Are Sex and Estrogen Modulators of Sympathetic Vascular Control in Resting and Contracting Skeletal Muscle?

The purpose of this thesis was to investigate whether sex and estrogen modulate sympathetic neurovascular control in resting and contracting skeletal muscle. This thesis also investigated the hypothesis that the effects of sex and estrogen on neurovascular control would be mediated by the signal molecule nitric oxide (NO). NO has many biological functions and has been shown to be a potent vasodilator, an inhibitor of sympathetic vasoconstriction in resting and contracting skeletal muscle (sympatholysis) and an inhibitor of efferent sympathetic nerve activity (SNA). NO is produced by the endothelial (eNOS), neuronal (nNOS) and inducible (iNOS) isoforms of the enzyme NO synthase (NOS). NOS enzyme expression appears to be sensitive to estrogen bioavailability and therefore NO bioavailability and the physiological functions of NO may be modulated by sex and estrogen status. An in vivo anesthetized rat preparation and vascular pharmacology were utilized in wild-type male and female rats and in female rats that had undergone a surgical procedure to chronically alter estrogen status. Vascular conductance was measured in the rat hindlimb at rest and during muscle contraction. The lumbar sympathetic chain was instrumented to evoke sympathetic vasoconstriction of the hindlimb skeletal muscle vasculature (Study 1 and 2) or record efferent SNA directed to the hindlimb skeletal muscle vasculature (Study 3) at rest and during electrically stimulated muscle contractions of the tricep surae muscle group. Study 1 investigated the hypothesis that sex modulates sympathetic vascular control, such that sympathetic vasoconstrictor responsiveness would be blunted and NO-mediated inhibition of sympathetic vasoconstriction would be augmented in female compared to male rats. Muscle contraction-mediated inhibition of sympathetic vasoconstriction (sympatholysis) was augmented in females compared to male rats. NOS blockade reduced sympatholysis in female, but not male rats, suggesting that the enhanced sympatholysis in female rats was partially mediated by NO. Study 2 investigated the hypothesis that estrogen status alters sympathetic vasoconstrictor responsiveness and modulates the contributions of nNOS- and eNOS derived NO to NO-mediated inhibition of sympathetic vasoconstriction at rest and during muscle contraction. Estrogen status did not alter vasoconstrictor responsiveness or sympatholysis in ovary intact (OI), ovariectomized (OVX) or ovariectomized and estrogen replaced (OVXE) female rats. Selective blockade of nNOS also did not alter vasoconstrictor responsiveness or sympatholysis, suggesting that nNOS-derived NO was not involved in NO-mediated inhibition of sympathetic vasoconstriction in female rats, regardless of estrogen status. Subsequent, non-selective NOS blockade reduced sympatholysis in all groups by a similar magnitude, suggesting that eNOS-mediated NO production was responsible for NO-mediated sympatholysis in female rats and was not influenced by estrogen status. Study 3 addressed the hypothesis that estrogen bioavailability would alter MSNA at rest and during muscle contraction through an NO dependent mechanism. Sympathetic nerve activity was recorded from the lumbar sympathetic chain (LSNA) in OI, OVX and OVXE rats at rest and during muscle contraction before and after non-selective NOS inhibition. Estrogen status did not alter efferent LSNA at rest or during muscle contraction. NOS inhibition elevated blood pressure and reduced efferent LSNA at rest and during muscle contraction in all groups. However, when blood pressure was normalized (to levels measured before NOS inhibition) by infusion of a vasodilator (hydralazine), LSNA at rest and during contraction were not different from LSNA measured in the control condition. These data suggest that efferent LSNA at rest and in response to muscle contraction was not a function of estrogen status and systemic NOS inhibition does not appear to alter efferent LSNA at rest and during exercise. This thesis provides evidence that sex modulates sympathetic vascular control and that sex differences in the regulation of sympathetic vasoconstriction in skeletal muscle are partly attributable to sex differences in NO mediated vascular control. Estrogen bioavailability did not alter NO-mediated vascular control or efferent LSNA at rest or in response to exercise. Thus, these data provide critical evidence that challenges the assertion that estrogen modulates NO bioavailability and sympathetic neurovascular control at rest and during exercise through an NO-dependent mechanism. These findings advance our understanding of how biological sex and estrogen influence the integrative control of blood pressure and skeletal muscle vascular resistance at rest and during exercise. The present findings have implication for future studies of vascular control in aging and conditions of altered estrogen status, such as menopause.