Haruhisa Hirakawa

Effects of systemic hypoxia on R-R interval and blood pressure variabilities in conscious rats

The effects of systemic hypoxia with different levels of CO2 on R-R interval (RRI) and systolic blood pressure (SBP) variabilities were investigated in conscious rats. Wistar rats chronically instrumented for the measurement of blood pressure, electrocardiogram, and renal sympathetic nerve activity (RSNA) were exposed to hypocapnic (Hypo), isocapnic (Iso), and hypercapnic (Hyper) hypoxia. On another day, the rats were treated with atropine and exposed to the same type of hypoxia. Sinoaortic denervation (SAD)-treated rats were exposed to Iso and Hyper, and RRI and SBP variabilities before and during hypoxia were analyzed using the maximum-entropy method with high resolution. With regard to RRI variability, very low frequency (VLF), low frequency (LF), and high frequency (HF) powers all decreased during Hypo, increased during Hyper, and did not change during Iso in intact rats. Changes during Hypo were attenuated by atropine, and those during Hyper were abolished by either atropine or SAD. The ratio of LF power to HF power decreased independently of increases in RSNA during each type of hypoxia. On the other hand, there were no changes in VLF, LF, or HF power in SBP variability during each type of hypoxia in intact rats. In atropine-treated rats, LF power increased during Iso and Hyper and HF power increased during each type of hypoxia. There was no difference in respiratory frequency among the three kinds of hypoxia in both intact and atropine-treated rats. The results suggest that arterial[Formula: see text] level rather than respiration frequency produces changes in powers of RRI variability through changes in parasympathetic nerve activity and that with regard to SBP variability, parasympathetic nerve activity masks changes in LF power that reflect an increase in RSNA and those in HF power that reflect a mechanical consequence of respiration.

Autonomic cardiovascular responses to hypercapnia in conscious rats: the roles of the chemo- and baroreceptors

The role of the autonomic nervous system, the central and peripheral chemoreceptors, and the arterial baroreceptors was examined in the cardiovascular response to hypercapnia in conscious rats chronically instrumented for the measurement of arterial blood pressure (ABP), heart rate (HR), and renal sympathetic nerve activity (RSNA). Rats were exposed to hypercapnia (6% CO2), and the cardiovascular and autonomic nervous responses in intact and carotid chemo- and/or aortic denervated rats were compared. In intact and carotid chemo-denervated rats, hypercapnia induced significant increases in mean ABP (MABP) and RSNA, and a significant decrease in HR. The HR decrease was reversed by atropine and eliminated by bilateral aortic denervation, which procedure, however, did not affect the MABP or RSNA response. Bilateral carotid chemo-denervation did not affect the baroreflex control of HR, although this control was attenuated by aortic denervation. Hypercapnia did not affect baroreflex sensitivity in intact rats. These results suggest that hypercapnia induces an increase in MABP due to an activation of sympathetic nervous system via central chemoreceptors and a decrease in HR due to a secondary reflex activation of the parasympathetic nervous system via arterial baroreceptors in response to the rise in ABP. In addition, carotid chemoreceptors do not play a major role in the overall cardiovascular response to hypercapnia in conscious rats. The mechanism responsible for the parasympatho-excitation may also involve CO2 induced aortic chemoreceptor simulation.

Suppression of hyperemia and DNA oxidation by indomethacin in cerebral ischemia

We investigated antioxidative activity and the effect of indomethacin, an agent that inhibits cyclooxygenase, on extracellular glutamate and cerebral blood flow in cerebral ischemia in gerbils. Pre-ischemic administration of indomethacin (5 mg/kg, i.p.) significantly rescued hippocampal CA1 neurons (9+/-6 cells/mm in the ischemia, 87+/-43 cells/mm in the indomethacin group, P<0.001). DNA fragmentation induced by ischemia was also examined using the terminal deoxynucleotidyl transferase-mediated UTP nick end labeling (TUNEL) method and indomethacin reduced TUNEL positive cells (140+/-21 in the ischemia, 99+/-31 in the indomethacin group, P<0.01). In addition, indomethacin attenuated the increase in hippocampal blood flow during reperfusion, but not increased extracellular glutamate by ischemia. Eight-hydroxydeoxyguanosine (8-OH-dG), a highly sensitive marker of DNA oxidation, was measured 90 min following ischemia using high-pressure liquid chromatography. Indomethacin significantly decreased the level of ischemia-induced 8-OH-dG in the hippocampus (P<0.05). These results suggest that indomethacin may protect neurons by attenuating oxidative stress and reperfusion injury in ischemic insult.

Brain neuronal nitric oxide synthase neuron-mediated sympathoinhibition is enhanced in hypertensive Dahl rats.

Objective To elucidate the role of central neurons containing neuronal nitric oxide synthase (nNOS neurons) in the sympathetic nervous system in hypertensive Dahl salt-sensitive (DS) rats. Design and methods Dahl rats were fed either a regular-salt (0.4% NaCl) or high-salt (8% NaCl) diet for 4 weeks. The effect of intracerebroventricular administration of S-methyl-L-thiocitrulline, a selective nNOS inhibitor, on renal sympathetic nerve activity was examined in chronically instrumented conscious DS rats. The activity and protein amount of brain nNOS was evaluated by enzyme assay and western blot analysis. The distribution and number of nNOS neurons in the brainstem were examined immunohistochemically in hypertensive and normotensive DS rats. Results S-methyl-L-thiocitrulline induced a larger increase in tonic renal sympathetic nerve activity generated before baroreflex-mediated inhibition in hypertensive DS rats than normotensive DS rats. Hypertensive DS rats showed increased nNOS activity in the brainstem, but not in the diencephalon or cerebellum. High nNOS activity was confirmed by an increase in the amount of nNOS protein. nNOS Neurons were localized in several nuclei throughout the brainstem; the dorsolateral periaqueductal gray, pedunculopontine tegmental nucleus, dorsal raphe nucleus, laterodorsal tegmental nucleus, lateral parabrachial nucleus, rostral ventrolateral medulla, nucleus tractus solitarius and raphe magnus. The number of nNOS neurons in these nuclei, except for the two raphes, was significantly greater in hypertensive than in normotensive DS rats. Conclusions These findings suggest that central nNOS-mediated sympathoinhibition may be enhanced in salt-sensitive hypertensive Dahl rats. The upregulated nNOS-mediated inhibition may occur in the central sympathetic control system generated before baroreflex-mediated inhibition.

Chemoreceptors in Autonomic Responses to Hypoxia in Conscious Rats

It has been suggested that the cardiovascular responses to hypoxia are produced as integrated interactions between the effects of the autonomic nervous system and the direct effect of hypoxia on the central nervous system (CNS) and the cardiovascular system (Daly, 1986; Marshall, 1994). However, hypoxic exposure has been delivered for relatively short periods of time before attaining a steady state in many works. Therefore, such responses may have resulted before integrated interactions began operating. There is also no information on the behavior of the autonomic nervous activities during hypoxia in conscious animals, which may be relevant to the study of an integrated function. In this study, the autonomic nervous activities are examined during the cardiovascular responses of the chemoreceptor-intact and of denervated conscious rats to systemic hypoxia, in order to determine the contribution of the arterial chemoreceptors to these responses.

Autonomic Cardiovascular Responses to Heme Oxygenase Inhibition in Conscious Rats

Carbon monoxide (CO) is produced in the course of heme degradation from biliverdin by heme oxygenase (HO) in various tissues, including the central nervous system. Recent studies suggest the inhibition of HO activity increases arterial pressure mediated by the autonomic nervous system. The present study was designed to investigate the autonomic regulation of cardiovascular responses to inhibition of endogenous CO production by the HO inhibitor Zinc deuteroporphyrin 2, 4-bis glycol (ZnDPBG) by using direct sympathetic nerve recordings in conscious, chronically instrumented rats. ZnDPBG induced increases in mean arterial pressure (MAP) (P<0.05) and renal sympathetic nerve activity (RSNA) (P<0.05) but no significant change in heart rate (P>0.05) in intact rats. In atropine-treated rats, ZnDPBG also induced increases in MAP (P<0.05) and RSNA (P<0.05) but no change in heart rate (P>0.05). In sinoaortic denervated rats, ZnDPBG induced increases in MAP (P<0.05), heart rate (P<0.05), and RSNA (P<0.05). ZnDPBG shifted the baroreflex curve for RSNA upward and to the right, which was characterized by increases in the maximum and minimum response and midpoint pressure without altering the maximum gain. These results indicate that inhibition of HO activity within the central nervous system causes sympathoexcitation, resulting in an increase in arterial pressure. We conclude that the CO/HO system plays an important role in cardiovascular regulation by modulating sympathetic tone.

Effect of water immersion on renal sympathetic nerve activity and arterial baroreflex in conscious rabbits with heart failure.

Several studies have indicated an interaction between cardiopulmonary mechanoreflex and arterial baroreflex. However, the contribution of cardiopulmonary mechanoreflex to an abnormal arterial baroreflex in chronic heart failure (CHF) has not been fully investigated. We examined the effect of the activation of cardiopulmonary mechanoreceptors induced by head-out water immersion (WI) on the arterial baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) in conscious rabbits with CHF induced by myocardial infarction. The arterial baroreflex sensitivity (BRS) of both HR and RSNA were decreased in CHF. WI induced a small decrease in RSNA in CHF compared to a sham-operated group (Sham), despite a similar increase in central venous pressure. WI did not affect BRS of HR or RSNA in either Sham or CHF. By averaging rectified RSNA recordings, we found that miniature RSNA in the control in CHF was higher than that in Sham. WI decreased the synchronized RSNA without changing miniature RSNA in either group. These results suggest that cardiopulmonary mechanoreflex control of RSNA is reduced in CHF, and that cardiopulmonary mechanoreflex has little effect on arterial baroreflex. An increase in miniature RSNA may reflect sympatho-excitation in CHF.

Cardiovascular and Respiratory Responses to Heme Oxygenase Inhibition in Conscious Rats

Many studies have indicated that endogenous carbon monoxide (CO) generated by the heme oxygenase (HO) system plays a physiological role as a second messenger (Maines, 1997). Although it has been suggested that the HO inhibitor-induced pressor response is mediated by the autonomic nervous system (Johnson et al. 1995), no direct evidence has been demonstrated by neural recordings. Furthermore, a recent study found that CO appears to modulate carotid body sensory activity (Prabhakar et al. 1995). However, the effect of HO inhibitor on tonic ventilatory regulation in conscious animals has not been examined. Accordingly, the main purpose of this study was to examine the respiratory response and the role of the autonomic nervous system in regulation of the cardiovascular response to the administration of zinc deuteroporphyrin 2, 4-bis glycol (ZnDPBG), an inhibitor of HO activity, in conscious rats. To assess the contribution of peripheral chemoreceptors (carotid and aortic bodies) to these responses, we also examined the effect of HO inhibition on respiration and the cardiovascular system in arterial chemo-denervated rats.

Effect of carbon dioxide on the structure of the carotid body: a comparison between normoxic and hypoxic conditions.

Laboratory for Anatomy and Physiology, Department of Sport and Medical Science, Kokushikan University, Tokyo, Department of Physiology, National Defense Medical College, Tokorozawa, Department of Medicine, Labour Welfare Corporation Ehime Rosai Hospital, Ehime, Department of Otorhinolaryngology, Yokohama City University School of Medicine, 3-9, Fukuura, Kanazawa-ku, Yokohama, and International Buddhist University, Osaka, Japan

Effect of CO2 on Cardiovascular Regulation in Conscious Rats

Carbon dioxide is known to have a significant effect on the cardiovascular and respiratory systems mainly by an action within the central nervous system (CNS), possibly on chemosensitive neurons on the surface of the medulla (Somerset al.,1989). CO2is also known to activate peripheral chemoreceptors. There are many reports on cardiovascular responses to hypercapnia in various experimental settings (Marshall, 1986, Rose et al., 1983, Somers et al., 1989, Walker, 1987). Although most studies indicated an increase in arterial blood pressure (ABP) during hypercapnia, the results of heart rate (HR) response were conflicting. We have previously reported that hypercapnia elicited cardiovascular changes due to an increase in sympathetic nerve activity (SNA) by the direct effect of CO2 on CNS and a simultaneous increase in parasympathetic nerve activity via an activation of peripheral chemoreceptors during hypoxia (Hirakawa et al., 1997). However, there is only a little information regarding a role of peripheral chemoreceptors in autonomic regulation of the cardiovascular system during hypercapnia without an interaction of hypoxia. In this study, we examined autonomic nervous activity and cardiovascular responses in intact and chemo-denervated conscious rats during normoxic hypercapnia.