The response of target cells to the steroid hormone aldosterone has been di
vided into acute nongenomic (<10 min) and sustained genomic (>10 min) actio
n. In the light of recent experiments this nomenclature does not hold anymo
re and should be abandoned. By applying atomic force microscopy (AFM) we ob
served in living endothelial cells that aldosterone induces cell volume inc
rease in less than 10 minutes. The cell nucleus was identified as the swell
ing site. Hormone-induced nuclear swelling can reach 15 to 28% of total cel
l volume dissipating within 30 minutes. This phenomenon could have function
al impact on flow resistance in small blood vessels. AFM-investigation of t
he intracellular signal pathway in nuclear envelope of aldosterone-injected
Xenopus laevis oocytes visualizes putative intracellular receptors (40 kD
granules) bound to nuclear pores 2 minutes after hormone injection, with su
bsequent macromolecule translocation into the nucleus. 15 minutes later mac
romolecules (800 kD plugs) appear in the central channels of the nuclear po
res. The plugs resemble ribonucleoproteins that carry the aldosterone-induc
ed mRNA to the ribosomes. We postulate that steroid-induced nuclear swellin
g is caused by a shift of receptors/transcription factors from cytoplasm in
to nucleoplasm followed by gene transcription. Nuclear volume returns to no
rmal when mRNA export through the nuclear pores is finished. Thus, steroid-
induced net-movements of macromolecules between intracellular compartments
initiate shifts in cell volume compensated by volume regulatory transporter
s and ion channels in the plasma membrane. Copyright (C) 2000 S. Karger AG
Basel.