Relationship between breathing and cardiovascular function at rest: sex-related differences

AIM: To compare relationships at rest between breathing rate, levels of muscle sympathetic nerve activity, total peripheral resistance and cardiac output among young men and women. METHODS: Recordings were made of respiratory movements, sympathetic nerve activity (peroneal microneurography), intra-arterial blood pressure, electrocardiogram, cardiac output (open-circuit acetylene uptake technique) in 19 healthy men (age 27 ± 2 years, mean ± SEM) and 17 healthy women (age 25 ± 1 years). Total peripheral resistance and stroke volume were calculated. Four minutes epochs of data were analysed. RESULTS: Breathing rates and sympathetic activity were similar in men and women but compared to men, women had significantly lower blood pressures, cardiac output and stroke volume. In men breathing rate correlated positively with sympathetic activity (r = 0.58, P < 0.05) but not in women (r = 0.12, P > 0.05). Furthermore, in men, respiratory rate correlated positively with total peripheral resistance (r = 0.65, P < 0.05) and inversely with cardiac output (r = −0.84, P < 0.05) and heart rate (r = −0.60, P < 0.05) but there were no such relationships in women (P > 0.05 for all). CONCLUSIONS: The positive relationship between breathing and sympathetic activity in men, and the inverse coupling of breathing to cardiac output and heart rate suggest that influences of respiration may be important not only for dynamic but also for ‘tonic’ cardiovascular function. The lack of relationships among these variables in women shows that there are fundamental differences in basic blood pressure regulation between the sexes

Blood pressure regulation in humans: calculation of an "error signal" in control of sympathetic nerve activity

Within an individual, diastolic blood pressure is negatively related to sympathetic burst incidence such that lower pressure is associated with high burst incidence. Our goal was to explore the utility of a calculation of a diastolic blood pressure “error signal” in the control of muscle sympathetic nerve activity in men and women. Baseline muscle sympathetic nerve activity was measured in healthy young men (n=22) and women (n=28). Women had significantly lower muscle sympathetic nerve activity than men (29 ± 3 vs 43 ± 2 bursts/100 heartbeats, p < 0.05). For each individual, the diastolic blood pressure at which there is a 50% likelihood of a muscle sympathetic nerve activity burst, the “T50” value, was calculated. Mean diastolic blood pressure was subtracted from the T50 blood pressure as an approximate error signal for burst activation. Error signal was negative in both sexes, indicating that diastolic blood pressure in both genders was higher than the diastolic blood pressure value associated with a 50% burst likelihood. However, average error signal was significantly larger in women (−4 ± 2 mmHg) than in men (−1 ± 0 mmHg; p < 0.05 vs women). We conclude that women operate at a mean diastolic blood pressure greater than their T50 compared to men and this may be a contributing factor to low basal muscle sympathetic nerve activity in women. The relationship between error signal and burst incidence may provide important insight into control of muscle sympathetic nerve activity across genders and in various populations

Mechanisms of augmented vasoconstriction induced by 5-hydroxytryptamine in aortic rings from spontaneously hypertensive rats

Background and purpose: To test whether development of enhanced vasoconstriction to 5-hydroxytryptamine (5-HT; serotonin) in SHR was temporally related to hypertension, elevated vascular superoxide (O2) levels, decreased NO bioavailability, or increased contractile effects of cyclooxygenase or rho-kinase and/or PKC. Experimental approach: We examined systolic blood pressure (SBP), vascular O2, and 5-HT-induced contractile responses of aortic segments from 4- and 8-week-old WKY and SHR. Key results: SBP was 35% higher in SHR than WKY at 4 weeks and 60% higher at 8 weeks. Contractile responses to 5-HT were similar in WKY and SHR at 4 weeks, but were markedly augmented in SHR at 8 weeks. The NO synthase inhibitor, L-NAME, enhanced contractile responses to 5-HT markedly in both strains at 4 weeks and in WKY at 8 weeks, but only very modestly in SHR at 8 weeks. These functional differences were associated with higher O2 levels in SHR versus WKY at 8 weeks, but not at 4 weeks. The rho-kinase inhibitor, Y-27632, and the PKC inhibitor, Ro 31-8220, each only modestly attenuated contractions in WKY and SHR in each age group, and their effects in each strain were more pronounced at 8 weeks. The cyclooxygenase inhibitor, indomethacin, had no effect on contractile responses. Conclusions and implications: Development of augmented vascular contractile responses to 5-HT in SHR is preceded by hypertension. It is associated with increased vascular O2 levels and reduced modulatory effects of NO, and is unlikely to be due to enhanced activity of rho-kinase, PKC or cyclooxygenase