Experimental Physiology –Research Paper: Glycopyrrolate abolishes the exercise‐induced increase in cerebral perfusion in humans

Abstract

Brain blood vessels contain muscarinic receptors that are important for cerebral blood flow (CBF) regulation, but whether a cholinergic receptor mechanism is involved in the exercise‐induced increase in cerebral perfusion or affects cerebral metabolism remains unknown. We evaluated CBF and cerebral metabolism (from arterial and internal jugular venous O2, glucose and lactate differences), as well as the middle cerebral artery mean blood velocity (MCA Vmean; transcranial Doppler ultrasound) during a sustained static handgrip contraction at 40% of maximal voluntary contraction (n= 9) and the MCA Vmean during ergometer cycling (n= 8). Separate, randomized and counterbalanced trials were performed in control (no drug) conditions and following muscarinic cholinergic receptor blockade by glycopyrrolate. Glycopyrrolate increased resting heart rate from ∼60 to ∼110 beats min−1 (P < 0.01) and cardiac output by ∼40% (P < 0.05), but did not affect mean arterial pressure. The central cardiovascular responses to exercise with glycopyrrolate were similar to the control responses, except that cardiac output did not increase during static handgrip with glycopyrrolate. Glycopyrrolate did not significantly affect cerebral metabolism during static handgrip, but a parallel increase in MCA Vmean (∼16%; P < 0.01) and CBF (∼12%; P < 0.01) during static handgrip, as well as the increase in MCA Vmean during cycling (∼15%; P < 0.01), were abolished by glycopyrrolate (P < 0.05). Thus, during both cycling and static handgrip, a cholinergic receptor mechanism is important for the exercise‐induced increase in cerebral perfusion without affecting the cerebral metabolic rate for oxygen.