Masao Yamasaki

Spaceflight alters the fiber composition of the aortic nerve in the developing rat.

Hydrostatic pressure gradients due to the gravitational force in blood vessels disappear under conditions of microgravity during spaceflight, and the ability of the baroreceptor reflex to control arterial pressure and blood distribution may be altered. We hypothesized, on the basis of the results obtained in our previous experiments using the head-down tilt method in rats and rabbits, that the range of increase in arterial pressure caused by animal behavior narrows under conditions of microgravity, affecting the development of high-threshold unmyelinated fibers in the rat aortic nerve which sends signals from baroreceptors located in the aortic wall to the reflex center. We verified this hypothesis using 9-day-old rat neonates housed with their dams for 16 days on the space shuttle Columbia in outer space (STS-90, Neurolab Mission). Age-matched neonatal rats with the dams remained on the ground as controls. After breeding was carried out in the three experimental groups (FLT, spaceflight; AGC, asynchronous ground control; VIV, vivarium ground control), specimens of the 25-day-old rats were excised and five left aortic nerves in each group were examined by electron microscopy. The number of aortic unmyelinated fibers was significantly less in the FLT group than in each ground control (mean+/-S.D.; 139+/-37 in the FLT, 207+/-36 in the AGC, 283+/-121 in the VIV; P<0.05), which may be related to the weakness of the baroreceptor reflex under conditions of microgravity in space. This result may contribute to understanding of the several cardiovascular issues which occur under microgravity and after reexposure to gravity in human.

Pulmonary C-fibers elicit both apneusis and tachypnea in the rabbit

The effects of phenylbiguanide (PBG) on phrenic nerve and pulmonary C-fibers were studied in anesthetized spontaneously breathing rabbits with unilateral vagotomy. Right atrial injections of PBG at low (10 micrograms/kg) and high (100 micrograms/kg) dose resulted in a shallow tachypnea and apneusis followed by tachypnea, respectively, and these effects were blocked by procaine treatment of the vagus nerve. Also, the injection of PBG (100 micrograms/kg) still evoked the rapid shallow breathing preceded by apneusis in carotid chemoreceptor-denervated animals. Vigorous stimulation of pulmonary C-fibers by PBG (100 micrograms/kg) coincided with apneusis and the response was followed by a more modest increase in activity associated with tachypnea. Administration of PBG (10 micrograms/kg) into the right atrium caused an increase in pulmonary C-fiber activity associated with tachypnea. However, a small dose of PBG injected into the aortic circulation had no effect on the C-fiber activity but did inhibit respiration. These results suggest that the stimulation of pulmonary C-fibers via PBG injection can produce both inspiratory apnea and tachypnea.

Substance P antagonist does not block the stimulation of rapidly adapting pulmonary stretch receptors by ammonia

We investigated the effects of the substance P (SP) blocker [D-Pro2,D-Trp7,9]-SP on the response of rapidly adapting pulmonary stretch receptors (RARs) to SP administered into the right atrium, or ammonia vapor inhaled into the lungs in anesthetized, spontaneously breathing rabbits. Right atrial administration of SP (0.3, 1.0, and 3.0 µg/kg) caused an increase in the RAR activity, and this increase became more prominent as the dose of SP was increased. The RARs increased their activity following inhalation of vapor from 5 and 10% ammonia solutions, and the increase was concentration dependent. The excitatory responses of RAR activity to SP at different doses were greatly diminished or completely blocked by administration of the selective SP antagonist (300 and 500 µg/kg). However, the ammonia-induced RAR stimulation was not significantly altered by prior treatment with the SP blocker (300 and 500 µg/kg). These results suggest that the stimulation of RARs by ammonia does not occur as a result of the release of SP from sensory nerves in the airways and lungs.

Evidence for the presence of function of the inhibitory M2 receptors in the rabbit airways and lungs.

We investigated to determine whether or not the inhibitory M2-receptors function in the rabbit lung and heart. Rabbits were anesthetized, vagotomized, paralyzed and ventilated. Administration of gallamine, an M2-receptor antagonist, augmented an increase of PT produced by vagal stimulation with or without simultaneous administration of histamine and the increases were dose-dependent. Conversely, prior treatment with pilocarpine, an M2-receptor agonist, reduced these responses in a dose-dependent manner. The PT responses to histamine injection only were not significantly altered by administration of either gallamine or pilocarpine. The remaining bronchoconstrictor responses to the three stimuli in the presence of gallamine or pilocarpine were completely blocked by atropine. In another series of experiments, gallamine treatment enhanced bronchoconstriction evoked by vagal stimulation but reduced acetylcholine (ACh)-induced bronchoconstriction. These opposite responses were dose-dependent for gallamine. The results suggest that there are inhibitory M2-receptors in the parasympathetic nerves innervating the lungs in the rabbit. Furthermore, gallamine treatment that completely blocked bradycardia evoked by ACh administration reduced vagally-mediated bradycardia. This implies that gallamine appears to have an antagonistic action on muscarinic receptors in the rabbit heart.

H1- and H2-receptor influences of histamine and ammonia on rapidly adapting pulmonary stretch receptor activities

Studies were designed to establish the difference of the excitatory mechanisms between histamine and ammonia on rapidly adapting pulmonary stretch receptors (RARs). We therefore examined the responses of RARs to histamine administered as an aerosol and ammonia inhaled as a vapor before and after aerosol administration of mequitazine or cimetidine in spontaneously breathing rabbits. The excitatory responses of RARs to aerosols of histamine at different concentrations were completely blocked by administration of aerosol mequitazine but potentiated by aerosol cimetidine. However, the increases of RAR activity produced by inhalation of ammonia vapor at different concentrations were not significantly affected by aerosol administration of either a H1-receptor blocker or a H2-receptor antagonist. These results suggest that the stimulation of RARs by aerosol histamine occurs as a result of the interaction between H1 (excitatory)- and H2 (inhibitory)-receptor effects, whereas these two receptor effects do not contribute to the ammonia-induced RAR stimulation.

Developmental changes in functional characteristics of aortic baroreceptor afferents in rats

To investigate postnatal developmental changes in functional characteristics of the afferent pathway of the aortic baroreceptor reflex, the responses of aortic nerve activity (ANA) to blood pressure (BP) changes elicited by phenylephrine and sodium nitroprusside administration were tested in 3‐, 8‐ and 20‐week‐old male rats under chloralose (60 mg kg−1i.p.) and urethane (600 mg kg−1i.p.) anaesthesia. The function curve of ANA in response to BP changes showed a sigmoid shape that shifted to the right from 3 to 8 weeks of age. The maximal activity and maximal gain of the aortic nerve, which were calculated by a logistic function analysis, were significantly higher in 20‐week‐old rats (maximal activity, 532 ± 47% of baseline; maximal gain, 7.9 ± 0.8% of baseline mmHg−1; n= 9) than in 3‐week‐old rats (maximal ANA, 268 ± 25% of baseline, P < 0.001; maximal gain, 4.9 ± 0.5% of baseline mmHg−1, P < 0.01, n= 9) and 8‐week‐old rats (maximal ANA, 309 ± 18% of baseline, P < 0.001; maximal gain, 4.9 ± 0.3% of baseline mmHg−1, P < 0.01, n= 11). These results suggest that the operating point of aortic baroreceptor afferents is reset to the higher pressure level during development from 3 to 8 weeks of age and, thereafter, the afferent gain increases from 8 to 20 weeks of age. This functional change may be an important factor to prevent an excess increase of BP, which would result in pathophysiological problems.

Cholinergic and H1-receptor influences of histamine on slowly adapting pulmonary stretch receptor activity in the rabbit

Afferent impulses of slowly adapting pulmonary stretch receptors (SARs) were obtained by dissecting fine slips from the left vagus nerve (LVN) and by leaving the rest of the nerve intact. In the same SAR preparation, changes of the receptor activity in response to right atrial injections of histamine (10 and 60 micrograms/kg) were successively examined before and after atropine (1 mg/kg), partial vagal efferent ablation, and mequitazine (1 mg/kg) in 10 rabbits. Administration of histamine led to an increase in the SAR activity, and this effect became more pronounced by increasing the dose of histamine. Atropine treatment diminished the responses of SARs to histamine at different doses. Partial vagal efferent ablation produced by denervation of the rest of the intact LVN slightly reduced the response of SARs to histamine at 10 micrograms/kg but had no significant effect on the SAR response to 60 micrograms/kg histamine. In the absence of vagal afferent and efferent activities on the left side, mequitazine, a potent H1-receptor blocker, completely blocked low- and high-dose effects of histamine on SARs. We compared the responses of the receptor activity to aerosol histamine (1 and 4%) and to topical application of histamine (0.1 ml, 0.025% and 0.1%) in six SAR preparations. The magnitude and duration of increased SAR activity became more prominent by increasing the concentration of histamine. The firing pattern and discharge rate of SARs following aerosol or intratracheal administration of histamine were similar to those after intra-atrial histamine. In addition, we also examined the excitatory responses of SAR activity to right atrial injections of histamine at 10 and 60 micrograms/kg before and after topical administration of atropine (0.1 ml, 1%, n = 6) or mequitazine (0.1 ml, 1%, n = 6) in 12 SAR preparations. Intratracheal atropine diminished the response of SARs to 10 micrograms/kg of histamine but had no significant effect on the response of SARs to histamine at 60 micrograms/kg. All the responses of SARs to histamine were completely blocked by topical application of mequitazine. These results suggest that the change of SAR activity produced by histamine at 10 micrograms/kg occurs mainly as a result of the release of acethylcholine (ACh) via the vagovagal reflex and that the activation of H1-receptors of the airway smooth muscle contributes importantly to the response of SARs to histamine at 60 micrograms/kg.

Effects of calcium channel and H1-receptor blockers on the responses of slowly adapting pulmonary stretch receptors to histamine in vagotomized rabbits

We studied the effects of calcium channel antagonists (verapamil and nifedipine) and H1-receptor blockers (mequitazine) on changes in the slowly adapting pulmonary stretch receptors (SARs) located below the carina in response to right atrial injections of histamine (60 and 80 µg/kg) in anesthetized artificially ventilated rabbits with bilateral vagotomy. After histamine was injected into the right atrium, the SARs became more active during expiration but decreased their activity during inspiration. These changes were more pronounced by increasing the dosage of histamine. However, administration of histamine had no significant effect on tracheal pressure (PT). Verapamil treatment (1 mg/kg) did not alter the SAR response to histamine, whereas the responses of SARs to histamine at different dosages were significantly diminished by treatment with nifedipine (1 mg/kg). Mequitazine (1 mg/kg), a potent H1-receptor blocker, blocked completely all the responses of SAR activity to histamine. These results suggest that the effect of histamine 60–80 µgg/kg on SAR activity is mediated by the activation of H1-receptors of the peripheral airway smooth muscle and that this activation, at least in part, involves the opening of calcium channels of the airway smooth muscle.

Effects of atropine on the responses of rapidly adapting pulmonary stretch receptors and dynamic lung compliance to sodium cyanide-induced hyperpnea

The responses of tracheal pressure (PT) and heart rate (HR) to electrical stimulation of the peripheral cut-ends of the vagus nerves (10-15 V, 10 Hz, 1 ms) to activate both myelinated and non-myelinated fibers were examined before and after administration of atropine (1 mg/kg) in artificially ventilated, bilaterally vagotomized rabbits. Vagal stimulation caused an increase in PT and a decrease in HR. The PT response to vagal stimulation was completely blocked by atropine which significantly reduced the bradycardia evoked by the stimulation. In other series of experiments, we also examined the responses of rapidly adapting pulmonary stretch receptors (RARs) and dynamic lung compliance (Cdyn) to intravenous injections of sodium cyanide (NaCN, 20 and 30 micrograms/kg) before and after administration of atropine (1 mg/kg) in spontaneously breathing rabbits with vagal efferent activities. Administration of NaCN led to an increase in RAR activity associated with a decrease in Cdyn, and these two effects became more prominent by increasing the dose of NaCN. Atropine treatment did not significantly alter the responses of RARs and Cdyn to the injections of NaCN with different doses. These results suggest that the changes of RAR activity and Cdyn produced by hyperpnea due to NaCN administration are not influenced by the vagally mediated bronchoconstriction.

Cardiovascular regulation during upright standing behavior in conscious rats.

Although rats often show an upright standing behavior the cardiovascular response during the behavior has not yet been fully clarified. In this study we quantified the activity of upright standing behavior in rats using infrared beam detectors and measured cardiovascular variables during the behavior. Rats demonstrated a high level of upright standing activity as they showed the upright posture more than 500 times per day at 10 weeks of age. The average upright standing duration time was less than 10s. Arterial pressure slightly decreased while heart rate increased in response to the behavior and these responses were not affected by sino-aortic denervation. Our results indicate that other mechanisms such as the vestibulo-cardiovascular reflex may completely compensate the lack of the baroreceptor reflex to maintain cardiovascular homeostasis in response to acute positional changes in rats. Moreover rats demonstrate complex integrative mechanisms maintaining cardiovascular homeostasis against the upright standing behavior which frequently occurs in rats.