Coronary vascular growth matches IGF-1-stimulated cardiac growth in fetal sheep

Abstract

As loss of contractile function in heart disease could often be mitigated by increased cardiomyocyte number, expansion of cardiomyocyte endowment paired with increased vascular supply is a desirable therapeutic goal. Insulin-like growth factor 1 (IGF-1) administration increases fetal cardiomyocyte proliferation and heart mass, but how fetal IGF-1 treatment affects coronary growth and function is unknown. Near-term fetal sheep underwent surgical instrumentation and were studied from 127 to 134 d gestation (term = 147 d), receiving either IGF-1 LR3 or vehicle. Coronary growth and function were interrogated using pressure-flow relationships, an episode of acute hypoxia with progressive blockade of adenosine receptors and nitric oxide synthase, and by modeling the determinants of coronary flow. The main findings were that coronary conductance was preserved on a per-gram basis following IGF-1 treatment, adenosine and nitric oxide contributed to hypoxia-mediated coronary vasodilation similarly in IGF-1-treated and Control fetuses, and the relationships between coronary flow and blood oxygen contents were similar between groups. We conclude that IGF-1-stimulated fetal myocardial growth is accompanied by appropriate expansion and function of the coronary vasculature. These findings support IGF-1 as a potential strategy to increase cardiac myocyte and coronary vascular endowment at birth.

Keywords: adenosine; angiogenesis; contracture; coronary autoregulation; developmental origins; nitric oxide.

Figure 1.

The experimental timeline.

Figure 2.. Coronary pressure-flow relationship example.

A pressure-flow relationship example from an IGF-1-treated fetal sheep. At day 0 (127 d gestational age [GA]) and at day 7 (134 dGA, term=147 dGA) after chronic IGF-1 treatment, autonomic responses were blocked and vascular occluders were used to transiently increase or decrease coronary driving pressure. Then a maximally-vasodilating dose of adenosine was administered to the coronary circulation, and the pressure manipulations were repeated. The slope of each relationship is the coronary conductance, and the difference between the slopes at a given pressure, on a given day, is the coronary reserve.

Figure 3.. Coronary pressure-flow relationships.

The coronary conductance, normalized to weight of myocardium perfused, was not different between groups at resting “baseline” flow, or maximal hyperemia on A) day 0, or B) day 7 of IGF-1 or vehicle treatment. C) Coronary reserve was reduced within the IGF-1 group between days 0 and 7. Control n=12, IGF-1 n=8. Data were analyzed by 2-way repeated measures ANOVA followed, if justified, by the Holm-Šidák multiple comparisons test (P<0.05). Different from *same-group day 0. Data are shown as mean ± SD.

Figure 4.. Blood O₂ contents and coronary hemodynamics during acute hypoxia on study day 7.

A) Arterial O₂ content (CaO₂) in Control and IGF-1-treated fetal sheep during acute hypoxia, and with adenosine receptor (AR) and nitric oxide synthase (NOS) blockade. B) Coronary sinus O₂ content (CvO₂) during acute hypoxia and maximum AR and NOS blockade. C) The difference between CaO₂ and CvO₂, which is the O₂ extracted by the myocardium. D) Coronary extraction ratio, the percent of O₂ extracted from arterial blood by the myocardium. E) Coronary conductance during acute hypoxia. F) Cardiac work, calculated as the double product. G) Myocardial O₂ delivered as a function of cardiac work during acute hypoxia, and with AR and NOS blockade. H) Myocardial O₂ consumption, and I) myocardial O₂ consumption per work. Number of animals is shown within symbols. Different from *Control, †Hypoxia timepoint within-group by the Holm-Šidák multiple comparisons test following a significant interaction term by 2-way mixed effects analysis (a symbol and bar denotes differences at this level), and significant ‡linear trend across columns (P<0.05). Data are shown as mean ± SD.

Figure 5.. Example of spontaneous transient increases in circumflex coronary flow in a fetal sheep.

A LabChart window displaying (top-bottom) fetal amniotic pressure, arterial pressure (corrected for amniotic pressure), venous pressure (corrected for amniotic pressure), raw coronary flow, mean coronary flow, and heart rate in a fetus of an awake, calmly resting ewe. In this example, fetal coronary flow rises, following an elevation in amniotic compartment pressure (peak-peak interval approximately 4.5 min). Flow is elevated for approximately 11 minutes. Note that perfusion pressure and work did not change, as both arterial and venous pressures followed amniotic pressure, and heart rate did not increase. Time scale for right window, 1 division = 0.5 s; on left 1 division = 60 s.

Figure 6.. O₂ contents and coronary flows obtained during simultaneous sampling of arterial and coronary sinus blood, and measurements of hemodynamics.

A) The relationship between arterial O₂ content (CaO₂) and coronary sinus O₂ content (CvO₂). Coronary flow as a function of B) CaO₂ and C) CvO₂. D) Coronary flow as a function of the difference in between CaO₂ and CvO₂. Individual measurements are plotted as raw data.