As an athlete, you might be excused for not knowing what adenosine triphosphate (ATP) is. After all, not many of us are chemists or biologists. That I am not either will become evident below.
ATP is a nucleotide triphosphate and related to the molecules that make up our DNA and RNA, the building blocks of life. More importantly for us, perhaps, is that APT captures chemical energy from the foods we eat and converts it into energy for cellular functions, such as when our muscles contract during exercise. When you go for a run, cycle or swim, lift weights, ski or climb mountains, ATP in your muscles produces the energy to sustain your activity.
ATP requires oxygen, so as your exercise intensity increases, so does the need for more oxygen in your muscle cells. The most readily available source of that oxygen is from your blood. As your muscles use up more ATP energy, waste products such as lactic acid and carbon dioxide begin to accumulate. Chemical receptors, sensitive to pH in the muscle cells, detect the increasing acidity caused by the build-up of lactic acid and carbon dioxide. Sensory nerve fibers associated with the sympathetic nervous system – mechanoreceptors and metaboreceptors – carry messages from the muscles to the medulla oblongata in the brain stem, which is responsible for cardiovascular and respiratory regulation. This triggers the release of adrenaline (epinephrine) from the adrenal glands. The adrenaline binds to the Beta-1 adrenergic receptors of the heart muscle which increases the flow of calcium to the cells of the heart muscle. The calcium causes the cells to depolarize and repolarize more rapidly, which makes the heart beat faster and stronger. Consequently, more oxygen is delivered to the muscles to meet the exercise intensity demands. At the same time, the respiratory center in the brain responds to the levels of decreasing oxygen and increasing carbon dioxide in the bloodstream. Your rate of breathing increases, expelling more carbon dioxide and increasing oxygen intake into the bloodstream. The breakdown of the ATP in the muscle cells releases adenosine which acts like a vasodilator to increase blood flow in the vascular system. And you keep pushing ahead, remembering to fuel and hydrate so the ATP won’t go hungry.
But what if you are taking a beta blocker such as bisoprolol? Since the beta blocker binds to the Beta-1 adrenergic receptors of the heart muscle, it prevents the released adrenaline from doing so. As the dosage of beta blocker increases, a higher percentage of the receptors are blocked. Blocked receptors prevent the heart from beating as fast (negative chronotropic effect) or as strongly (negative inotropic effect) as it might have otherwise. Without that increase in the delivery of oxygen and nutrients, your muscles might start feeling tired and sluggish. Restricted from receiving enough oxygen and nutrients to meet the exercise demands, less ATP energy is produced and your muscles “bonk.” As well, the levels of carbon dioxide and lactic acid remain high, and the muscles may begin to ache. Muscle fatigue sets in.
You might also find that breathing is a challenge. To compensate for the mismatch between oxygen supply and demand, you may start breathing faster and feel short of breath. However, because the heart isn’t pumping sufficient oxygen to supply the muscles in the quantity or speed necessary, you’re left panting. Your lungs are doing their part but your heart isn’t keeping up the pace.
The bright side is that beta blockers protect the heart. As for their side effects, with consistent and progressive training, and patience, the mitochondria in the muscle cells can become more numerous and therefore better utilize the oxygen that is available for ATP production. Training can also make the mitochondria become more efficient at extracting oxygen from the blood, from about 25% at rest to over 75% during strenuous exercise, depending on your fitness level. Over time, the heart also adapts by increasing stroke volume, pumping more blood per beat. The heart and muscles can adapt to the mechanism beta blockers use to protect the heart, for many people, when they do what they do best: stubbornly persisting in their training. The extent of that adaptation depends on the type of beta blocker — selective or non-selective — and the dosage, and your unique physiological characteristics and determination.
So your running and cycling friends may smirk when climbing a hill, share their heart rates and yours is the lowest, as if that’s an advantage or an artificial sign of a strong cardiovascular system. You’re working harder at the low heart rate.