Vermeulen, Tyler D.

Our understanding of altered neuro‐cardiovascular control following long‐duration inactivity via head‐down bed rest (HDBR) is incomplete. Here, we retrospectively analyzed two long‐duration HDBR studies, including female (n=17; 32±5 years) and male (n=16; 29±7 years) cohorts, to determine whether HDBR 1) alters spontaneous cardiovagal baroreflex sensitivity (cBRS) similarly between males and females and 2) alters spontaneous baroreflex control of muscle sympathetic action potentials (AP) in males. Heart rate (HR; electrocardiogram) and arterial blood pressure (BP; finger‐plethysmography) were continuously measured during 5‐minutes of supine rest prior to and following 60‐days bed rest in a ‐6° head‐down position. Spontaneous cBRS was assessed by the relationship between R‐R interval and systolic BP analyzed using the linear regression sequence method. Recordings of multi‐unit muscle sympathetic nerve activity (peroneal microneurography) were obtained and a continuous wavelet transform detected APs which were grouped into AP clusters with similar morphology (n=10, all males). Baroreflex threshold for AP cluster discharge was assessed by linear regression analysis of the relationship between AP cluster discharge probability and diastolic BP. No change in resting HR (64±9 versus 66±9 /min, P=0.12) or mean BP (91.2±9.0 versus 90.6±9.2 mmHg, P=0.73) occurred after HDBR. Reductions in cBRS after HDBR (‐4.1±9.4 ms/mmHg, Time effect: P<0.01) were similar between males and females (Time‐by‐Sex effect: P=0.51) and not related to changes in total blood volume (r 2 =0.045, P=0.24). Grouped AP clusters demonstrated heterogenous arterial baroreflex threshold gain according to AP cluster morphology (Cluster effect: P<0.01) which was unaltered following HDBR (Time‐by‐Cluster effect: P=0.43). In summary, males and females similarly attenuate cBRS following 60‐days of HDBR and novel evidence indicates HDBR does not alter spontaneous baroreflex control of muscle sympathetic APs in males.

AbstractReducing the work of breathing during exercise improves locomotor muscle blood flow and reduces diaphragm and locomotor muscle fatigue and is thought to be the result of a sympathetically mediated reflex.AimThe aim of this study was to assess muscle sympathetic nerve activity (MSNA) when the work of breathing is experimentally lowered during dynamic exercise.MethodsHealthy subjects (n = 12; age = 29 ± 9 years) performed semi‐recumbent cycling trials at 40%, 60%, and 80% of peak workload. Exercise trials consisted of spontaneous breathing, reduced work of breathing (proportional assist ventilator), followed by further spontaneous breathing (post‐ventilator). MSNA was recorded from the median nerve.ResultsThere was no difference in work of breathing between PAV and post‐PAV at 40% peak work. At 60% peak work, the ventilator significantly (P < 0.05) reduced work of breathing (103 ± 39 vs 144 ± 47 J min−1), sympathetic nerve activity (35 ± 5 vs 42 ± 8 burst min−1), and (2.4 ± 0.5 vs 2.6 ± 0.5 L min−1) without influencing ventilation (86 ± 9 vs 82 ± 10 L min−1; P > 0.05), for PAV and post‐PAV respectively. During 80% peak work (n = 8), the ventilator significantly (P < 0.05) reduced work of breathing (235 ± 110 vs. 361 ± 150 J min−1), MSNA (48 ± 7 vs 54 ± 11 burst min−1), and (2.9 ± 0.6 vs 3.2 ± 0.7 L min−1) but not ventilation (121 ± 20 vs 123 ± 20 L min−1; P > 0.05), for PAV and post‐PAV respectively. There was a significant relationship between MSNA and (P < 0.0001) with a significant interaction due to the ventilator (P < 0.05).ConclusionLowering the normally occurring work of breathing during exercise results in commensurate reductions in MSNA. Our findings provide evidence of a sympathetically mediated vasoconstrictor effect emanating from respiratory muscles during exercise.