Changes in forearm muscle temperature alter renal vascular responses to isometric handgrip

Abstract

The purpose of the present study was to examine the effect of heating and cooling the forearm muscles on renal vascular responses to ischemic isometric handgrip (IHG). It was hypothesized that heating and cooling the forearm would augment and attenuate, respectively, renal vascular responses to IHG. Renal vascular responses to IHG were studied during forearm heating at 39°C ( n = 15, 26 ± 1 yr) and cooling at 26°C ( n = 12, 26 ± 1 yr). For a control trial, subjects performed the experimental protocol while the forearm was normothermic (∼34°C). Muscle temperature (measured by intramuscular probe) was controlled by changing the temperature of water cycling through a water-perfused sleeve. The experimental protocol was as follows: 3 min at baseline, 1 min of ischemia, ischemic IHG to fatigue, and 2 min of postexercise muscle ischemia. At rest, renal artery blood velocity (RBV; Doppler ultrasound) and renal vascular conductance (RVC = RBV/mean arterial blood pressure) were not different between normothermia and the two thermal conditions. During ischemic IHG, there were greater decreases in RBV and RVC in the heating trial. However, RBV and RVC were similar during postexercise muscle ischemia during heating and normothermia. RVC decreased less during cooling than in normothermia while the subjects performed the ischemic IHG protocol. During postexercise muscle ischemia, RVC was greater during cooling than in normothermia. These results indicate that heating augments mechanoreceptor-mediated renal vasoconstriction whereas cooling blunts metaboreceptor-mediated renal vasoconstriction.

Modulation of muscle sympathetic nerve activity to muscle heating during dynamic exercise

“…The main finding of this study was that blood flow modulates the effect of muscle heating on MSNA responses to DHG (study 1). However, consistent with our previous findings using isometric handgrip (12,20), muscle heating augmented MSNA responses during ischemic DHG (study 2).…”

“…By occluding blood flow during exercise, we were able to significantly augment exercise-mediated increases in MSNA during muscle heating, and thus confirm our previous results (12,20). Moreover, our results clearly implicate blood flow as a potent modifier of heat-related MSNA responses during dynamic exercise.…”

“…We have previously reported that both muscle heating and cooling alters MSNA responses to ischemic forearm exercise (20,21). Muscle heating augmented exercise-induced increases in MSNA, whereas cooling delayed the increases in MSNA (12,20,21). With muscle heating, the augmented response appeared to be mediated by sensitization of muscle mechanoreceptors by increased metabolite production.…”

“…We have demonstrated that muscle heating increases MSNA responses to ischemic isometric handgrip (IHG) (12,20). In addition to elevated MSNA responses, mean arterial pressure (MAP) and heart rate (HR) were augmented during IHG.…”

Modulation of muscle metaboreceptor activation upon sweating and cutaneous vascular responses to rising core temperature in humans

“…Therefore, we speculate that the afferent signals from muscle metaboreceptors would augment the activity of thermoregulatory center when this center is already activated by increasing core or skin temperatures. In addition, it has been suggested that an elevation of muscle temperature would augment the responsiveness of muscle afferents (26,39), implying that the elevated muscle temperature associated with passive heating would influence sweating response caused by muscle metaboreceptors in the present study.…”