The contractile properties of the mesenteric collecting lymphatics of the rat were analyzed under control conditions and during periods of enhanced lymph formation using in vivo microscopic techniques. Pressure and diameter were simultaneously monitored in microscopic collecting lymphatics, and lymphatic pump function was analyzed in accordance with basic principles of cardiac mechanics. The lymphatic contractile cycle was divided into two phases of systole and four phases of diastole. Under control conditions, lymphatics contracted with a frequency of 6.4 +/- 0.61 beats/min and ejected approximately 67% of their end-diastolic volume. Ten minutes after the rate of lymph formation was elevated by plasma dilution, end-diastolic diameter, contraction frequency, ejection fraction, and stroke volume increased. Pressure in the lymphatic network became less pulsatile in high lymph flow states. Contractility, an index of inotropic changes in lymphatic pump, was unaltered when lymph flow was increased by plasma dilution. Furthermore, the maximal shortening velocity of lymphatic smooth muscle did not change during periods of enhanced lymph flow. Thus it appears that passive increases in the rate of lymph formation exert few, if any, inotropic effects on the lymphatic pump. The augmented stroke volume and contraction frequency appear to result mainly from intrinsic stretch-dependent mechanisms set in motion by elevated preload. These data represent the first comprehensive characterization of both the flow-generating and muscle characteristics of intact collecting lymphatics and provide a basis for future studies on the physiological regulation of lymphatic contraction.

Xanthine oxidase-derived oxidants and leukocytes have been implicated in the microvascular injury associated with reperfusion of ischemic intestine. The objective of this study was to determine whether xanthine oxidase-derived oxidants play a role in the leukocyte-microvascular interactions initiated by ischemia-reperfusion. Adherence and extravasation of leukocytes were monitored in cat mesenteric venules subjected to 1 h of ischemia (blood flow reduced to 20% of control) and reperfusion. Leukocyte rolling velocity, vessel diameter, and red cell velocity were also measured in control (untreated) animals and in animals pretreated with either allopurinol or superoxide dismutase. The responses of venular blood flow, wall shear rate, and leukocyte rolling velocity to ischemia and reperfusion did not differ between the three experimental groups. In control animals, 1 h of ischemia was associated with significant adherence and extravasation of leukocytes with reperfusion greatly enhancing these responses. Allopurinol treatment did not alter the responses to ischemia per se, yet it largely prevented the further increment in adherence and extravasation associated with reperfusion. Superoxide dismutase treatment attenuated the leukocyte responses elicited by both ischemia and reperfusion. Our observations that both allopurinol and superoxide dismutase attenuate reperfusion-induced leukocyte adherence and extravasation are consistent with the hypothesis that xanthine oxidase-derived oxidants initiate the leukocyte infiltration induced by reperfusion of ischemic intestine.

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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.