Apelin increases contractility in failing cardiac muscle

Abstract

Apelin, a ligand for apelin-angiotension receptor-like 1 (APJ), has recently been shown to be a potent positive inotropic agent in normal hearts. In humans, levels of apelin have been shown to rise in early-stage heart failure and to fall in late-stage heart failure. In this study, we tested the hypothesis that apelin augments contraction directly in failing rat cardiac muscle. Right ventricular heart failure secondary to pulmonary hypertension was induced by exposing the rats to hypoxia (10% O(2) inhaled air) for 14-16 weeks. Trabeculae were dissected and mounted between a force transducer and a motor arm, superfused with Krebs-Henseleit (K-H) solution (pH 7.4, 22 degrees C), and loaded with fura-2. Both force development and [Ca(2+)](i) transient amplitude increased in a dose-dependent manner in the presence of Apelin-12 (10 approximately 70 nM, [Ca(2+)](o)=0.5 mM) in failing muscles as compared to control (36+/-7% vs. 7.4+/-5% at 70 nM, P<0.05). Also, [Ca(2+)](i) transients increased up to 18.4+/-9.5% as compared to control (4.5+/-1.9%, P<0.05). The increases in contraction in the presence of apelin were also maintained over a range of external Ca(2+) (0.5-2.0 mM). Steady-state force-[Ca(2+)](i) relation of the failing muscles reveals decreased maximal Ca(2+)-activated force (F(max)) (51.45+/-5.3 vs. 98.5+/-11.5 mN/mm(2), P<0.001), with no changes in Ca(2+) required for 50% of maximal activation (Ca(50)) (0.45+/-0.07 vs. 0.30+/-0.04 muM, P>0.05) and Hill coefficient (4.60+/-0.73 vs. 3.17+/-0.92, P>0.05). Apelin (70 nM) had no effect on the steady-state force-[Ca(2+)](i) relation in failing muscles (F(max): 63.03+/-3.5 mN/mm(2); Ca(50): 0.50+/-0.08 microM; Hill coefficient: 4.73+/-0.89). These results indicate that apelin exerts a selective positive inotropic action in failing myocardium. The increased force development is the result of increased [Ca(2+)](i) transients rather than changes in myofilament calcium responsiveness.

Figure 1

Representative tracings of twitch force (A) and [Ca²⁺]_i transients (B) from control and failing trabeculae. The muscles were superfused with K-H solution and stimulated at 0.5 Hz. [Ca²⁺]=2.0 mM, temp 22 °C. Note that the failing muscle generated lower amplitudes of twitch force and Ca²⁺ transient. Pooled data of twitch force (C) and amplitudes of [Ca²⁺]_i transients (D) of control and failing muscles at varied external [Ca²⁺]s. Force development and Ca²⁺ transients remain depressed at all [Ca²⁺]_os tested (P<0.014). n = 9 in each group.

Figure 2

Time to peak force (A) and time to peak Ca²⁺ transient (B) of control and failing muscles at varied [Ca²⁺]_os. The times to peak for both twitch force and [Ca2+]_i transient are significantly prolonged (p<0.014). The relaxation times from peak to 50% of peak levels of force (C) and Ca²⁺ transients (D) are also significantly prolonged in each group (P<0.014). n = 9.

Figure 3

Representative tracings of twitch force and [Ca²⁺]_i transient in a control (A) and failing (B) trabeculae before and after apelin exposure. Apelin had minimal effect on contraction and [Ca²⁺]_i transient in the control muscle. Apelin increased amplitudes of force development and [Ca²⁺]_i transient without affecting the time course of force and [Ca²⁺]_i transient. Pooled data of increases of developed forces (C) and [Ca²⁺]_i transients (D) after varied doses of apelin in control and failing muscles. The failing muscles were more responsive to apelin than control muscles. [Ca²⁺] = 0.5 mM, temperature: 22 °C, n=6 in each group.

Figure 4

Pooled data of peak force versus peak [Ca²⁺]_i transient of failing muscles before and after apelin treatment (70 nM). Varied levels of twitch forces were achieved by exposing the muscles to different [Ca²⁺]_os. A linear relationship between force and [Ca²⁺]_i was assumed and fit by least-squares minimization. n = 4 in each group.

Figure 5

Steady-state force-[Ca²⁺]_i relations of control, failing, and failing muscles treated with apelin (70 nM). The failing muscles exhibited depressed maximal Ca²⁺-activated force (P<0.001 vs. control) with no changes in Ca²⁺ required for 50% activation and Hill coefficient as compared with control muscles. Apelin did not change the force-[Ca²⁺]_i relation in failing muscles. See text for details. n = 8 control, n=5 failing, and n=6 failing + apelin.