Margaret D. Brown

Angiogenesis and improved left ventricular function as a consequence of long-term bradycardia were first demonstrated in normal hearts, either electrically paced (rabbits, pigs) or treated with a selective sinus blocking drug alinidine (rats). Here we review the evidence that chronic heart rate reduction can have similar effects in the heart with compromised vascular supply, due to either hypertensive or haemodynamic overload hypertrophy (rats, rabbits) or ischaemic damage (rats, rabbits, pigs). Bradycardia induced over several weeks increased capillarity in all hypertrophied hearts, and in border and remote left ventricular myocardium of infarcted hearts. In some, but not all cases, coronary blood flow was improved by heart rate reduction, suggesting enlargement of the resistance vasculature in some circumstances. Cardiac or left ventricular function indices, which were depressed by hypertrophy or ischaemic damage, were preserved or even enhanced by chronic heart rate reduction. The expansion of the capillary bed in the vascularly compromised heart induced by bradycardia may be stimulated by mechanical stretch of the endothelium and/or VEGF activated by chamber dilation and myocyte stretch. The increased number of capillaries and more homogeneous distribution of capillary perfusion would support the better pump function, even in the absence of higher coronary flow. The beneficial impact of chronic heart rate reduction on myocardial angiogenesis and function in cardiac hypertrophy and infarction may be major factor in the success of beta-blockers in treatment of human heart failure.

Objectives: To develop a model of peripheral arterial disease (PAD) in rat skeletal muscle with sustained impairment of microcirculatory perfusion, and to ascertain whether increased muscle activity can reverse the impairment.Methods: Three weeks after iliac ligation in rats, the ipsilateral femoral artery was ligated (double ligation, DL), and in some animals, muscle activity was increased by electrical stimulation for 2 weeks (10 Hz, 15 min on, 85 mins off, 7 times per day). Diameter changes of precapillary arterioles to vasoactive agonists and capillary perfusion (flow intermittency, capillary red cell velocity [V_rbc], and diameters) were measured in extensor digitorum longus muscle and compared with 5 weeks iliac only ligation (single ligation, SL) and controls. Total muscle endothelial nitric oxide synthase (eNOS) was estimated by Western blotting.Results:Whereas single ligation increased intermittency of capillary flow with little effect on V_rbc and shear stress, DL completely eliminated increases in V_rbc and shear stress after muscle contractions. Arterial dilation to sodium nitroprusside was attenuated similarly in SL and DL; in SL, acetylcholine induced constriction and bradykinin an attenuated dilation, but in DL vessels were unresponsive to either. Chronic stimulation returned all microcirculatory parameters in DL to normal and increased levels of eNOS protein by 75%.Conclusions: Femoral artery ligation following iliac ligation impairs arteriolar vasodilator capacity, capillary perfusion, and shear‐dependent function of microcirculatory endothelium more than iliac ligation alone and is more representative of long‐standing ischemia in PAD. Chronic intermittent electrical stimulation can normalize these derangements.

To establish the time course and extent of remodeling of terminal microcirculation in ischemic rat skeletal muscle during prolonged low flow that does not lead to inflammation. One common iliac artery was ligated via laparotomy in adult Sprague-Dawley rats and extensor digitorum longus (EDL) muscles removed at intervals (1, 2, and 5 weeks) postsurgery. Serial frozen EDL sections were stained to show capillaries (alkaline phosphatase), cell proliferation (antibody to proliferating cell nuclear antigen [PCNA]), terminal microvessels (antibodies to alpha-smooth muscle actin (alpha-SMA) or endothelial nitric oxide synthase [eNOS]), and macrophages (antibodies to infiltrating and resident macrophages). Total muscle eNOS protein was quantified by standard Western blotting techniques. Capillary proliferation was very limited in ischemic EDLs, with a modest 12% increase in the capillary/fiber ratio after 5 weeks, preceded at 2 weeks by increased numbers of PCNA-positive nuclei at capillary sites. There was no muscle necrosis or evidence of inflammation, based on macrophage staining. The number of terminal microvessels that were positive for alpha-SMA and <10 microm in diameter was fewer in ischemic EDLs at all time points, whereas the number of larger positive vessels was unchanged. eNOS-positive vessels <10 microm in diameter were stained similarly throughout ischemic muscles as the controls, and showed a similar increase in vessel/fiber ratio as the capillaries. The total eNOS protein level was similar to that in controls in ischemic EDLs after 1 and 2 weeks, but was 28% lower after 5 weeks. Prolonged, moderate flow reduction to skeletal muscles does not necessarily lead to inflammation or extensive capillary growth. Based on eNOS staining, the terminal microcirculation remains intact, but the loss of alpha-SMA immunoreactivity may indicate remodeling involving the "deinvestment" of microvessels by smooth muscle.