Workshop on Vascular Biology in Hypertension Transduction Mechanisms Involved in the Regulation of Myogenic Activity

Vascular smooth muscle has the ability to exist in a state of maintained partial constriction. This state of partial activation is initiated and/or maintained by the mechanical effects of distending pressure acting on the vascular wall. The intrinsic ability of vascular smooth muscle to respond to these mechanical forces is referred to as the myogenic mechanism. Within the past decade the signaling mechanisms responsible for mechanotransduction of myogenic phenomena have been the focus of extensive research. Two areas of active investigaThe intrinsic ability of vascular smooth muscle to maintain a partial state of contraction and to alter its state of contractile activation, either spontaneously or in response to changes in intravascular pressure, is commonly referred to as myogenic behavior. This behavior presents itself in several characteristic forms, including (1) myogenic tone (often referred to as basal, intrinsic, or spontaneous tone), the sustained contractile state intrinsic to vascular smooth muscle exhibited in the absence of neural or humoral influence; (2) the myogenic response, or the ability of vascular smooth muscle to constrict after an increase in intravascular pressure and, conversely, to dilate on lowering of intravascular pressure; and (3) vasomotion, the spontaneous, rhythmic oscillations in lumen diameter (see Johansson for review). Although myogenic behavior has been reported in conduit arteries and the facial vein,' myogenic tone and myogenic responses are more prevalent in the resistance vasculature, in particular, within the microcirculation. In fact, detailed studies have shown that within a vascular network, relative myogenic responsiveness increases with decreasing arteriolar size." Use of the term myogenic itself implies that mechanical stimuli act directly on vascular smooth muscle cells to elicit contraction. However, more recent evidence has shown that the vascular endothelium can modulate force production by vascular smooth muscle by virtue of its ability to release both relaxing and contracting factors in response to chemical and mechanical stimuli.' Thus, endothelium-derived vasoactive agents may play a role in regulating myogenic behavior and in some cases may be responsible for pressure-dependent changes in vasoconstrictor activity previously ascribed From the Microcirculation Research Institute, Department of Medical Physiology, College of Medicine, Texas A&M University Health Science Center, College Station. Correspondence to Gerald A. Meininger, PhD, Department of Medical Physiology, College of Medicine, Texas A&M University Health Science Center, Reynolds Medical Building, College Station, TX 77843-1114. tion include (1) the modulation of membrane ionic conductances by pressure/stretch and (2) the pressure/stretch —induced generation of second messengers known to be involved in vascular smooth muscle contraction. This review summarizes recent work aimed at understanding the mechanotransduction process in vascular smooth muscle. (Hypertension. 1994;23[part 2]:1096-1105.)