An estrogen metabolite, 2-methoxyestradiol, disrupts cardiac Microtubules and unmasks muscarinic inhibition of calcium current

Microtubules provide a chemical signaling function as well as structural support for heart cells. Microtubules modulate autonomic signaling in the heart, and their disruption by colchicine unmasks muscarinic inhibition of Ca(I-Ca) current. In this study, we compare the actions of the estrogen metabolite, 2-methoxyestradiol (2-ME), with those of colchicine on microtubule stability and chemical signal function in guinea pig-isolated ventricular myocytes. Like colchicine, 2-ME binds to microtubules and disrupts the cytoskeleton of cardiac myocytes. Incubation with 2-ME increased the soluble fraction of tubulin and decreased the polymerized fraction at concentrations ranging from 10 to 100 mu M. 2-ME was less potent than colchicine in causing microtubular disruption. Treatment with 2- ME for up to 4 h was accompanied by a progressive increase of I-Ca amplitude. There was no change in the rates of I-Ca inactivation. Carbachol, which has no effect on I-Ca in untreated ventricular myocytes, inhibited this current in the presence of 2- ME. The extent of inhibition increased with incubation time in 2- ME such that carbachol completely removed the increment of I-Ca by the estrogen metabolite. The results illustrate the important role of microtubules in modulating cardiac autonomic signaling.