**8. Molecular and cellular signaling**

by exposed VSM and the presence of physical obstructions such as intimal plaque promotes VSM cell proliferation and migration as underpinnings of deleterious vessel growth and remodeling [14,15]. This pathogenic feed-forward mechanism has poor implications for proper

In addition to the essential VSM and autonomic neural mechanisms, a functional endothelium is equally important for appropriate blood flow control and modulation. VEC dysfunction is increasingly accepted as a common trait of nearly all forms of CVD and is often the initial insult in CVD pathogenesis. Though environmental and genetic factors consistently contribute to these disease states as well, the onset of vascular endothelial dysfunction can also be caused by smoking, hypertension, and/or diabetes [1]. Contrary to this evidence, Horvath and colleagues argue that arterial stiffness is not directly related to a properly functioning endo‐ thelium [9]. This is true in the sense that the mechanical elasticity of the vessel may compress or expand due to pressure changes alone, but a functional endothelium is nonetheless important for its contribution to baroreceptor function and is often a target for CVD prevention. It is well described that reduced NO bioavailability within the vasculature has a direct influence on VEC dysfunction [5]. Nonetheless, in the absence of an intact or functional endothelium, the VSM layer is still capable of receiving NO from alternate NOS isoforms independent of the endothelial layer. For example, inducible nitric oxide synthase (iNOS) either from circulating cells or platelets or from local adventitial nerves can provide VSM with bioavailable NO independent of endothelial contribution. Also, since iNOS is located within VSM it can intrinsically synthesize NO, in turn facilitating vasorelaxation in autocrine fashion. It should be noted though that the canonical pathway of vasodilation begins with VEC

In sum, hemodynamics and intrinsic and extrinsic neural control of blood flow including sensory input to the carotid sinus and the ensuing baroreflex pathway are essential, yet can become compromised under many disease states. The causes of this dysfunction are not entirely understood, but research suggests that there is a direct link between autonomic output and neural integrity and vascular function/dysfunction and the establishment and/or mainte‐ nance of CVD. Thus, the preservation of the baroreceptor reflex within the carotid sinus as well as intrinsic/extrinsic neural control mechanisms are essential for ensuring adequate

Several treatment strategies currently exist to control and/or minimize symptoms associated with CVD. First, often the most prudent forms of CVD therapy involve lifestyle modifications, and many of these are based on known risk factors for CVD. Specific lifestyle choices may

vessel function and tissue perfusion.

**6. Vascular endothelium**

14 Muscle Cell and Tissue

activation as described.

**7. Treatment strategies**

arterial blood supply to the brain in CVD prophylaxis.

Mechanistically, a wide variety of molecular and cellular signaling pathways and signal transduction processes have been theorized as serving key regulatory roles in the dedifferen‐ tiation and phenotypic conversion of VSM cells and subsequent vessel wall remodeling that leads to CVD. Among these, cyclic nucleotides and their predominantly kinase-driven downstream pathways represent significant regulatory factors in determining vascular physiology and pathology. Cyclic nucleotide signaling consists of purine and pyrimidine cyclic monophosphates, many of which have characteristics that allow them to serve as biologically important second messengers. The family of purine cyclic nucleotides consist of the wellknown adenosine 3´,5´-cyclic monophosphate (cyclic AMP) and guanosine 3´,5´-cyclic monophosphate (cyclic GMP) and the lesser known inosine 3´,5´-cyclic monophosphate (cyclic IMP) and xanthosine 3´,5´-cyclic monophosphate (cyclic XMP). These signals operate primarily through protein kinase-driven phosphorylation events on downstream targets to exert functional control over a wide variety of cellular processes in a myriad of mammalian tissues. In the cardiovascular system, cyclic AMP and cyclic GMP are ubiquitous and are established as critical second messengers with abilities to regulate crucial homeostatic and pathophysio‐ logical functions. There also exists the less characterized family of cyclic pyrimidine nucleoti‐ des cytidine 3´,5´-cyclic monophosphate (cyclic CMP), uridine 3´,5´-cyclic monophosphate (cyclic UMP), and thymidine 3´,5´-cyclic monophosphate (cyclic TMP); however, their physiological/pathophysiological importance in VSM and/or during CVD is not well under‐ stood.

As mentioned, several molecules exist in this wide family of cyclic nucleotides that are considered to be true second messengers. A second messenger is defined as being generated by a first-messenger-regulated enzyme, being activated by targeted effector proteins, exerting defined biological effects, being degraded by specific inactivation mechanisms, and whose effects can be duplicated by membrane-permeable analogues and/or bacterial nucleotidyl cyclase toxins. [17] Accordingly, both cyclic AMP and cyclic GMP are considered the tradi‐ tional second messengers, being generated by membrane-bound and/or soluble cyclase enzymes, exerting many effects via intracellular kinases and/or nonkinase targets such as ion channels, have broad biological effects, are inactivated by a family of phosphodiesterases (PDEs), and whose effects can be mimicked by specific analogues or toxins. Also, new theories suggest that the pyrimidines cyclic CMP and cyclic UMP are in fact emerging second messen‐ gers in that they also fulfill these criteria [17,18].

The most recognized cyclic nucleotide second messengers are cyclic AMP and cyclic GMP which are synthesized and operate in similar mechanistic fashion. Cyclic AMP is generated through several different avenues including adenylate cyclase (AC) stimulation by direct agonists or following beta-stimulation or through G protein-coupled receptor activation. Following stimulation of AC, adenosine triphosphate (ATP) is dephosphorylated to produce cyclic AMP and PPi. Similarly, following activation of GC through natriuretic peptides (activating particulate GC) or from the gaseous ligands (NO) or carbon monoxide (CO) which activate soluble GC, GTP is dephosphorylated to yield cyclic GMP and PPi. Cyclic AMP and cyclic GMP operate largely through downstream phosphorylation events on Ser/Thr or tyrosine (Tyr) residues on target proteins and serve diverse roles in normal vascular physiol‐ ogy and homeostasis and during the pathogenesis of CVD. The preferred target proteins for cyclic AMP and cyclic GMP are protein kinase A (PKA) and PKG, respectively [19]. Much of the work from our lab over the past few years has focused on cyclic AMP and cyclic GMP and their abilities to target downstream phosphorylatable substrates mainly through PKA and PKG to elicit functional control over a variety of physiological and pathophysiological parameters elemental to CVD. Some of our latest studies have identified the Ser/Thr kinases PKA, PKG, protein kinase C (PKC), and the metabolic gauge AMP-activated protein kinase (AMPK) as biologically important regulators of VSM proliferation, migration, and chemotaxis; ECM and MMP balance; and cellular apoptosis or necrosis using a variety of experimental approaches and commercially available rodent VSM cells, primary rodent VSM cells, and human primary VSM cells. We have observed capacity of Ser/Thr-specific protein phospha‐ tases (PPs) in moderating these kinase activities and in maintaining proper phosphorylative balance. We have also witnessed abilities of the Ser/Thr PPs to elicit control over VSM growth independent of direct kinase involvement. Additionally, many of these studies performed in cultured cells have been validated in whole animal models of injury-induced VSM growth, and these findings have verified biological ability of these cyclic nucleotide systems to operate in a whole body setting. Lastly, we have solidified these observations obtained in rodent models by recapitulating them in human primary coronary artery VSM preparations, thus adding translational relevance to these basic science findings. Certainly, the capacity of cyclic AMP and cyclic GMP and their phosphorylatable kinase and PPi targets to control deleterious growth of VSM is significant and of particular importance for VSM-specific CVD. These observations provide important new perspectives on vessel wall biology and cell signaling and also highlight potential new targets that could be used to combat CAD and PAD and associated vascular occlusive disorders. Figure 4 depicts chemical structures of cyclic AMP and cyclic GMP, respectively. **FIGURE 4**

**Figure 4.** Chemical structures of cyclic AMP and cyclic GMP, respectively.
