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A Possible Role of the Glomus Cell in Controlling Vascular Tone of the Carotid Labyrinth of Xenopus Laevis

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A Possible Role of the Glomus Cell in Controlling Vascular Tone of the Carotid Labyrinth of Xenopus Laevis Tohoku J. exp. Med., 1987, 151, 395-408 A Possible Role of the Glomus Cell in Controlling Vascular Tone of the Carotid Labyrinth of Xenopus laevis TATSUMIKUSAKABE, * KAZUKOISHII and KOSEI ISHIIt Department of Physiology, Fukushima Medical College, Fukushima 960 KUSAKABE,T., ISHII,K. and Isxu, K. A Possible Role of the Glomus Cell in Controlling Vascular Tone of the Carotid Labyrinth of Xenopus laevis. Tohoku J. exp. Med., 1987, 151(4), 395-408 To clarify the physiological significance of the g-s connection (intimate apposition of the glomus cell to the smooth muscle) in the Xenopus carotid labyrinth, experiments were carried out morphologically and physiologically. Results obtained are as follows. 1. Efferent electrical stim- ulation of the glossopharyngeal nerve resulted in concentrating dense-cored vesicles on the peripheral region of the glomus cell, and a decrease of vesicles as a whole. 2. In the carotid labyrinth perfused artificially, outflow of the internal and the external carotid arteries decreased with administration of catecholamines (adrenaline, noradrenaline and dopamine). 3. Acetylcholine reduced only the internal outflow. This response was depressed by atropine, hexamethonium and phenotolamine, whereas accelerated by propranolol. 4. Sodium cyanide reduced the internal outflow without affecting the external outflow, and its effect is depressed by phentolamine. From these results, a possibility that the glomus cell participates in controlling the blood flow in the labyrinth through the intervention of the g-s connection was discussed. Xenopus laevis ; carotid labyrinth ; glomus cell ; catecholamine ; arterial chemoreceptor The carotid labyrinth of amphibia has a chemoreceptor function (Ishii et al. 1966) and contains glomus cells similar to those of many animal species in fine structure (Rogers 1963; Ishii and Oosaki 1969; Poullet-Krieger 1973). Recent- ly, an electron microscopic study of Xenopus laevis carotid labyrinth (Ishii and Kusakabe 1982) revealed that some of glomus cells were intimately connected with the smooth muscle in the labyrinth (g-s connection). Moreover, efferent electrical stimulation of the glossopharyngeal nerve resulted in facilitation of exocytosis of dense-cored vesicles at the g-s connection. These findings suggest that some of glomus cells may affect the vascular tone of the labyrinth. In Rana temporaria, Banister et al. (1967) confirmed fluorescent cells in the carotid laby- rinth, and by perfusion experiment Banister et al. (1975) have proposed that these Received December 3, 1986; accepted for publication February 23, 1987. Present address : Department of Anatomy, Yokohama City University School of Medicine, Yokohama, 236, Japan ; tWatari Nakaecho 9-2, Fukushima, 960, Japan. 395 396 T. Kusakabe et al. fluorescent cells might be responsible for controlling blood flow in its vascular network. Their results have been discussed by Smith et al. (1981) based on the perfusion and tissue culture experiments of the labyrinth of Bufo marinus. In the present study, the distribution of dense-cored vesicles and its change after nerve stimulation were morphologically studied, and vascular responses to some sympathomimetic substances and arterial chemoreceptor stimulants were inves- tigated in the perfusion experiments. Results suggested that the glomus cell participates in controlling the blood flow in the carotid labyrinth. MATERIALS AND METHODS Electron microscopy. Eight male Xenopus laevis weighing 40-60 g were used for electron microscopic study. The region of the carotid labyrinth was exposed on both sides of pithed animals. The right glossopharyngeal nerve was cut just below the jugular ganglion and isolated from surrounding tissues. Cutting all its branches with the carotid nerve left intact, the nerve was subjected to electrical stimulation. The left labyrinth was kept intact for the control. Electrical stimulation was done by rectangular wave pulses for 1 min in the following parameters ; 2 volts, 1 msec, 20 Hz. A thin nylon tube was inserted into the aortic trunk to perfuse the carotid labyrinth. After stimulation the labyrinths on both sides were washed with heparinized Ringer solution, then perfused for 10 min with 2% glutaraldehyde in 0.1 mole cacodylate buffer (pH L3) at a pressure of 50 cmHZO. Then, the labyrinths were removed from the body and immersed in the fresh fixative for 3 hr. The specimen was divided into small blocks and postfixed in osmium tetroxide buffered with 0.1 mole cacodylate for 3 hr and embedded in Epon 812. Sections of 11am in thickness were stained with toluidine blue for light microscopy. Ultrathin sections were serially cut from the block in which glomus cells were confirmed by light microscope in advance, and stained with 1% uranyl acetate and Reynold's lead solution. Observation was done by a Hitachi Fig. 1. Diagram of the arrangement for artificial perfusion of the carotid laby- rinth. R1 and R2, reservoir bottles ; T1, T2 and T3, three-way cocks ; S, small compartment (0.25 ml) partitioned by TZ and T3 in which stimulants were filled. With turning the three way cock (T1) stimulants reach the labyrinth. labyr, carotid labyrinth ; c.c.a, common carotid artery ; e.c.a, external carotid artery ; i.c.a, internal carotid artery. Xenopus Carotid Labyrinth Controlling Blood Flow 397 HU-11 electron microscope. To measure distribution of dense-cored vesicles in the cyto- plasm, photographs of the glomus cell were classified in 3 parts ; the part containing nucleus in its cytoplasm (N), without nucleus (M), and the cell processes (P). The density of vesicles per l ,um2was counted in each regions and the mean densities were compared among regions, and between the control and the stimulated groups. The significance of differences was tested by Student's t-test. Perfusion experiments. Twenty three pithed animals of both sexes weighing 40-60 g were used. As shown in Fig. 1, the experimental arrangement was composed of two reservoir bottles. Both bottles were filled with about 100 ml of modified Ringer solution and connected with a three-way cock (T1) ; the one (R1) directly, the other (R2) with insertion of another 2 three-way cocks (T2, T3), and joined to a fine nylon tube inserted into the common carotid artery. It was designed to perfuse the labyrinth with a fixed volume of stimulants stored in the space between T2 and T3 (ca. 0.25 ml) by turning the cock T1. By flowing Ringer solution in the control bottle (R1) the labyrinth was washed up. In the case of testing effects of some blockers on responses to stimulant substances, both bottles (R1, R2) were filled with the saline containing the blocker in a certain concentration, and the stimulant substance diluted with the blocker containing saline was put in the small compartment between T1 and T2. Thus, a fixed volume of stimulant was given during administration of the blocking agent. Fine stainless steel tubes were inserted into both the internal and the external carotid arteries, respectively, and drops escaping from the tip of them were led to a drop-counter and recorded on magnetic tape for the computer analysis. The pressure of perfusion was selected between 15-20 cmH20. For comparison, responses to stimulation were quantitatively expresed according to the formula R.O.E = (Y-.-X)/X (rario of effect), where X is the mean drop number per min during the control period for 2 min and Y is the mean drops per min for 3-5 min or more from 1 min after starting the stimulation. Thus, R.O.E was positive for vasoconstriction and negative for vasodilata- tion. All these procedures were processed by a minicomputor (TI-980B). The composition of Ringer solution was as follows : NaCI, 112 mM ; KCI, 3.0 mM ; CaCl2, 2.2 mM ; NaHC03, 1.2 mM ; glucose, 0.6 mM ; albumin (fraction V), 5 g/liter ; heparin, 2,000 u/liter. RESULTS Electron microscopicobservation Distribution of dense-coredvesicles in the intact glomus cell. In micrographs taken from 8 carotid labyrinths of the control side, the number of pictures of glomus cells were 184 in total including all N-, M- and P- regions. Among these 3 regions there is a dissimilarity in the density of vesicles in the cytoplasm. As shown in Fig. 3A, the mean density was minimum in N(7.43± 2,l0/,u m2, n = 74), maximum in P (21.94 + 8.73/p m2, n = 41), and that in M(9.31+ 3.30/,um2, n = 69) lay between N and P. The ratio of the density in N to that in P was about 3. All of these differencesof mean densities among these regions were significant (p < 0.01). These results suggested that dense-coredvesicles have a tendency to aggre- gate in the glomus cell process. Distribution of dense-coredvesicles after efferentstimulation of the glossophar- yngeal nerve. Efferent stimulation of the glossopharyngeal nerve resulted in a significant (p <0.01) decrease of vesicles in the glomus cell (Fig. 2). On 101 micrographic figures of the glomus cell mean densities of vesicles in each regions were measured as 1.99+ 0.50/,um2 (n -42) in N, 3.14± 1.60/,um2 (n = 40) in M and 398 T. Kusakabe et al. Fig. 2. Electronmicrograms showing the change of the number of dense-cored vesicles in the glomus cell after efferent stimulation of the carotid nerve, a, control ; b, after stimulation. Fig 3. Densities of dense-cored vesicles in N, M and P. A, control ; B, after simulation. 8.43 + 2.62/1am2 (n =19) in P. As summarized in Fig. 3B, vesicles in all 3 regions decreased with nerve stimulation as a whole, and the inequality existing in regions was augmented ; density ratio N to P increased from 3 to 4. Xenopus Carotid Labyrinth Controlling Blood Flow 399 Perfusion experiments of the carotid labyrinth Effects of catecholamines The glomus cells contain some catecholamines in their granules and are intimately apposed with the smooth muscles which are one of structural elements of the labyrinth.
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