Effects of chronic lesions of the anteroventral third ventricle region on baroreceptor reflex function in conscious rats

Tempol Reverses the Negative Effects of Morphine on Arterial Blood-Gas Chemistry and Tissue Oxygen Saturation in Freely-Moving Rats

We have reported that pretreatment with the clinically approved superoxide dismutase mimetic, Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl), blunts the cardiorespiratory depressant responses elicited by a subsequent injection of fentanyl, in halothane-anesthetized rats. The objective of the present study was to determine whether Tempol is able to reverse the effects of morphine on arterial blood-gas (ABG) chemistry in freely-moving Sprague Dawley rats. The intravenous injection of morphine (10 mg/kg) elicited substantial decreases in pH, pO₂ and sO₂ that were accompanied by substantial increases in pCO₂ and Alveolar-arterial gradient, which results in diminished gas-exchange within the lungs. Intravenous injection of a 60 mg/kg dose of Tempol 15 min after the injection of morphine caused minor improvements in pO₂ and pCO₂ but not in other ABG parameters. In contrast, the 100 mg/kg dose of Tempol caused an immediate and sustained reversal of the negative effects of morphine on arterial blood pH, pCO₂, pO₂, sO₂ and Alveolar-arterial gradient. In other rats, we used pulse oximetry to determine that the 100 mg/kg dose of Tempol, but not the 60 mg/kg dose elicited a rapid and sustained reversal of the negative effects of morphine (10 mg/kg, IV) on tissue O₂ saturation (SpO₂). The injection of morphine caused a relatively minor fall in mean arterial blood pressure that was somewhat exacerbated by Tempol. These findings demonstrate that Tempol can reverse the negative effects of morphine on ABG chemistry in freely-moving rats paving the way of structure-activity and mechanisms of action studies with the host of Tempol analogues that are commercially available.

Tachycardic responses to stimulation of β-adrenoceptors in the brain parenchyma in conscious rats

This study aimed to investigate how stimulation of β-adrenoceptors in the anteroventral third ventricular region (AV3V; a pivotal forebrain area for autonomic functions) and other brain regions affects heart rate (HR) in conscious rats. Topical injections of the β-adrenergic agonist isoproterenol (Isop) into the AV3V caused dose-related and reversible increases in HR. Only its highest dose utilized significantly affected blood pressure (BP), inducing a decrease. The tachycardia due to AV3V Isop lasted significantly longer than that elicited by hypotension, and was inhibited by AV3V administration of the β-adrenergic antagonist propranolol or systemic infusion of a ganglion blocker hexamethonium. Plasma noradrenaline indicative of sympathetic nerve activity increased in parallel with rises in HR after the AV3V application of Isop. When Isop was locally injected into various brain regions other than the AV3V, region-related effectiveness in provoking tachycardia was observed that tended to be large in limbic structures and the hypothalamic paraventricular nucleus. No region responded to Isop applications with decreases in HR. These results suggest that β-adrenoceptors in the AV3V and other brain regions may be able to produce tachycardia by enhancing, at least in part, the efferent sympathetic nerve activity controlling cardiac function.

Dietary Effects of Structured Lipids and Phytosteryl Esters on Cardiovascular Function in Spontaneously Hypertensive Rats

This study examined the dietary effects of sesame oil (SO)-based structured lipids (SL) and phytosteryl esters (PE) on cardiovascular function in conscious spontaneously hypertensive rats (SHR) fed high-fat (HF) diets (20% w/w fat). The dietary groups were as follows: normal diet (4.5% w/w fat), SO, SO fortified with PE (SOP), SL, and SL fortified with PE (SLP). Mean arterial blood pressures were similar in all groups, whereas resting heart rates (HR) were higher in all HF-fed groups. The pressor responses to the alpha1-adrenoceptor agonist, phenylephrine (5 microg/kg), were similar in all groups. However, the pressor responses to phenylephrine (10 microg/kg) were diminished in SO- or SL-fed SHR, whereas they were not diminished in SOP- or SLP-fed SHR. The depressor responses elicited by the nitric oxide (NO) donor, sodium nitroprusside (5 and 10 microg/kg), were not diminished in HF-fed rats. Baroreflex-mediated changes in HR were variously decreased in the HF-fed groups, and this decrease tended to be greater in SOP and SLP than in SO and SL groups. The depressor and tachycardic responses elicited by the beta-adrenoceptor agonist, isoproterenol, were equivalent in all groups. The depressor responses elicited by the endothelium-dependent agonist, acetylcholine (0.1 microg/kg), and the hypertension elicited by the NO synthesis inhibitor, NG-nitro-L-arginine methylester (25 micromol/kg), were similar in all groups. These findings demonstrate that (1) HF diets increase resting HR and impair baroreflex function in SHR, whereas they do not obviously affect endothelium-dependent vasodilation, and (2) fortification with PE may be deleterious to cardiovascular function (eg, baroreflex activity) in SHR.

Peripheral hyperosmolality reduces cardiac baroreflex sensitivity

The effects of acute increases in peripheral osmolality on cardiac baroreflex sensitivity were tested in conscious, unrestrained rats by fitting values for arterial blood pressure and heart rate during acute pressor and depressor responses to a four-parameter, sigmoid logistic function curve. Cardiac baroreceptor reflex function curves were produced before and following 30 min intravenous (i.v.) infusion of 2.5 M NaCl, an equipressor concentration of phenylephrine, isotonic saline, or 2.5 M NaCl following central administration of an angiotensin-converting enzyme (ACE) inhibitor. Hypertonic saline infusion increased blood pressure, reduced heart rate and baroreflex sensitivity (gain and range), and shifted the baroreflex function curve to a higher operating blood pressure. These effects were prevented or attenuated by prior central administration of an ACE inhibitor. Isotonic volume expansion did not alter baroreflex responses. Finally, phenylephrine produced similar changes in blood pressure but did not alter cardiac baroreflex responses. These data demonstrate that acute increases in peripheral osmolality and/or sodium decrease cardiac baroreflex sensitivity through generation of central angiotensin II, independent of changes in extracellular fluid volume and blood pressure.