Aldosterone- and progesterone-membrane-binding proteins: new concepts of nongenomic steroid action

n the classical theory of steroid action steroids penetrate into cells and bind to intracellular receptors resulting in modulation of nuclear transcription and protein synthesis within hours. In addition, rapid actions of steroids have been identified, which are incompatible with the classic model of steroid action. Specific binding sites for aldosterone and progesterone have been reported in membrane preparations of liver, vascular smooth muscle cells and kidney. These sites are discussed to be involved in rapid nongenomic steroid actions, such as the rapid activation of the Na(+)/H(+) exchanger and elevation of [Ca(2+)]i in vascular smooth muscle cells by aldosterone. In addition, rapid progesterone-induced increases of [Ca(2+)]i have been reported in spermatozoa. A high affinity progesterone-membrane binding protein from porcine liver has been identified and cloned. The derived amino acid sequence showed no significant identity with any functional protein suggesting a binding site completely different to classic progesterone receptors. These binding sites are possibly involved in rapidly induced meiotic maturation of amphibian oocytes and the spermatozoan acrosome reactions as evidenced by recent studies, where the progesterone induced acrosome reactions and calcium signaling was blocked by a specific antibody raised against the membrane binding site for progesterone. In addition to data on specific steroid binding and rapid steroid signaling in vitro, results of nongenomic steroid effects in vivo are presented and their physiological relevance are discussed in the review.