Nongenomic steroid actions: completing the puzzle. Aldosterone as an example

In the common unidimensional theory of steroid action, steroids bind to intracellular receptors and modulate nuclear transcription and thus protein synthesis. These genomic steroid effects, being characterized by their delayed onset of action and their dependence on transcription and protein synthesis, have been known for several decades. In contrast, very rapid actions of steroids, which are considered to be of nongenomic origin, have been recognized more widely and characterized in detail only during the past ten years. Specific rapid effects of steroids and related hormones like vitamin D3 and thyroid hormones on cellular function involve a conventional second messenger cascade which in most cases includes phospholipase C, phosphoinositide turnover, intracellular pH and intracellular calcium ([Ca2+]i), and protein kinase C. Furthermore, binding sites in membranes have been characterized exposing binding features compatible with an involvement in rapid-steroid signaling. Characteristics of putative membrane receptors are completely different from those of classic intracellular steroid receptors; this also includes the inability of classic steroid receptor antagonists to inhibit those rapid nongenomic steroid actions. The physiological and pathophysiological relevance of these effects is still largely unclear, but their existence has been proven recently even under in vivo conditions. New drugs modulating nongenomic steroid actions may find applications in various areas such as the cardiovascular and central nervous systems, infertility and electrolyte homeostasis. This short review focuses mainly on the nongenomic actions of aldosterone and their cardiovascular implications.