Abstract
Exercise facilitates cerebral lactate uptake, likely by increasing arterial lactate concentration and hence the diffusion gradient across the blood-brain barrier. However, nonspecific β-adrenergic blockade by propranolol has previously reduced the arterio-jugular venous lactate difference (AVLac) during exercise, suggesting β-adrenergic control of cerebral lactate uptake. Alternatively, we hypothesized that propranolol reduces cerebral lactate uptake by decreasing arterial lactate concentration. To test that hypothesis, we evaluated cerebral lactate uptake taking changes in arterial concentration into account. Nine healthy males performed incremental cycling exercises to exhaustion with and without intravenous propranolol (18.7 ± 1.9 mg). Lactate concentration was determined in arterial and internal jugular venous blood at the end of each workload. To take changes in arterial lactate into account, we calculated the fractional extraction (FELac) defined as AVLac divided by the arterial lactate concentration. Arterial lactate concentration was reduced by propranolol at any workload (P < 0.05), reaching 14 ± 3 and 11 ± 3 mmol·l−1 during maximal exercise without and with propranolol, respectively. Although AVLac and FELac increased during exercise (both P < 0.05), they were both unaffected by propranolol at any workload (P = 0.68 and P = 0.26) or for any given arterial lactate concentration (P = 0.78 and P = 0.22). These findings support that while propranolol may reduce cerebral lactate uptake, this effect reflects the propranolol-induced reduction in arterial lactate concentration and not inhibition of a β-adrenergic mechanism within the brain. We hence conclude that cerebral lactate uptake during exercise is directly driven by the increasing arterial concentration with work rate.
NEW & NOTEWORTHY During exercise the brain consumes lactate as a substitute for glucose. Propranolol has previously attenuated this cerebral lactate uptake, suggesting a β-adrenergic transport mechanism. However, in the present study, we demonstrate that the fractional extraction of arterial lactate by the brain is unaffected by propranolol throughout incremental exercise to exhaustion. We conclude that cerebral lactate uptake during exercise is passively driven by the increasing arterial concentration, rather than by a β-adrenergic mechanism within the brain.