The smooth muscle cell directly drives the contraction of the vascular wall and hence regulates the size of the blood vessel lumen. We review here the current understanding of the molecular mechanisms by which agonists, therapeutics, and diseases regulate contractility of the vascular smooth muscle cell and we place this within the context of whole body function. We also discuss the implications for personalized medicine and highlight specific potential target molecules that may provide opportunities for the future development of new therapeutics to regulate vascular function.

In vitro experiments showing the activation of the myosin phosphatase via heterophilic leucine zipper interactions between its targeting subunit (MYPT1) and cGMP-dependent protein kinase I suggested a pathway for smooth muscle relaxation (Surks, H. K., Mochizuki, N., Kasai, Y., Georgescu, S. P., Tang, K. M., Ito, M., Lincoln, T. M., and Mendelsohn, M. E. (1999) Science 286, 1583-1587). The relationship between MYPT1 isoform expression and smooth muscle responses to cGMP signaling in vivo has not been explored. MYPT1 isoforms that contain or lack a C-terminal leucine zipper are generated in birds and mammals by cassette-type alternative splicing of a 31-nucleotide exon. The avian and mammalian C-terminal isoforms are highly conserved and expressed in a tissue-specific fashion. In the mature chicken the tonic contracting aorta and phasic contracting gizzard exclusively express the leucine zipper positive and negative MYPT1 isoforms, respectively. Expression of the MYPT1 isoforms is also developmentally regulated in the gizzard, which switches from leucine zipper positive to negative isoforms around the time of hatching. This switch coincides with the development in the gizzard of a cGMP-resistant phenotype, i.e. inability to dephosphorylate myosin and relax in response to 8-bromo-cGMP after calcium activation. Furthermore, association of cGMP-dependent protein kinase I with MYPT1 is detected by immunoprecipitation only in the tissue that expresses the leucine zipper positive isoform of MYPT1. These results suggest that the regulated splicing of MYPT1 is an important determinant of smooth muscle phenotypic diversity and the variability in the response of smooth muscles to the calcium desensitizing effect of cGMP signaling.Smooth muscle contraction is initiated by the phosphorylation of the regulatory myosin light chain (MLC 20 ) 1 by the calcium/calmodulin-dependent activation of the myosin light chain kinase (MLCK) (1). Relaxation is effected by the dephosphorylation of MLC 20 by the smooth muscle myosin phosphatase (SMMP). Complexity is brought to this system by accessory proteins and signaling pathways that regulate the smooth muscle contractile state (reviewed in Refs. 2 and 3). The SMMP is a target of signals that are positive and negative modulators of smooth muscle tone. SMMP is a heterotrimeric protein composed of the 37-kDa catalytic subunit (PP1c␦), the 130/133-kDa myosin targeting subunit (MYPT1, also referred to as MBS), and the 21-kDa M21 subunit (4 -6). MYPT1 targets the catalytic subunit to MLC 20 (7,8) and in this way confers substrate specificity to the phosphatase, whereas the function of the M21 subunit is unknown. Activation of the Rho kinase signaling pathway leads to phosphorylation of the MYPT1 subunit, resulting in inhibition of myosin phosphatase activity and an increase in smooth muscle tone (9 -13). This signaling pathway is thought to determine the calcium-sensitizing effect of ␣-adrenergic stimulation, for example, in which greater force is produced at a given calcium concentration th...

Background In vitro studies suggest that phosphorylation of titin reduces myocyte/myofiber stiffness. Titin can be phosphorylated by cGMP activated protein kinase (PKG). Intracellular cGMP production is stimulated by B-type natriuretic peptide (BNP) and degraded by phosphodiesterases (PDE) including PDE-5A. We hypothesized that a PDE-5A inhibitor (sildenafil) alone or in combination with BNP would increase left ventricular (LV) diastolic distensibility by phosphorylating titin. Methods and Results 8 elderly dogs with experimental hypertension (OH) and 4 young normal (YN) dogs underwent measurement of the end-diastolic pressure (EDP) volume (EDV) relationship (EDPVR) during caval occlusion at baseline, after sildenafil and after BNP infusion. To assess diastolic distensibility independent of load/extrinsic forces, the EDV at a common EDP on the sequential EDPVRs was measured (LV capacitance). In a separate group of dogs (n=7 OH and 7 YN), serial full thickness LV biopsies were harvested from the beating heart during identical infusions to measure myofilament protein phosphorylation. Plasma cGMP increased with sildenafil and further with BNP (7.31±2.37 to 26.9±10.3 to 70.3±8.1 pmol/ml, P<0.001). LV diastolic capacitance increased with sildenafil and further with BNP (51.4±16.9 to 53.7±16.8 to 60.0±19.4 ml, P<0.001). Changes were similar in OH and YN dogs. There were no effects on phosphorylation of troponin I, troponin T, phospholamban, or myosin light chains −1 or −2. Titin phosphorylation increased with sildenafil and BNP, whereas titin-based cardiomyocyte stiffness decreased. Conclusion Acute cGMP enhancing treatment with sildenafil and BNP improves LV diastolic distensibility in vivo, in part, by phosphorylating titin.