Effects of acute anoxia on heart function in crucian carp: importance of cholinergic and purinergic control.
The objective of this study was to characterize the effects of acute anoxia on contractile and electrical activity in the heart of an anoxia-tolerant fish species, the crucian carp (Carassius carassius L.). Responses of atrial and ventricular tissue or isolated cells to NaCN, adenosine, and carbachol were determined to examine the effects of anoxia on cardiac performance and to clarify the possible role of local purinergic modulation and parasympathetic nervous control in the function of the anoxic fish heart. The contractility of the crucian carp heart is strongly decreased by acute anoxia. A rapid reduction in cardiac contractility is attained by reflex bradycardia and suppression of atrial contractility. These responses are mediated by muscarinic cholinergic receptors through the opening of inwardly rectifying potassium channels and are likely to protect the cardiac muscle from hypoxic/anoxic damage. The depletion of tissue oxygen content also directly depresses heart rate and cardiac force. Ultimately, an increase in cytosolic Ca(2+) concentration occurs that activates sarcolemmal Ca(2+) extrusion through the Na(+)-Ca(2+)-exchange and generates an inward exchange current with consequent depolarization of the resting membrane potential and possible cell death. At physiological concentration, the effects of adenosine on contractile and electrical activity were relatively weak, suggesting that the purinergic system is not involved in the acute anoxia response of the crucian carp heart. (+info)
Inhibition of baroreflex vagal bradycardia by selective stimulation of arterial chemoreceptors in rats.
We reported recently that hypoxia inhibits baroreflex vagal bradycardia (BVB) in rats and that this inhibition persists following chemoreceptor denervation. However, since it is possible that hypoxia also affects the central processing of chemoreceptive input, the existence of chemoreceptor-mediated inhibition of BVB cannot be ruled out. Therefore, we have studied whether selective chemoreceptor activation affects BVB in normoxic conditions. In pentobarbital-urethane-anaesthetized, succinylcholine-immobilized, artificially ventilated rats, BVB was provoked by electrical stimulation of the aortic depressor nerve. Arterial chemoreceptors were selectively activated by intracarotid injection of a minute amount of sodium cyanide. Cyanide injection consistently increased blood pressure while changing heart rate variably. BVB was inhibited in a dose-dependent manner. This inhibition, as well as changes in blood pressure and heart rate, was abolished following transection of the carotid sinus nerve (CSN) ipsilateral to the injection. Spinal cord transection at the C2 level did not affect the inhibition. On the other hand, intracarotid cyanide had no effect on bradycardia elicited by electrical stimulation of a peripheral cut end of the cervical vagus nerve. We conclude that chemoreceptor activation definitely inhibits BVB and that this inhibition is mediated by the CSN, and predominantly occurs in the central nervous system. The possibility is suggested that severe hypoxia suppresses not only BVB but also the chemoreceptor-mediated inhibition of BVB, both via the direct, central action. (+info)
Transient, reversible apnoea following ablation of the pre-Botzinger complex in rats.
1. In some anaesthetized preparations, eupnoea is eliminated following a blockade or destruction of neurons in a rostral medullary pre-Botzinger complex. 2. Neurons in this region might underlie the neurogenesis of eupnoea, or be the source of an input which is necessary for eupnoea to be expressed. If the latter, any apnoea following ablation of the pre-Botzinger complex might be reversed by an augmentation in 'tonic input.' Contrariwise, this apnoea should be permanent if the neuronal activities of the pre- Botzinger complex are an exclusive generator of the eupnoeic rhythm. 3. Decerebrate, vagotomized, paralysed and ventilated adult rats were studied. Efferent activity of the phrenic nerve was recorded as an index of ventilatory activity. 4. Blockade or destruction of neuronal activities of the pre-Botzinger complex by unilateral and/or bilateral injections of muscimol or kainic acid eliminated eupnoea only transiently. Eupnoea returned following activation of the peripheral chemoreceptors and spontaneously over time. 5. Results do not support the concept that neuronal activities of the pre-Botzinger complex play an exclusive role in the neurogenesis of eupnoea in vivo. Rather, these neuronal activities appear to provide a tonic input to the ponto-medullary circuit which generates eupnoea and/or appear to be one component of this circuit. (+info)
Nitric oxide increases persistent sodium current in rat hippocampal neurons.
1. The effects of nitric oxide (NO) donors on whole-cell, TTX-sensitive sodium currents and single sodium channels in excised patches were examined in rat hippocampal neurons. The whole-cell sodium current consisted of a large transient component (INa,t) and a smaller, inactivation-resistant, persistent component (INa,p). 2. In acutely dissociated neurons, the amplitude of the whole-cell INa, p increased by 60-80 % within a few minutes of exposure to either of two NO donors, sodium nitroprusside (SNP, 100 microM) or S-nitroso-N-acetyl-DL-penicillamine (SNAP, 100 microM). 3. The amplitude of INa,t was not changed significantly by the same concentrations of SNP and SNAP, indicating that NO had a selective effect on INa,p. 4. Both NO donors significantly increased the mean persistent current in excised inside-out patches from cultured hippocampal neurons. SNP at 10-100 microM increased average mean persistent current at a pipette potential (Vp) of +30 mV from -0.010 +/- 0.014 pA (control) to -2.91 +/- 1.41 pA (n = 10). SNAP at 3-100 microM increased the average mean inward current in six inside-out patches from -0.07 +/- 0.02 to -0.30 +/- 0.08 pA (Vp = +30 mV). 5. The increase in persistent Na+ channel activity recorded in inside-out patches in the presence of SNP or SNAP could be reversed by the reducing agent dithiothreitol (DTT, 2-5 mM) or by lidocaine (1-10 microM). 6. The average mean current recorded in the presence of SNP was 10-fold higher than that elicited by SNAP. The time delay before an increase was observed was shorter with SNP (4.0 +/- 0.8 min, n = 8) than with SNAP (8.4 +/- 1.6 min, n = 7). 7. A component of the SNP molecule added on its own, 5 mM sodium cyanide (NaCN), increased mean current in excised inside-out patches (Vp = +30 mV) from -0.06 +/- 0.04 to -0.58 +/- 0.21 pA (n = 19). This increase in channel activity could be blocked by 10 microM lidocaine and 2-5 mM DTT. 8. These results suggest that NO may directly increase the activity of neuronal persistent Na+ channels, but not transient Na+ channels, through an oxidizing action directly on the channel protein or on a closely associated regulatory protein in the plasma membrane. (+info)
A cytochrome bb'-type quinol oxidase in Bacillus subtilis strain 168.
The aerobic respiratory system of Bacillus subtilis 168 is known to contain three terminal oxidases: cytochrome caa(3), which is a cytochrome c oxidase, and cytochrome aa(3) and bd, which are quinol oxidases. The presence of a possible fourth oxidase in the bacterium was investigated using a constructed mutant, LUH27, that lacks the aa(3) and caa(3) terminal oxidases and is also deficient in succinate:menaquinone oxidoreductase. The cytochrome bd content of LUH27 can be varied by using different growth conditions. LUH27 membranes virtually devoid of cytochrome bd respired with NADH or exogenous quinol as actively as preparations containing 0.4 nmol of cytochrome bd/mg of protein but were more sensitive to cyanide and aurachin D. The reduced minus oxidized difference spectra of the bd-deficient membranes as well as absorption changes induced by CO and cyanide indicated the presence of a "cytochrome o"-like component; however, the membranes did not contain heme O. The results provide strong evidence for the presence of a terminal oxidase of the bb' type in B. subtilis. The enzyme does not pump protons and combines with CO much faster than typical heme-copper oxidases; in these respects, it resembles a cytochrome bd rather than members of the heme-copper oxidase superfamily. The genome sequence of B. subtilis 168 contains gene clusters for four respiratory oxidases. Two of these clusters, cta and qox, are deleted in LUH27. The remaining two, cydAB and ythAB, encode the identified cytochrome bd and a putative second cytochrome bd, respectively. Deletion of ythAB in strain LUH27 or the presence of the yth genes on plasmid did not affect the expression of the bb' oxidase. It is concluded that the novel bb'-type oxidase probably is cytochrome bd encoded by the cyd locus but with heme D being substituted by high spin heme B at the oxygen reactive site, i.e. cytochrome b(558)b(595)b'. (+info)
Effects on breathing of carotid body denervation in neonatal piglets.
The purpose of these studies was to test the hypothesis that carotid chemoreceptor activity is necessary for postnatal maturation of the ventilatory control system. By using a lateral surgical access, 17 piglets were carotid body denervated (CBD) and 14 were sham denervated at 3-25 days of age. After surgery, there was no irregular breathing in any group. There was no significant hypoventilation when CBD was performed at less than 5 days of age (n = 5) and only a mild (arterial PCO(2) 5 Torr; P < 0.05) to moderate, transient (arterial PCO(2) 8 Torr; P < 0.5) hypoventilation in piglets denervated at 10-15 (n = 6) and 20-25 (n = 6) days of age, respectively. Three weeks after surgery, both breathing of a hypoxic gas mixture and jugular venous NaCN injections elicited a hyperpnea in the CBD piglets that was attenuated compared with that in sham CBD piglets. In the CBD piglets, there was no response to injections of NaCN in the carotid arteries, but there was a response to NaCN injected into the proximal descending aorta, suggesting the residual peripheral chemosensitivity was of aortic origin. Carotid chemoreceptor-intact piglets had carotid and aortic NaCN chemosensitivity by 2 days of age. The carotid response persisted for the 40 days of the study, but the aortic reflex persisted only until approximately 8 days of age. We conclude that 1) the major effect of CBD per se in neonatal piglets is age-dependent hypoventilation and 2) there is a high degree of plasticity in peripheral chemosensitivity in neonates that may contribute to minimizing the changes in breathing after CBD. (+info)
MCC-134, a novel vascular relaxing agent, is an inverse agonist for the pancreatic-type ATP-sensitive K(+) channel.
The effects of a novel vasorelaxant agent, MCC-134 (1-[4-(1H-imidazol-1-yl)benzoyl]-N-methyl-cyclobutanecarbothioamide++ +), were examined on reconstituted ATP-sensitive K(+) (K(ATP)) channels, which are composed of an inwardly rectifying K(+) channel, Kir6.2, and three types of sulfonylurea receptors (SUR): SUR1, SUR2A, and SUR2B. Each type of K(ATP) channel was heterologously expressed in human embryonic kidney 293T cells. The expressed K(ATP) channel currents were measured with the whole-cell configuration of the patch-clamp method. MCC-134 activated the SUR2B/Kir6.2 channel, was a weak activator of the SUR2A/Kir6.2 channel, but did not activate the SUR1/Kir6.2 channel. MCC-134 suppressed SUR1/Kir6.2 channel currents that had been fully activated by either diazoxide or NaCN, whereas it did not affect the fully activated SUR2A/Kir6.2 or SUR2B/Kir6.2 channel currents. Thus, MCC-134, which is a relatively effective opener of the vascular smooth muscle type (SUR2B) of K(ATP) channel, is an antagonist of the pancreatic type (SUR1) of K(ATP) channel. Therefore, depending on the subtype of SUR, a pharmacological agent can cause either activation or inhibition of K(ATP) channel activity. (+info)
Presenilin-1 mutation increases neuronal vulnerability to focal ischemia in vivo and to hypoxia and glucose deprivation in cell culture: involvement of perturbed calcium homeostasis.
Many cases of early-onset inherited Alzheimer's disease (AD) are caused by mutations in the presenilin-1 (PS1) gene. Studies of cultured neural cells suggest that PS1 mutations result in perturbed cellular calcium homeostasis and may thereby render neurons vulnerable to apoptosis. In light of evidence that metabolic impairment plays a role in AD, that cerebral ischemia may be a risk factor for AD, and that individuals with AD have increased morbidity and mortality after stroke, we examined the impact of a PS1 mutation on neuronal vulnerability to ischemic injury. We report that the extent of brain injury after focal cerebral ischemia reperfusion is increased, and behavioral outcome is worsened, in PS1 mutant knock-in mice compared to wild-type mice. Cultured cortical neurons from PS1 mutant mice exhibit increased vulnerability to glucose deprivation and chemical hypoxia compared to their wild-type counterparts. Calcium imaging studies demonstrated enhanced elevation of intracellular calcium levels after glucose deprivation and chemical hypoxia in neurons from PS1 mutant mice. Agents that block calcium release from IP(3)- and ryanodine-sensitive stores (xestospongin and dantrolene, respectively) protected against the endangering action of the PS1 mutation. Our data suggest that presenilin mutations may promote neuronal degeneration in AD by increasing the sensitivity of neurons to age-related ischemia-like conditions. The data further suggest that drugs that stabilize endoplasmic reticulum calcium homeostasis may prove effective in suppressing the neurodegenerative process in AD patients. (+info)