Vasoregulatory Responses To Sudden Changes In Forearm Perfusion Pressure During Moderate Forearm Exercise

Abstract

Exercising muscle blood flow is thought to be tightly regulated to metabolic demand. The time course of vasoregulatory responses to a disruption of steady state exercising muscle blood flow remains unclear. PURPOSE We tested the hypothesis that sudden changes in local forearm perfusion pressure during moderate intensity forearm handgrip exercise would alter forearm blood flow (FBF) and result in rapid compensatory changes in forearm vascular conductance (FVC, vasoregulatory responses) to restore FBF. METHODS 12 healthy young subjects (7 female,5 male) lay supine and were instrumented for brachial artery FBF (Doppler ultrasound), arterial blood pressure (ABP; arterial tonometry) and heart rate (HR;ECG). Exercise consisted of 30% MVC rhythmic isometric handgrip contractions (2 s /2 s contraction/relaxation duty cycle). Rapid changes in arm position between above (AH) and below (BH) heart level (forearm perfusion pressure difference of ∼30 mmHg) during steady state exercise occurred during a 2 s contraction period. Exercise then continued for 4 min in the new arm position. Vasoregulatory responses were examined by analysing cardiac cycles during relaxation between contractions. FVC during exercise was calculated using forearm level ABP. RESULTS AH to BH transition: an immediate increase in FBF (ml/min ±SE) (419.5±56.6 to 708.7±68.5, p < 0.05) and FVC (602.1±94.4 to 709.5±85.4, p < 0.05) was followed by rapid restoration of FVC (ml/min/100 mmHg ±SE) (609.6±88.9, P=1.00) with no further change, and incomplete restoration of FBF (597.0±72.1, p < 0.001). BH to AH transition:an immediate decrease in FBF (551.0±68.6 to 349.1±52.7, p < 0.001) and FVC (540.0±71.3 to 468.3±74.3, P=0.017) was followed by rapid restoration of FVC (511.8±68.5, P=0.970) with no further change, and incomplete restoration of FBF (409.6±54.3, p < 0.001). CONCLUSION Changes in perfusion pressure during exercise result in an immediate change in FBF and perfusion pressure-mediated changes in resistance vessel distension (altered FVC). Rapid acting vasoregulatory mechanism(s) (likely myogenic) respond to restore FVC, with no further vasoregulatory adjustments such that FBF is not restored. Supported by NSERC; Canada Foundation for Innovation; Ontario Innovation Trust