Effects of anti‐oestrogens and β‐estradiol on calcium uptake by cardiac sarcoplasmic reticulum

Abstract

1 Tamoxifen and a group of structurally similar non-steroidal, triphenolic compounds inhibit the oestrogen receptor. In addition to this action, these anti-oestrogens are known to inhibit some types of plasma membrane ion channels and other proteins through mechanisms that do not appear to involve their interactions with the estrogen receptor but could be the result of their eect on membrane lipid structure or¯uidity. 2 We studied the eects of b-estradiol and three anti-oestrogens (tamoxifen, 4-hydroxytamoxifen and clomiphene) on Ca 2+ uptake into sarcoplasmic reticulum (SR) vesicles isolated from canine cardiac ventricular tissue. 3 The antiestrogens all inhibit SR Ca 2+ uptake in a concentration-dependent manner (order of potency: tamoxifen 4 4-hydroxytamoxifen 5 clomiphene). Although these compounds rapidly inhibit net Ca 2+ uptake they do not have a similar rapid eect on the ATPase activity of the SR Ca pump. b-estradiol has no eect on Ca 2+ uptake nor does it alter the inhibitory action of tamoxifen on the SR. 4 The dierences in the eects of b-estradiol and the anti-oestrogens on cardiac SR Ca 2+ uptake do not correlate with dierences in the ways in which these compounds have been reported to interact with membrane lipids. Our results are consistent, however, with direct eects on a membrane protein (possibly an SR Cl 7 or K + channel).

Effects of Phytoestrogens on Sarcoplasmic/Endoplasmic Reticulum Calcium ATPase 2a and Ca2+Uptake into Cardiac Sarcoplasmic Reticulum

“…At the highest concentration tested (30 M), genistein did not affect SR Ca 2ϩ uptake, whereas phloretin, at the same concentration, inhibited uptake to approximately 70% of control. It was also shown in a previous study (Dodds et al, 2001) that ␤-estradiol does not affect Ca 2ϩ uptake into canine cardiac SR vesicles nor does it alter the inhibitory effect of the antiestrogen tamoxifen on Ca 2ϩ uptake. It is thus unlikely that the effects of phloretin (or phloridzin) on SR Ca 2ϩ uptake are mediated through estrogen receptors.…”

“…Ca 2ϩ uptake into cardiac SR vesicles was measured with fura-2 as described previously (Kargacin and Kargacin, 1994, 1998a,b, 2000Dodds et al, 2001) using either a SPEX CMX fluorimeter (SPEX Industries, Edison, NJ) or a fluorimeter (Photon Technologies International, Lawrenceville, NJ) with the excitation wavelength alternated between 340 and 380 nm. Fluorescence emission was measured at 510 nm; 340/380 fluorescence ratios were determined every second.…”

“…Inhibition of Ca 2ϩ uptake by tamoxifen was not mimicked by ␤-estradiol and ␤-estradiol did not alter the effects of tamoxifen (Kargacin et al, 2000;Dodds et al, 2001), indicating that the action of tamoxifen is not mediated by interaction with estrogen receptors. In previous work with phytoestrogens, it was shown (Shoshan and MacLennan, 1981) that quercetin directly inhibits the ATPase activity of the SR Ca 2ϩ pump.…”

“…In previous studies (Kargacin et al, 2000;Dodds et al, 2001), we showed that the mixed estrogen antagonists/agonists tamoxifen, 4-hydroxytamoxifen, and clomiphene inhibit Ca 2ϩ uptake into cardiac sarcoplasmic reticulum (SR). Inhibition of Ca 2ϩ uptake by tamoxifen was not mimicked by ␤-estradiol and ␤-estradiol did not alter the effects of tamoxifen (Kargacin et al, 2000;Dodds et al, 2001), indicating that the action of tamoxifen is not mediated by interaction with estrogen receptors.…”

Inhibitory Effects of the Antiestrogen Agent Clomiphene on Cardiac Sarcolemmal Anionic and Cationic Currents

“…Nevertheless, evidence is accumulating that some actions assigned to antiestrogens do not involve an interaction with the estrogen receptor (Zhang et al, 1994;Voets et al, 1995;Manolopoulos et al, 2000;Dodds et al, 2001). Recently, Maertens et al (2001) have shown that both the cis-and trans-isomers of CLM inhibit both volume-regulated anion channels (VRACs) and cell proliferation in cultured pulmonary artery endothelial cells (CPAEs).…”