Takehiko Azuma

The response of lymphatic smooth muscles to vasoactive substances

A study was made of the isotonic response of bovine mesenteric lymphatics to several physiological vasoactive substances. Contractions of lymphatic smooth muscles were induced by serotonin (5-HT), prostaglandin F2α (PGF2α), noradrenaline (NA), histamine, dopamine and acetylcholine (ACh). The smooth muscles were particularly sensitive to 5-HT. Excepting PGF2α no other substances could equal 5-HT in the magnitude of the maximum response. The majority of 5-HT receptors seemed to be the D receptors. The decreasing order of the contractile responses was as follows: 5-HT>PGF2α>NA>histamine>dopamine>ACh. The contractile response to ACh was observed only in specimens involving valvular region. It was very likely that, in the lymphatics, there were 2 kinds of receptors for catecholamines, i.e. α and β receptors, and the stimulation of the former induced smooth muscle contraction and that of the latter relaxation. A difference was noticed between the responses of valvular and intervalvular segments to NA. Relaxations of lymphatic smooth muscles were induced not only by isoproterenol but also by adenosine and adenine nucleotides. The decreasing order of the relaxant responses was as follows: ISP>adenosine >ATP>ADP>cyclic AMP≧AMP. The relaxant responses to adenine nucleotides tended to reduce with decrease in the number of high energy phosphates.

Distensibility of the vein: from the architectural point of view.

By MEANS of a universal tensile testing instrument, stress-strain relationships were studied on longitudinal and circumferential strips excised from various portions of the caval vein and from the cervical, axillary, iliac and femoral veins of dogs. In order to investigate the mechanical properties from the architectural point of view, the relationships were also studied on the nuchal ligament, tendon and intestinal smooth muscle bundle which are composed mainly of elastin, collagen and smooth muscle fibers, respectively. Histological examinations were also carried out with these vascular and non-vascular specimens. The results obtained were summarized as follows: (1) the circumferential distensibility of the veins in low stress range is determined mainly by mechanical characteristics of smooth muscle fibers; (2) the circumferential distensibility of the veins approaches the distensibility of collagen fibers asymptotically in high stress range; (3) longitudinal distensibility of the veins depends on the fractional content of tight elastin fibers.

Innervation of bovine mesenteric lymphatics: From the histochemical point of view

Abstract Histochemical studies were carried out on the distribution of adrenergic and cholinergic nerves in bovine mesenteric lymphatics. Most of the catecholamine-specific fluorescent nerve fibers demonstrated by the glyoxylic acid method encircled these vessels, although a few of them ran spirally or longitudinally. The fluorescent nerve fibers were present not only in the adventitia but also in the smooth muscle layers. The peak wavelengths of excitation and emission spectra of the fluorescence analyzed by means of a microepifluorescence spectrophotometer were about 415 and 465 nm, respectively. Both the values agreed with those of norepinephrine. Most of the cholinesterase-positive fibers stained by the copper thiocholine method were longitudinal in direction. The pattern of their distribution was similar to that of the catecholamine-containing nerve fibers, though the distribution density of the former fibers was less than the latter. Labeling was not affected by 10 −4 M iso-OMPA in the presence of acetylthiocholine iodide as a substrate—an indication that the staining of these fibers is specific for cholinesterase activity. These results suggest that the adrenergic and cholinergic nerve fibers are distributed in the smooth muscle layers as well as in the adventitia of bovine mesenteric lymphatics.

Vasa vasorum within the media of bovine mesenteric lymphatics.

Summary Smooth muscles in bovine mesenteric lymphatics were well-developed and arranged in three layers, i.e., the internal longitudinal, intermediate circumferential, and external longitudinal. The outermost one was much thicker than the other two. Blood capillaries were found within the smooth muscle layers as well as in the ad-ventitia. These blood capillaries may be essential for maintaining vigorous rhythmic contractions of the lymphatic smooth muscles which act as a driving force for the propulsion of lymph. The presence of numerous mitochondria and glycogen granules seemed to reflect a high metabolic activity of the smooth muscle cells. The authors are indebted to Dr. T. Suganuma and Mr. R. Ichikawa for their technical advice and assistance.

Variegated effects of prostaglandins on spontaneous activity in bovine mesenteric lymphatics

The effects of PGs (PGA2, PGB2, PGE2, PGF2 alpha, PGI2, and 6-keto PGF1 alpha) were investigated quantitatively in insolated bovine mesenteric lymphatics for elucidating the possible significance of PGs in lymph propulsion. 10(-8) M PGF2 alpha, PGA2, and PGB2 produced recognizable increases of frequency and amplitude of spontaneous contractions. The positive chrono- and inotropic responses were enhanced with increasing concentration of the PGs from 10(-8) to 10(-6) M. Both PGE2 and PGI2 in a concentration range from 10(-8) to 10(-6) M caused dose-related reductions in the amplitude of spontaneous contractions. No decrease in contraction rhythm was observed by the administration of PGI2 and PGE2 at concentrations lower than 5 X 10(-7)M. All of the above-mentioned responses induced by PGF2 alpha, PGA2, PGB2, PGI2, and PGE2 were not inhibited by pretreatment with alpha- and beta-adrenergic blocking agents, muscarinic antagonist, antihistamics, and serotonic antagonist. These findings suggest that PGF2 alpha, PGA2, and PGB2 in a low concentration may facilitate lymph flow of bovine mesenteric lymphatics in the living body, since the spontaneous contractions of lymphatic smooth muscle may drive lymph centripetally in the presence of direction valves. On the other hand, PGI2 and PGE2 may cause an inhibition of lymph transport.

Numerical Analysis of Blood Flow in the Vertebral Artery

Abnormal hemodynamic forces associated with distortions of blood vessel lumen have been thought to play an important role in the pathogenesis of focal vascular lesions. In the vertebral artery, segments located between osseous rings are ectatic compared with those surrounded by the rings. Based on the assumption that arterial blood flow was quasi-steady, this work was undertaken to investigate the structure of flow through arterial models with one or two sinusoidal stenoses. Numerical analysis was performed by an integral-momentum method. The validity of the method was examined by comparison of experimental data so far reported with theoretical results. Velocity and wall shear stress distributions were explored in a model with two stenoses simulating a part of the vertebral artery. The ectatic segments of the vertebral artery have been known as predilection sites for atherosclerotic lesions. The present study suggested that the ectatic wall was under unstable shear stresses, the direction of which was dependent upon the magnitude of the Reynolds number.

Electrical activity of lymphatic smooth muscles.

Summary Membrane action potentials of bovine mesenteric lymphatics were recorded simultaneously with isometric contractions by the use of the sucrose gap method. The action potentials, about 3 sec in duration, always had one-to-one correspondence to the contraction waves. Each action potential consisted of three phases, i.e., initial slow depolarization, spike, and slow repolarization. The application of nor-adrenalin elicited a slight but long-lasting depolarization superimposed by frequent discharges of action potentials. The administration of acetylcholine in a relatively high concentration to the preparations induced a transient depolarization followed by the occurrence of action potentials. From the effects of tetrodotoxin, manganese, calcium-free environment, and barium chloride on the spontaneous contractions, it was suggested that calcium current may probably play a major role in producing spike discharge in bovine lymphatic smooth muscles.

Bradykinin‐induced contractions of bovine mesenteric lymphatics.

The mode of action of bradykinin (BK) on bovine mesenteric lymphatics was investigated by recording isometric tensions and action potentials in the isolated longitudinal segments. Addition of BK in concentrations from 10(‐10) to 4 X 10(‐6) M caused dose‐related tonic contractions. BK in a low concentration accelerated the rhythm of action potential discharges in the spontaneously beating preparations and elicited frequent discharges of action potentials and a rapid rise in smooth muscle tone associated with phasic contractions. BK in high concentrations (more than 10(‐7) M) caused a further rise of tension in the preparations which had already been depolarized in a high‐K solution. The contraction induced by 4 X 10(‐9) M‐BK in the standard solution was abolished in a Ca‐free environment or in the presence of a Ca‐antagonist, 10(‐4) M‐D‐600, though more than 50% of the contraction caused by 10(‐6) M‐BK still remained in both circumstances. In a Ca‐free solution containing 1 mM‐EGTA (Ca‐free standard solution), 10(‐6) M‐BK caused a slight contraction even after high‐K‐induced contractions were completely blocked. The contractile response to 10(‐6) M‐BK in the Ca‐free standard solution was augmented after activation of beta‐receptors. It is concluded that the BK‐induced contractions may be closely related to an increased Ca influx through the membrane and release of membrane‐bound and intracellular Ca. The increased uptake of Ca into the BK‐sensitive intracellular store may contribute to the relaxing effect of beta‐agonist.

Characteristics of secondary flow in steady and pulsatile flows through a symmetrical bifurcation

Steady and pulsatile flow in a glass model simulating an arterial bifurcation was investigated by flow visualization techniques. Secondary flow generated at the bifurcation has a similar pattern to a vortex, called the horseshoe vortex, produced around a wall-based protuberance in a circular tube. The same flow disturbance was clearly observed during the decelerating phase of pulsatile flow. The vortex produces a stagnation point on the top and bottom wall just upstream from the bifurcation apex. When aluminium dust was suspended in the test fluid perfusing the blood vessel model, particles deposited over an area spreading from the stagnation point to the lateral corners of the bifurcation. Comparison between the present results and topographical patterns of atherosclerosis reported in the literature suggests that it is in such low shear regions that lipid deposition tends to occur most.

Vortex Generation in Pulsatile Flow Through Arterial Bifurcation Models Including the Human Carotid Artery

Visualization experiments were performed to elucidate the complicated flow pattern in pulsatile flow through arterial bifurcations. Human common carotid arteries, which were made transparent, and glass-models simulating Y- and T-shaped bifurcations were used. Pulsatile flow with wave forms similar to those of arterial flow was generated with a piston pump, elastic tube, airchamber, and valves controlling the outflow resistance. Helically recirculating flow with a pattern similar to that of the horseshoe vortex produced around wall-based protuberances in circular tubes was observed in pulsatile flow through all the bifurcations used in the present study. This flow type, which we shall refer to as the horseshoe vortex, has also been demonstrated to occur at the human common carotid bifurcation in steady flow with Reynolds numbers above 100. Time-varying flows also produced the horseshoe vortex mostly during the decelerating phase. Fluid particles of dye solution approaching the bifurcation apex diverged, divided into two directions perpendicularly, and then showed helical motion representing the horseshoe vortex formation. While this helical flow was produced, the stagnation points appeared on the wall upstream of the apex. Their position was dependent upon the flow distribution ratio between the branches in the individual arteries. The region affected by the horseshoe vortex was smaller during pulsatile flow than during steady flow. Lowering the Reynolds number together with the Womersley number weakened the intensity of helical flow. A separation bubble, resulting from the divergence or wall roughness, was observed at the outer or inner wall of the branch vessels and made the flow more complicated.