The endogenous peptide apelin potently improves cardiac contractility and reduces cardiac loading in vivo

Abstract

Objective: The endogenous peptide apelin is differentially regulated in cardiovascular disease but the nature of its role in cardiac function remains unclear.

Methods: We investigated the functional relevance of this peptide using ECG and respiration gated magnetic resonance imaging, conductance catheter pressure-volume hemodynamic measurements, and echocardiography in vivo. In addition, we carried out histology and immunohistochemistry to assess cardiac hypertrophy and to localize apelin and APJ in the adult and embryonic mouse heart.

Results: Intraperitoneal injection of apelin (300 microg/kg) resulted in a decrease in left ventricular end diastolic area (pre: 0.122+/-0.007; post: 0.104+/-0.005 cm(2), p=0.006) and an increase in heart rate (pre: 537+/-20; post: 559+/-19 beats per minute, p=0.03). Hemodynamic measurements revealed a marked increase in ventricular elastance (pre: 3.7+/-0.9; post: 6.5+/-1.4 mm Hg/RVU, p=0.018) and preload recruitable stroke work (pre: 27.4+/-8.0; post: 51.8+/-3.1, p=0.059) with little change in diastolic parameters following acute infusion of apelin. Chronic infusion (2 mg/kg/day) resulted in significant increases in the velocity of circumferential shortening (baseline: 5.36+/-0.401; 14 days: 6.85+/-0.358 circ/s, p=0.049) and cardiac output (baseline: 0.142+/-0.019; 14 days: 0.25+/-0.019 l/min, p=0.001) as determined by 15 MHz echocardiography. Post-mortem corrected heart weights were not different between apelin and saline groups (p=0.5) and histology revealed no evidence of cellular hypertrophy in the apelin group (nuclei per unit area, p=0.9). Immunohistochemistry studies revealed APJ staining of myocardial cells in all regions of the adult mouse heart. Antibody staining, as well as quantitative real time polymerase chain reaction identified expression of both APJ and apelin in embryonic myocardium as early as embryonic day 13.5.

Conclusions: Apelin reduces left ventricular preload and afterload and increases contractile reserve without evidence of hypertrophy. These results associate apelin with a positive hemodynamic profile and suggest it as an attractive target for pharmacotherapy in the setting of heart failure.

Figure 1

Changes in cardiovascular function following intraperitoneal injection of apelin¹² in C57Bl6 mice. Mice (n=9) were anesthetized with isoflurance and warmed to 36–37 degrees before magnetic resonance imaging. Electrocardiography revealed a significant increase in HR following apelin injection (Panel A). ECG and respiration gated cine magnetic resonance images of the left ventricle taken in short axis reveal a significant reduction in left ventricular end diastolic area (Panel C) with an upward trend in ejection fraction (Panel B). Panel D shows example images of end diastole (left) and end systole (right) from pre (above) and post (below) apelin injection.

Figure 2

Pressure-volume hemodynamics in response to acute apelin infusion. Anesthetized, ventilated C57Bl6 mice underwent placement of a specialized conductance catheter along the long axis of the left ventricle via a sub-diaphragmatic approach (Panel D). The pressure-volume relationship at different levels of preload, facilitated by a 5 second manual occlusion of the inferior vena cava, is illustrated. Loops were recorded at baseline (example series of loops from one animal, Panel A) and following 20 minutes of apelin infusion (example loops from the same animal following apelin, Panel B). Volume is expressed as Relative Volume Units (RVU). Mean data demonstrated that, after apelin infusion, ventricular elastance, the slope of the end systolic pressure-volume relationship, was increased (Panel C, a summary graph illustrating the mean increase in slope and intercept across all animals, p=0.018) along with preload recruitable stroke work (Panel E, mean±sem, p=0.056). Steady state end systolic pressure was lower following apelin indicating a reduction in afterload (Panel F, mean±sem, p=0.02).

Figure 3

Effect of chronic apelin infusion in C57Bl6 mice. Long axis and short axis views of the left ventricle with Doppler sampling of the outflow tract were used to estimate left ventricular contractility in vivo (Panel A). The velocity of circumferential shortening (Panel C) and cardiac output (Panel D) were significantly increased from baseline following two weeks of ^PYRapelin¹³ infusion. Systolic blood pressure as determined by tail cuff was also lower but this did not reach significance (Panel B).

Figure 4

Immunohistochemistry of apelin and APJ in the developing and adult mouse heart. Specific immunolocalization of both proteins in the developing myocardium revealed very similar patterns of expression as early as embryonic day 13.5 (Panels A, B: A – atrium, V – ventricle, li – liver). Real time quantitative RT-PCR of isolated heart mRNA suggested that the relative contribution to the total myocardial RNA by these transcripts remains relatively constant through late gestation and adulthood (Panel C). In the adult mouse heart, immunolocalization of APJ expression was identified in association with both atrial and ventricular myocardial cells (Panels D, E). A control slide without the addition of the secondary antibody is shown in Panel F.