Androgens are powerful non-genomic inducers of calcium sensitization in visceral smooth muscle

Abstract

Androgens are recognized as genotropic inducers of a number of physiological functions mainly associated with the development of sexual characteristics. However, as in the case of estrogens, the number of studies evidencing androgen actions in non-reproductive tissues has steadily grown over the past years. Here, we show that androgens acutely (∼30 min) alter the frequency spectrum of peristaltic activity of intestinal smooth muscle and augment the amplitude agonist-induced contractile activity. Maximal stimulation occurred at physiological concentrations of androgens with EC 50 values in the picomolar range. Androgen-induced potentiation was prevented by preincubation with androgen receptor (AR) antagonists but unaffected by cycloheximide plus actinomycin D, indicating that potentiation was mediated by ARs via a non-genomic mechanism. The effects of androgens were mimicked by polyamines and were completely blocked by inhibitors of polyamine synthesis. Using ionomycin-permeabilized intestinal smooth muscle preparations, we demonstrate that androgens exert their effects by inducing a mechanism of sensitization to calcium and not by altering intracellular calcium homeostasis. Correspondingly, the potentiation of mechanical activity induced by androgens was accompanied by an increase in the phosphorylation of the regulatory myosin light chain (LC 20) within the same time-course than calcium sensitization and mechanical potentiation. The pursuit of potential signalling pathways linking androgen receptor activation with calcium sensitization revealed that mechanical potentiation of intestinal muscle by androgens involve activation of the Rho pathway, whose downstream effector, Rho-associated kinase (ROCK), is eventually responsible for displacement of the phosphorylation/dephosphorylation state of LC 20 towards its phosphorylated form.