Effects of K+ channel blockers on K+ channels, membrane potential, and aldosterone secretion in rat adrenal zona glomerulosa cells.
The hypothesis that angiotensin II (ANG II)-induced aldosterone secretion is mediated through inhibition of plasma membrane K+ channels was examined by measuring the effects of K+ channel blockers on K+ currents, membrane potential, and aldosterone secretion in rat adrenal glomerulosa cells. Effective K+ channel blockers were identified and studied using patch clamp methods on isolated glomerulosa cells in cell culture. Extracellular Cs+ (2-20 mm) caused a voltage-dependent inhibition of macroscopic K+ currents, exhibiting an apparent Kd of 2 mM for blockade of K+ current at membrane potentials near the K+ equilibrium potential. Outward K+ current opposed the Cs+ block, imparting a steep voltage dependence to this block. In single channel studies Cs blocked inward, but not outward, unitary currents through ANG II-regulated weakly voltage-dependent K+ channels, which are thought to control resting membrane potential. Cs+ reversibly depolarized the resting membrane potential at concentrations greater than or equal to the apparent Kd for K+ conductance inhibition (> or =2 mM). Depolarization consisted of a slow, maintained phase proportional to Cs+ concentration superimposed with 2- to 5-mV transient depolarizing events. Cs+ induced a Ca2+-dependent stimulation of aldosterone secretion in acutely dissociated cells, exhibiting an EC50 of approximately 3 mM. Maximal Cs+-induced secretion was quantitatively similar to 1 nM ANG II- or 8 mM K+-induced secretion. Cs+-induced secretion was not additive with that of ANG II. K+ channel blockers that did not inhibit weakly voltage-dependent K+ channels at rest (quinidine, apamin, and charybdotoxin) did not cause depolarization or stimulate aldosterone secretion. Furthermore, charybdotoxin did not significantly affect ANG II-induced aldosterone secretion, indicating that Ca2+-dependent maxi-K+ channels did not contribute to the control of aldosterone secretion in acutely dissociated cells. These data strongly support involvement of weakly voltage-dependent K+ channels in ANG II-induced aldosterone secretion, but also implicate roles for other channel classes in controlling membrane potential during ANG II-induced aldosterone secretion.
Lotshaw
D P
DP
Department of Biological Sciences, Northern Illinois University, DeKalb 60115, USA.
D000450:Aldosterone / Q000378:metabolism*; D000804:Angiotensin II / Q000494:pharmacology; D000818:Animals; D001030:Apamin / Q000494:pharmacology*; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology*; D018999:Charybdotoxin / Q000494:pharmacology*; D004594:Electrophysiology; D005260:Female; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology; D011802:Quinidine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D015384:Zona Glomerulosa / Q000166:cytology / Q000378:metabolism*
D026902:Potassium Channel Blockers; D015221:Potassium Channels; D000804:Angiotensin II; D018999:Charybdotoxin; D002586:Cesium; D001030:Apamin; D000450:Aldosterone; D011802:Quinidine; D011188:Potassium