L. B. Bell

Renal sympathetic and heart rate baroreflex function in conscious and isoflurane anaesthetized normotensive and chronically hypertensive rabbits.

1. Baroreflex control of heart rate (HR) has been studied in normotensive (NT) and hypertensive (HT) awake and anaesthetized animals and man, but baroreflex control of sympathetic nerve activity has not been well studied. We investigated baroreflex control of HR and renal sympathetic nerve activity (RSNA) over a wide range of arterial pressure (AP) in conscious and isoflurane (ISO) anaesthetized NT and HT rabbits.

INFLUENCE OF CARDIAC AFFERENTS ON TIME-DEPENDENT CHANGES IN THE RENAL SYMPATHETIC BAROREFLEX OF CONSCIOUS RABBITS

1. The stability of the renal sympathetic baroreflex and nasopharyngeal reflex, and the role of cardiac sensory receptors, was studied in conscious rabbits over a 5 h experimental period.

Baroreflex Modulation by Isoflurane Anesthesia in Normotensive and Chronically Hypertensive Rabbits

Publisher Summary Hemodynamic instability has been well documented in both humans and animals in hypertensive and is presumed to be at least partly because of impaired baroreflex control of efferent sympathetic activity. Control of arterial pressure (AP) is further complicated during anesthesia, as anesthetics attenuate arterial baroreflexes. The study presented in this chapter has been designed to investigate the individual effects of isoflurane (ISO) anesthesia and chronic hypertensive (HT) on baroreflex control of heart rate (HR) and efferent sympathetic nerve activity. In addition, the integrative effects of HT and anesthesia are discussed. A couple of studies in the chapter have demonstrated that increases in the level of ISO in already anesthetized animals produce progressive depression of the sympathetic baroreflex. In addition, the upper plateau and range of the AP-renal efferent sympathetic nerve activity (RSNA) baroreflex has been significantly reduced at all levels of ISO. It is also demonstrated that the AP-RSNA baroreflex remains functional during ISO anesthesia, although the ability to respond to transient falls in AP is decreased with increasing levels of ISO. It is showed that the range and slope of the HR baroreflex do not differ between normotensive (NT) and HT rabbits. The results of the present study indicate that the range and slope of the AP-RSNA baroreflex is not altered by hypertension in conscious rabbits. In the present study the resting RSNA is not different between conscious NT and HT rabbits. But resting HR in the present study has been significantly lower in HT rabbits than in NT rabbits. The results of the present study indicate that ISO anesthesia depressed both baroreflex curves to the same degree in NT and HT rabbits. The only difference is that baroreflex curves in HT rabbits have been displaced to the right along the pressure axis.

Respiratory alterations with intrapericardial procaine in the conscious rabbit

1 Intrapericardial procaine, used to produce cardiac nerve blockade in both conscious and anaesthetized animals, has been reported to also produce changes in respiration. This study systematically investigated the effects of two doses of intrapericardial procaine on respiration in the conscious rabbit. 2 Rabbits were pre‐instrumented with a chronic diaphragm electromyogram (dEMG) recording electrode and intrapericardial catheter. Arterial pressure, heart rate, dEMG and respiratory excursions (recorded with a pneumograph) were monitored in the conscious rabbit before and after intrapericardial and intravenous infusion of 2 and 5% procaine. Efficacy of cardiac nerve blockade was tested by intravenous infusion of phenyl biguanide. Arterial blood gases were determined at rest and during changes in respiration. 3 Following a low dose of intrapericardial procaine (12 mg), dEMG and respiratory excursions increased (65 ± 13 and 65 ± 13%, respectively) with no change in breathing frequency or arterial blood gases. Following a high dose of intrapericardial procaine (30 mg), four of six animals exhibited a similar response. However, four of the six rabbits also exhibited a second type of response pattern characterized by a further increase in respiratory efforts (65 ± 13%), abolition of dEMG, and a mild hypoxaemia. 4 Intravenous infusion of a low dose of procaine was without effect, whereas intravenous infusion of a high dose of procaine produced minor behavioural responses. 5 In four additional anaesthetized rabbits, it was demonstrated that high doses of intrapericardial procaine anaesthetized the phrenic nerve to produce the observed alterations in respiration. 6 We conclude that if intrapericardial procaine is used to block cardiac nerves in conscious rabbits, it should be used in a low concentration and at the lowest possible total dose to avoid complications due to changes in respiration.

Effects of Isoflurane on Regulation of Capacitance Vessels under Normotensive and Chronically Hypertensive Conditions

Publisher Summary This chapter summarizes several studies that demonstrate that significant differences exist in the reflex regulation of mesenteric capacitance veins in chronic renal wrap hypertensive versus normotensive rabbits. The heart rate response to aortic depressor nerve stimulation and to graded hypoxia has also been equally attenuated by isoflurane in both groups of animals. The two groups of animals also differ significantly in the baseline responses to isoflurane administration. In both groups of animals, 1.5% inhaled isoflurane significantly reduces heart rate and mean arterial pressure and dilated mesenteric capacitance vein diameter. However, in the hypertensive animals, the isoflurane-mediated reduction in arterial pressure has been significantly greater and the isoflurane-mediated increase in mesenteric vein diameter has been significantly less than that observed in the normotensive animals. The present study of the interaction between a volatile anesthetic and the hypertensive condition focused on changes in reflex responses that are mediated primarily by an elevation of sympathetic neural efferent activity. It has been observed that an inhibition of baroreflex and chemoreflex changes in heart rate, blood pressure, and mesenteric vein diameter during the administration of 0.75 minimum alveolar concentration (MAC) isoflurane in normotensive control animals. In addition, isoflurane has been shown to inhibit the relaxation effect of endothelium-derived nitric oxide on vascular smooth muscle cells in in vitro mesenteric vein ring preparations. In the hypertensive animals, the inhibitory effect of isoflurane on the heart rate responses to chemoreflex activation, on the blood pressure responses to both baroreflex and chemoreflex activation and on the mesenteric venodilation in response to aortic depressor nerve stimulation has been the same as in the normotensive control animals. Finally, it should be noted that the action of different anesthetic agents on sympathetically mediated reflex responses, particularly reflex regulation of venous tone, may vary.