PRESSURE AND FLOW-DEPENDENT VASCULAR TONE

Most small arteries are partially constricted in vivo. After excluding neurogenic, metabolic, and circulating as well as local hormonal influences, a sizeable component of tone persists which is commonly called basal tone. In the absence of such tone, cardiac output would be insufficient to maintain the circulation. This review focuses on the contribution of stretch, induced by changes in transmural pressure, and flow acting through shear forces exerted at the blood vessel wall interface, to basal tone. Evidence concerning the cellular processes that may be activated by these physical forces - the mechanotransducing systems - are discussed. The involvement of the endothelium and the role of change in membrane potential are evaluated and lead to the conclusion that pressure and flow effects do not depend exclusively on the release of endothelial factors nor the activation of voltage-gated Ca2+ channels. Stretch/pressure-induced changes in tone show distinctive pharmacological profiles. They are dependent on extracellular calcium and yet in many instances are only weakly affected by organic Ca2+-entry inhibitors. Flow-dependent vascular effects, both constrictor and dilator, are both exquisitely sensitive to changes in extracellular Na+ and appear to be related to its transmembrane gradient. Stretch/pressure cause activation of protein kinase C, an intracellular modulator of Ca2+-dependent contractile processes. The existence of separate and distinctive cellular sensing and responding systems to pressure and flow raise the possibility that the smooth muscle tone of the vascular system can be influenced independently by the pressure and rate of flow of the blood.