Myosin phosphatase is a target for signaling pathways that modulate calcium sensitivity of force production in smooth muscle. Myosin phosphatase targeting subunit 1 (MYPT1) isoforms are generated by cassette-type alternative splicing of exons in the central and 3' portion of the transcript. Exclusion of the 3' alternative exon, coding for the leucine zipper (LZ)-positive MYPT1 isoform, is associated with the ability to desensitize to calcium (relax) in response to NO/cGMP-dependent signaling. We examined expression of MYPT1 isoforms and smooth muscle phenotype in normal rat vessels and in a prehepatic model of portal hypertension characterized by arteriolar dilation. The large capacitance vessels, aorta, pulmonary artery, and inferior vena cava expressed predominantly the 3' exon-out/LZ-positive MYPT1 isoform. The first-order mesenteric resistance artery (MA1) and portal vein (PV) expressed severalfold higher levels of MYPT1 with predominance of the 3' exon-included/LZ-negative isoform. There was minor variation in the presence of the MYPT1 central alternative exons. Myosin heavy and light chain splice variants in part cosegregated with MYPT1 isoforms. In response to portal hypertension induced by PV ligature, abundance of MYPT1 in PV and MA1 was significantly reduced and switched to the LZ-positive isoform. These changes were evident within 1 day of PV ligature and were maintained for up to 10 days before reverting to control values at day 14. Alteration of MYPT1 expression was part of a complex change in protein expression that can be generalized as a modulation from a phasic (fast) to a tonic (slow) contractile phenotype. Implications of vascular smooth muscle phenotypic diversity and reversible phenotypic modulation in portal hypertension with regards to regulation of blood flow are discussed.

Physiological roles of endogenous nitric oxide (NO) in the lymphatic pump activity of rat mesenteries in vivo were evaluated using an intravital video microscope system. Changes in the pumping frequency (F), the end diastolic diameter (EDD), and the end systolic diameter (ESD) of the mesenteric lymph microvessels were measured with the microscope system and then the pump flow index (PFI) was calculated. A 15-min superfusion of 30 microM N(omega)-nitro-L-arginine methyl ester (L-NAME) in the mesenteries caused significant increases of F and PFI and a significant decrease of the EDD and ESD. Simultaneous superfusion of 1 mM L-arginine with 30 microM L-NAME produced a significant reversal of the L-NAME-mediated increase of F and decrease of ESD. A 15-min superfusion of 100 microM aminoguanidine caused no significant effects on F, EDD, and ESD of the mesenteric lymph vessels in vivo. These findings suggest that endogenous NO has physiologically modulated the lymphatic pump activity in rat mesentery in vivo and that the production and release of NO may be mediated by constitutive NO synthase but not by inducible NO synthase.

Shirasawa, Yuichi, and Joseph N. Benoit. Stretchinduced calcium sensitization of rat lymphatic smooth muscle. Am J Physiol Heart Circ Physiol 285: H2573-H2577, 2003. First published August 28, 2003 10.1152/ajpheart.00002. 2003The relationships between smooth muscle calcium and isometric tension generation to spontaneous lymphatic pump activity and its modulation by stretch equivalent from 0 to ϳ6 cmH2O were investigated. Excised preparations of the rat thoracic duct were mounted on a wire myograph and loaded with the calcium-sensitive fluorochrome indo-1. Calcium-dependent fluorescence and isometric force were simultaneously recorded. The thoracic duct segments developed spontaneous rhythmic contractile activity. Each contraction was preceded by an increase in intracellular calcium. When the vessels were normalized and stabilized at a preload equal to 3 cmH2O, the peak generation in tension occurred 0.70 Ϯ 0.11 s after that of calcium. Incremental stretch enhanced the frequency of the phasic activity and amplitude of isometric force generation but not the basal calcium level or the amplitude of the calcium transient. These findings suggest that stretch enhances lymphatic pump activity by increasing the pacemaker activity and the calcium sensitivity of the contractile apparatus.

These findings demonstrated that olmesartan could restore night-time blood pressure fall, as seen with diuretics and sodium restriction, possibly by enhancing daytime sodium excretion. Since nocturnal blood pressure is a strong predictor of cardiovascular events, olmesartan could relieve cardiorenal load through normalization of circadian blood pressure rhythm besides having powerful ability to block the renin-angiotensin system.