Rationale Catecholamines increase cardiac contractility, but exposure to high concentrations or prolonged exposures can cause cardiac injury. A recent study demonstrated that a single subcutaneous injection of isoproterenol (ISO; 200 mg/kg) in mice causes acute myocyte death (8-10%) with complete cardiac repair within a month. Cardiac regeneration was via endogenous cKit+ cardiac stem cell (CSC)-mediated new myocyte formation. Objective Our goal was to validate this simple injury/regeneration system and use it to study the biology of newly forming adult cardiac myocytes. Methods and Results C57BL/6 mice (n=173) were treated with single injections of vehicle, 200mg/kg or 300mg/kg ISO, or with two daily doses of 200mg/kg ISO for 6 days. Echocardiography revealed transiently increased systolic function and unaltered diastolic function 1 day after single ISO injection. Single ISO injections also caused membrane injury in about 10% of myocytes but few of these myocytes appeared to be necrotic. Circulating troponin I levels after ISO were elevated, further documenting myocyte damage. However, myocyte apoptosis was not increased after ISO injury. Heart weight to body weight ratio and fibrosis were also not altered 28 days after ISO injection. Single or multiple dose ISO injury was not associated with an increase in the percentage of 5-ethynyl-2’-deoxyuridine (EdU)-labeled myocytes. Furthermore, ISO injections did not increase new myocytes in cKit+/Cre × R-GFP transgenic mice. Conclusions A single dose of ISO causes injury in about 10% of the cardiomyocytes. However, most of these myocytes appear to recover and do not elicit cKit+ cardiac stem cell (CSC)-derived myocyte regeneration.

This “Controversies in Cardiovascular Research” article evaluates the evidence for and against the hypothesis that the circulating blood level of Growth Differentiation Factor 11 (GDF11) decreases in old age and that restoring normal GDF11 levels in old animals rejuvenates their skeletal muscle and reverses pathological cardiac hypertrophy and cardiac dysfunction. Studies supporting the original GDF11 hypothesis in skeletal and cardiac muscle have not been validated by a number of independent groups. These new studies have either found no effects of restoring normal GDF11 levels on cardiac structure and function or have shown that increasing GDF11 or its closely related family member GDF8 actually impairs skeletal muscle repair in old animals. One possible explanation for what appears to be mutually exclusive findings is that the original reagent used to measure GDF11 levels also detected many other molecules so that age dependent changes in GDF11 are still not well known. The more important issue is if increasing blood [GDF11] repairs old skeletal muscle and reverses age-related cardiac pathologies. There are substantial new and existing data showing that GDF8/11 can exacerbate rather than rejuvenate skeletal muscle injury in old animals. There is also new evidence disputing the idea that there is pathological hypertrophy in old C57bl6 mice and that GDF11 therapy can reverse cardiac pathologies. Finally, high [GDF11] causes reductions in body and heart weight in both young and old animals, suggestive of a cachexia effect. Our conclusion is that elevating blood levels of GDF11 in the aged might cause more harm than good.

Rationale: Possible beneficial effects of Growth Differentiation Factor 11 (GDF11) on the normal, diseased, and aging heart have been reported, including reversing aging induced hypertrophy. These effects have not been well validated. High levels of GDF11 have also been shown to cause cardiac and skeletal muscle wasting. These controversies could be resolved if dose-dependent effects of GDF11 were defined in normal and aged animals as well as in pressure overload induced pathological hypertrophy. Objective: To determine dose-dependent effects of GDF11 on normal hearts and those with pressure overload induced cardiac hypertrophy. Methods and Results: 12–13-week-old C57BL/6 mice underwent transverse aortic constriction (TAC) surgery. One-week post TAC, these mice received recombinant GDF11 at one of 3 doses: 0.5 mg/kg, 1.0 mg/kg, or 5.0 mg/kg for up to 14 days. Treatment with GDF11 increased plasma concentrations of GDF11 and p-SMAD2 in the heart. There were no significant differences in the peak pressure gradients across the aortic constriction between treatment groups at one-week post-TAC. Two weeks of GDF11 treatment caused dose-dependent decreases in cardiac hypertrophy as measured by HW/TL ratio, myocyte cross sectional area, and LV mass. GDF11 improved cardiac pump function while preventing TAC-induced ventricular dilation and caused a dose-dependent decrease in interstitial fibrosis (in vivo), despite increasing markers of fibroblast activation and myofibroblast transdifferentiation (in vitro). Treatment with the highest dose (5.0mg/kg) of GDF11 caused severe body weight loss, with significant decreases in both muscle and organ weights and death in both sham and TAC mice. Conclusions: Although GDF11 treatment can reduce pathological cardiac hypertrophy and associated fibrosis while improving cardiac pump function in pressure overload, high doses of GDF11 cause severe cachexia and death. Use of GDF11 as a therapy could have potentially devastating actions on the heart and other tissues.