Randomized controlled trials - mechanistic studies of testosterone and the cardiovascular system

Testosterone deficiency is common in men with cardiovascular disease (CVD), and randomized placebo-controlled trials (RCTs) have reported beneficial effects of testosterone therapy on exercise-induced cardiac ischemia in chronic stable angina, functional exercise capacity, maximum oxygen consumption during exercise (VO_2max) and muscle strength in chronic heart failure (CHF), shortening of the Q-T interval, and improvement of some cardiovascular risk factors. Testosterone deficiency is associated with an adverse CV risk profile and mortality. Clinical and scientific studies have provided mechanistic evidence to support and explain the findings of the RCTs. Testosterone is a rapid-onset arterial vasodilator within the coronary circulation and other vascular beds including the pulmonary vasculature and can reduce the overall peripheral systemic vascular resistance. Evidence has demonstrated that testosterone mediates this effect on vascular reactivity through calcium channel blockade (L-calcium channel) and stimulates potassium channel opening by direct nongenomic mechanisms. Testosterone also stimulates repolarization of cardiac myocytes by stimulating the ultra-rapid potassium channel-operated current. Testosterone improves cardiac output, functional exercise capacity, VO_2maxand vagally mediated arterial baroreceptor cardiac reflex sensitivity in CHF, and other mechanisms. Independent of the benefit of testosterone on cardiac function, testosterone substitution may also increase skeletal muscle glucose metabolism and enhance muscular strength, both factors that could contribute to the improvement in functional exercise capacity may include improved glucose metabolism and muscle strength. Testosterone improves metabolic CV risk factors including body composition, insulin resistance, and hypercholesterolemia by improving both glucose utilization and lipid metabolism by a combination of genomic and nongenomic actions of glucose uptake and utilization expression of the insulin receptor, glucose transporters, and expression on regulatory enzymes of key metabolic pathways. The effect on high-density lipoprotein-cholesterol (HDL-C) differs between studies in that it has been found to fall, rise, or have no change in levels. Testosterone replacement can suppress the levels of circulating pro-inflammatory cytokines and stimulate the production of interleukin-10 (IL-10) which has anti-inflammatory and anti-atherogenic actions in men with CVD. No effect on C-reactive protein has been detected. No adverse effects on clotting factors have been detected. RCTs have not clearly demonstrated any significant evidence that testosterone improves or adversely affects the surrogate markers of atherosclerosis such as reduction in carotid intima thickness or coronary calcium deposition. Any effect of testosterone on prevention or amelioration of atherosclerosis is likely to occur over years as shown in statin therapy trials and not months as used in testosterone RCTs. The weight of evidence from long-term epidemiological studies supports a protective effect as evidenced by a reduction in major adverse CV events (MACEs) and mortality in studies which have treated men with testosterone deficiency. No RCT where testosterone has been replaced to the normal healthy range has reported a significant benefit or adverse effect on MACE nor has any recent meta-analysis.