Oral L-tyrosine supplementation augments the vasoconstriction response to whole body cooling in older adults

Abstract

L-tyrosine is the primary substrate for noradrenaline biosynthesis within sympathetic axon terminals. Under stressful conditions requiring increased catecholamine production, axonal L-tyrosine concentration may limit the full expression of the sympathetic effector response and this may be particularly evident in older adults. We hypothesize that oral L-tyrosine supplementation will increase the sympathetic response to whole-body cooling and muscle metaboreflex activation. In a randomized, double-blind design, eleven young (Y = 24 ± 1 years) and eleven older (O = 68 ± 4 years) participants ingested either 150 mg kg⁻¹ of L-tyrosine or placebo prior to commencing 30 minutes of whole-body cooling to induce a gradual decline in skin temperature from 34 to 30.5°C. Laser Doppler flux (LDF) was measured at the ventral forearm and cutaneous vascular conductance (CVC) was calculated as CVC = LDF/mean arterial pressure and expressed as a percent change from baseline (%ΔCVC). Two minutes of static handgrip exercise (35% MVC) followed by 3 minutes of post-exercise ischaemia was implemented before and toward the end of the cooling bout. Tyrosine supplementation did not affect blood pressure or heart rate responses to exercise or post-exercise ischaemia. However, the blunted VC response to whole-body cooling in older adults (Placebo: Y = 39 ± 5; O = 16 ± 2 %ΔCVC; P < 0.05) was augmented after tyrosine supplementation (Tyrosine: Y = 40 ± 4; O = 32 ± 5 %ΔCVC; P < 0.05). These results suggest that L-tyrosine bioavailability may limit thermoregulatory function in an older population.

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