Effects of alpha-adrenoceptor-stimulating drugs on baroreceptor reflexes in conscious cats

Arterial baroreceptor reflex counteracts long-term blood pressure increase in the rat model of renovascular hypertension

INTRODUCTION

The present study tested the hypothesis that long-term effects of baroreceptor activation might contribute to the prevention of persistent arterial blood pressure (BP) increase in the rat model of renovascular hypertension (HTN).

METHODS

Repetitive arterial baroreflex (BR) testing was performed in normo- and hypertensive rats. The relationship between initial arterial BR sensitivity and severity of subsequently induced two-kidney one-clip (2K1C) renovascular HTN was studied in Wistar rats. Additionally, the time course of changes in systolic BP (SBP) and cardiac beat-to-beat (RR) interval was studied for 8 weeks after the induction of 2K1C renovascular HTN in the rats with and without sinoaortic denervation (SAD). In a separate experimental series, cervical sympathetic nerve activity (cSNA) was assessed in controls, 2K1C rats, WKY rats, and SHR.

RESULTS

The inverse correlation between arterial BR sensitivity and BP was observed in the hypertensive rats during repetitive arterial BR testing. The animals with greater initial arterial BR sensitivity developed lower BP values after renal artery clipping than those with lower initial arterial BR sensitivity. BP elevation during the first 8 weeks of renal artery clipping in 2K1C rats was associated with decreased sensitivity of arterial BR. Although SAD itself resulted only in greater BP variability but not in persistent BP rise, the subsequent renal artery clipping invariably resulted in the development of sustained HTN. The time to onset of HTN was found to be shorter in the rats with SAD than in those with intact baroreceptors. cSNA was significantly greater in the 2K1C rats than in controls.

CONCLUSIONS

Arterial BR appears to be an important mechanism of long-term regulation of BP, and is believed to be involved in the prevention of BP rise in the rat model of renovascular HTN.

Oral clonidine reduces the requirement of prostaglandin E1 for induced hypotension

PURPOSE

To determine the effects of preanesthetic oral clonidine on the dose of prostaglandin EI (PGEI) required to produce hypotension during anesthesia.

METHOD

Oral placebo, 75 microg or 150 microg clonidine were administered 60 min prior to induction of anesthesia. Anesthesia was maintained with O2:N2O (30:70) and isoflurane 1.0%. After hemodynamic stabilization, an infusion of prostaglandin EI was started (0.05 microg x kg(-1) x min(-1)) and the rate of infusion was adjusted to maintain mean arterial pressure (MAP) between 60-70 mm Hg during operation.

RESULTS

Duration of hypotension in placebo, 75 microg and 150 microg preanesthetic oral clonidine treated groups were 132+/-46, 117+/-37 and 129+/-56 min, respectively. The PGEI requirement in each group were 1563+/-180 (28.6+/-3.2), 594+/-197 (10.8+/-3.6) and 283+/-30 (5.5+/-3.6) microg (microg x kg(-1)), respectively. In addition, blood loss in each group were 1461+/-389, 805+/-240 and 931+/-40 ml, respectively.

CONCLUSION

Preanesthetic oral clonidine decreased the dose of PGEI required to produce hypotension, and decreased the blood loss during operation.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Possible mechanisms underlying the pressure responses evoked in conscious cats by emotional stress

The character of baroreceptor reflexes was studied in conscious cats both under resting conditions and during emotional stress. During naturally evoked emotional tension 32 cats showed a two-phase change in arterial pressure: an initial hypotension was followed by a rise in arterial pressure. The pressor component was accompanied by a suppression of the baroreceptor reflex evoked by phenylephrine (i.v.). Electrical stimulation of the hypothalamic "defence area" also resulted in a raised arterial pressure and a suppression of the baroreceptor reflex. 3-7 days after bilateral section of the carotid sinus and aortic nerves 5 cats showed a reduction in arterial pressure only when confronted by a dog. When a reflex response to phenylephrine began to return, the depressor responses seemed to diminish and pressor responses reappeared. "Mild" (50-100 microA) stimulation of the hypothalamic "defence area" also caused a reduction of arterial pressure. However, a rise in arterial pressure occurred when the electrical stimulation of the hypothalamic "defence area" was increased. In 2 cats the pressor response caused by confrontation with a dog was not accompanied by any noticeable inhibition of the baroreceptor reflex. After cutting the sino-carotid and aortic nerves in these animals, emotional stress produced accentuated pressor responses. The results suggest that the presence of baroreceptor reflexes is a prerequisite for the development of hypertensive responses during emotional stress.

Effect of indapamide on the baroreceptor reflex in essential hypertension

The effect of chronic treatment with indapamide on blood pressure (BP), baroreceptor sensitivity (BRS) and vascular reactivity (VR) were investigated in 10 patients with essential hypertension. After 3 months of therapy with indapamide 2.5 mg/d the mean arterial pressure (MAP) had decreased from 135 +/- 6 to 112 +/- 2 mmHg (p less than 0.001); the heart rate (HR) had not changed, VR had decreased from 6.1 +/- 1.2 to 4.8 +/- 1.8 (pg . min . kg)-1 (p less than 0.05), and BRS had increased from 8.3 +/- 3.7 to 12.2 +/- 5.3 ms/mmHg (p less than 0.005), with a leftshift of the relationship between BP and heart period. An inverse correlation was found between the pre-treatment systolic blood pressure and the change in baroreceptor sensitivity after indapamide (r = 0.59; p less than 0.05). In conclusion, chronic treatment with indapamide enhances BRS and resets the reflex. The resetting may account for the lack of tachycardia at rest observed after treatment with indapamide. The mechanism by which indapamide interferes with the baroreceptor reflex requires further investigations.