Sympathetic modulation of blood pressure variability

Although sympathetic nervous activity (SNA) displays oscillations synchrono us with the heart beat and respiration, and between 0.1 -0.4 Hz, it is appa rent that each of these frequencies does not have the same effect on the va sculature. Frequencies above 1 Hz do not produce oscillations in the vascul ature but instead contribute to the mean level of vasoconstriction. Slower oscillations in SNA result in a cycle of vasoconstriction and vasodilation within the vasculature, the amplitude of which, generally decreases with in creasing frequency. Some studies indicate that within the same species, dif ferences exist in the frequency responses between vascular beds, such as th e skin and gut This differential responsiveness is also found between the m edullary and cortical vasculature regions of the rabbit kidney. Low-pass fi lter properties have been described in the iliac circulation of rats, and e vidence has been provided that noradrenaline reuptake mechanisms are not th e frequency limiting step of the vasculature response. Recent studies on is olated rat vascular smooth muscle cells suggest that sympathetic modulation of vascular tone is limited by the alpha -adrenoceptor signal transduction into the cells and not by an intrinsic inability of the cells to contract and relax at higher rates. (C) 2001 Lippincott Williams & Wilkins.