Pregnenolone sulphate-independent inhibition of TRPM3 channels by progesterone.
Bottom Line: Progesterone metabolites and 17β-oestradiol were also inhibitory but the effects were relatively small.Corticosteroids lacked effect.Relevance of TRPM3 or the progesterone effect to ovarian cells, which have been suggested to express TRPM3, was not identified.
Affiliation: Multidisciplinary Cardiovascular Research Centre, University of Leeds, Leeds, UK.Show MeSH
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Mentions: Although nifedipine is chemically distinct from pregnenolone sulphate, we could not be sure that it activated TRPM3 independently of the pregnenolone sulphate binding site. Therefore, we also sought TRPM3 activity in the absence of an exogenous agonist. Over-expressed human TRPM3 has been shown to have constitutive activity . However, in our tetracycline-inducible TRPM3 cells (in the presence of 1.5 mM extracellular Ca2+) such activity was not evident as a basal Ca2+ signal (Fig. 1b; Supplementary Fig. IV) or ionic current (Supplementary Fig. IV). Therefore we used a Ca2+ add-back protocol in which 5 mM Ca2+ was added to cells after a period in Ca2+-free solution (Fig. 6). This protocol revealed a Ca2+ response in TRPM3-expressing as well as control cells but the response was significantly larger in the presence of TRPM3 (Supplementary Fig. V). Therefore we used this signal to investigate effects of progesterone and dihydrotestosterone in the absence of an exogenous TRPM3 agonist. Progesterone inhibited the signal in TRPM3-expressing but not control cells (Fig. 6a, c), whereas dihydrotestosterone had no effect in either set of cells (Fig. 6b, d). The data support the hypothesis that progesterone inhibited TRPM3 independently of competition with an exogenous agonist, apparently acting as a mode-independent inhibitor. Dihydrotestosterone, by contrast, had an inhibitory effect that depended on the presence of the agonist pregnenolone sulphate.
Affiliation: Multidisciplinary Cardiovascular Research Centre, University of Leeds, Leeds, UK.