Abstract
17 beta-Oestradiol (E2) at 0.1-10 microM directly inhibited various tonic and phasic smooth muscle contractions. The mechanism(s) of oestrogen-induced inhibition of contraction was studied using intact and permeabilized strips and isolated single cells of smooth muscle. 2. In endothelium-denuded vascular smooth muscle, E2 attenuated high K(+)-induced force development and myosin light chain phosphorylation, and produced rapid and reversible relaxation. There were no significant differences in these inhibitory effects between tissue types (femoral artery vs. portal vein), species (rat vs. rabbit) or sexes. 3. The inhibitory potencies of several steroidal and non-steroidal oestrogen analogues were examined and their effects were for the most part stereo-specific. However, two steroids with negligible affinities for the nuclear oestrogen receptor also strongly inhibited high K(+)-induced contraction. 4. Genomic modulators including a protein synthesis inhibitor, an RNA synthesis inhibitor, and oestrogen receptor antagonists did not affect the inhibitory actions of E2. Inhibitors of cyclic nucleotide-dependent protein kinases did not reduce the E2 effect. 5. Ca2+ release from intracellular stores by agonists and by inositol 1,4,5-trisphosphate (IP3) does not appear to be modulated by E2. Neither pretreatment with ryanodine nor with thapsigargin affected the E2-induced inhibition of high K(+)-induced contraction. 6. E2 had no effect on either normal or GTP gamma S-increased Ca2+ sensitivity of the regulatory and contractile apparatus. 7. E2 and its analogues rapidly inhibited voltage-dependent L-type Ca2+ channel currents in isolated smooth muscle cells. Repetitive stimulation was not required for E2-induced inhibition of the currents. 8. This study strongly suggests that at pharmacological concentrations oestrogen primarily reduces Ca2+ influx through inhibition of L-type Ca2+ channels in a non-genomic manner and decreases myosin light chain phosphorylation and contraction of smooth muscle.