Abstract 19715: Silencing of Mineralocorticoid Receptors in the Heart Blunts the Anrep Effect
The stretch-induced increase of cardiac force is biphasic. A first abrupt increase (Farnk-Starling mechanism) is followed by a slower response that takes minutes to develop, which is believed to be the in vitro manifestation of the Anrep effect. This slow force response (SFR) is due to an increase in the intracellular Ca2+ transient. Although the cause of the increase in Ca2+ is controversial, an increased NHE1 activity as result of a stretch-triggered autocrine/paracrine mechanism was proposed to be crucial. Furthermore, based on pharmacological approaches we have recently suggested a role for mineralocorticoid receptors (MR) activation in the signaling pathway to the SFR. However, a recent report in smooth muscle cells called attention to the fact that MR inhibitors spironolactone or eplerenone can also affect signaling pathways that do not depend on MR activation. Therefore definitive evidence about the participation of MR in the SFR development needs of MR silencing by molecular techniques. In this study, an interference RNA sequence (small hairpin) able to mediate specific MR knockdown was incorporated into a lentiviral vector (l-shMR) and injected into the left ventricular wall of rat myocardium. Injection of a vector expressing a non-silencing sequence (scramble) served as control. Rats were sacrificed one month after injection. Myocardial MR protein expression was determined and the SFR as well as the stretch-triggered NHE1-mediated increase in pHi in the absence of bicarbonate was evaluated in isolated papillary muscles. Hearts with l-shMR showed reduced MR protein expression (100 ± 5.7 %, n=8, scramble vs. 57 ± 5.6 %, n=9, l-shMR, P<0.05) as well as inhibition of the SFR (in % of initial rapid phase: 125.9 ± 1.8, n=8, scramble, vs. 103.9 ± 1.3, n=8, l-shMR, P<0.05) and of the stretch-induced increase in pHi in bicarbonate free medium (ΔpH units: 0.156 ± 0.025, n=7, scramble, vs. 0.009 ± 0.015, n=10, l-shMR, P<0.05). These data provide unequivocal support to our proposal that MR activation is crucial in the stretch-triggered myocardial mechanism leading to NHE1 activation that determines the Anrep effect.
- © 2012 by American Heart Association, Inc.