, Ca2+-dependent activation, and ultimately, induction of steroidogenic enzymes. cAMP activates the second pathway through the conventional interaction with A-kinase followed by the induction of the same steroidogenic enzymes.
In recent studies, we have found that IAC K+ channels are directly activated by ATP and other nucleotides through the non-hydrolytic binding to the channel or associated protein. This is the first example of a K+ channel that is directly activated, rather than inhibited by ATP. IAC appears to be representative of a distinctive new class of K+ channel that acts as both a metabolic sensor and transducer. The control of K+ channel gating by metabolic factors, including cAMP and ATP, suggests a mechanism where the membrane potential of these secretory cells could be tightly coupled to the metabolic state of the cell, and modulated by iochemical signals originating at the cell membrane.
In other completed studies, we have found that ATP and adenosine inhibit IAC K+ channels and depolarize AZF cells through activation of multiple G protein-coupled nucleotide receptors. Apparently, these nucleotides are released from the adrenal medulla to synchronize the release of stress hormones, including catecholamines and cortisol.
We have also discovered that IAC K+ channels display a unique pharmacology combining sensitivity to conventional K+ channel blockers and to antagonists of cyclic nucleotide non-selective and to antagonists of cyclic nucleotide non-selective cation channels. In particular, IAC K+ channels are potently blocked by diphenyl-butylpiperidine antipsychotics, which also potently inhibit T-type Ca2+ channels. Having established that IAC is a unique K+ channel subtype that functions critically in steroidogenesis, we are continuing our study of these channels using a combination of patch clamp and molecular methods.