Abstract-The purpose of the present article is to provide an overview of plasma membrane steroid hormone receptors and their implications in nongenomic signaling. We especially focus on recent evidence supporting the notion of a possible membrane-associated aldosterone receptor, whether this receptor is different from the classic nuclear receptor, and the possible implications of such a receptor for nongenomic and genomic aldosterone effects in physiological and pathophysiological processes. (Hypertension. 2011;57:1019-1025.)Key Words: mineralocorticoid receptor Ⅲ aldosterone Ⅲ nongenomic effects Ⅲ plasma membrane A ldosterone is the major physiological regulator of Na ϩ and electrolyte balance and, hence, blood pressure homeostasis. In the kidneys, aldosterone mediates Na ϩ retention and K ϩ excretion via genomic effects through the cytosolic mineralocorticoid receptor (MR), which acts as a transcription factor. 1 In 1984, Moura and Worcel 2 described rapid (within minutes) effects of aldosterone on Na ϩ efflux from smooth muscle cells, and this effect could not be blocked by the transcription inhibitor actinomycin D. Later it was shown that aldosterone can elicit activation of Na ϩ /H ϩ exchanger and signaling cascades, such as mitogen activated protein kinases (MAPKs) or protein kinase C, through nongenomic mechanisms, and these effects may impact cellular functions, such as growth and differentiation. [3][4][5] Several excellent recent reviews have extensively described the physiology and biology of aldosterone action. 5-7
Evidence for a Nonclassic Aldosterone Receptor in the Plasma Membrane and MR-Independent Rapid SignalingInitially, the classic cytosolic MR protein was suggested to be involved in mediating both nongenomic and genomic aldosterone signaling 2,8 ; however, some studies reported that these nongenomic effects could not be blocked by MR antagonists such as spironolactone. In addition, although rapid, nongenomic effects were elicited by the mineralocorticoids aldosterone, deoxycorticosterone, and 9␣-fludrocortisone, these rapid effects could not be elicited by the glucocorticoids cortisol and corticosterone. 5 In analogy to other steroid hormones, such as estrogens, Wehling et al 9 hypothesized an aldosterone receptor that is distinct from the classic MR protein, located at the membrane, and responsible for nongenomic aldosterone effects. This view was initially supported by several studies demonstrating functional evidence for the possible existence of a plasma membranebound MR. For example, presumably membraneimpermeable, BSA-coupled aldosterone showed the same rapid effects as aldosterone, supporting the idea of a plasma membrane-bound receptor 10,11 ; however, aldosterone-BSA conjugates may undergo dissociation into hormone and BSA requiring critical and careful handling, 12 as shown for estrogen-BSA. 13 Additional support for a distinct membrane MR comes from recent work by Wildling et al. 14 Using endothelial cell swelling as an index of an aldosterone effect, it was shown that spironolact...