Abstract

Levosimendan is a pyridazinone-dinitrile derivative belonging to a new class of cardiac inotropic drugs, Ca⁺⁺ sensitizers. Levosimendan is also a vasodilator both in vitro and in vivo, but its mechanism is not well understood. The cardiac target protein of levosimendan, troponin C, is a Ca⁺⁺-binding EF-hand protein. This raises the possibility that levosimendan may also interact with smooth muscle EF-hand proteins, such as, calmodulin, the regulatory myosin light chains, or S100 proteins. We investigated the effects of levosimendan on [Ca⁺⁺]_i, and force in porcine coronary arteries, with receptor-mediated (U46619) or KCl stimulation. At high levels of stimulation, levosimendan decreased force without changing or increasing [Ca⁺⁺]_i, measured with the Ca⁺⁺-sensitive fluorescent probe fura-2 in the intact artery. With lower levels of U46619, levosimendan (1 μM) lowered force by 70% and reduced [Ca⁺⁺]_i by 38%. The relationship between force and [Ca⁺⁺]_i for KCl stimulation are significantly rightward shifted, indicating Ca⁺⁺desensitization by levosimendan. In contrast, the phosphodiesterase III inhibitor, milrinone, does not shift the force-Ca⁺⁺relations but elicits relaxation via lowering [Ca⁺⁺]_i. There was little change in pH_i, indicating that the Ca⁺⁺ desensitization by levosimendan was not attributable to decreasing pH_i. Levosimendan relaxes coronary arteries and lowers [Ca⁺⁺]_i by mechanisms different than milrinone. Our results indicate a lowering of [Ca⁺⁺]_i by levosimendan consistent with opening of potassium channels and a relaxation that is independent of [Ca⁺⁺]_i. Our evidence points to a novel mechanism that might involve the direct effect of levosimendan on the smooth muscle contractile or regulatory proteins themselves.