The chronic infusion of hexamethonium and phenylephrine to effectively clamp sympathetic vasomotor tone. A novel approach

Hypotensive effects of ghrelin receptor agonists mediated through a novel receptor

BACKGROUND AND PURPOSE

Some agonists of ghrelin receptors cause rapid decreases in BP. The mechanisms by which they cause hypotension and the pharmacology of the receptors are unknown.

EXPERIMENTAL APPROACH

The effects of ligands of ghrelin receptors were investigated in rats in vivo, on isolated blood vessels and on cells transfected with the only molecularly defined ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR1a).

KEY RESULTS

Three agonists of GHSR1a receptors, ulimorelin, capromorelin and CP464709, caused a rapid decrease in BP in the anaesthetized rat. The effect was not reduced by either of two GHSR1a antagonists, JMV2959 or YIL781, at doses that blocked effects on colorectal motility, in vivo. The rapid hypotension was not mimicked by ghrelin, unacylated ghrelin or the unacylated ghrelin receptor agonist, AZP531. The early hypotension preceded a decrease in sympathetic nerve activity. Early hypotension was not reduced by hexamethonium or by baroreceptor (sino-aortic) denervation. Ulimorelin also relaxed isolated segments of rat mesenteric artery, and, less potently, relaxed aorta segments. The vascular relaxation was not reduced by JMV2959 or YIL781. Ulimorelin, capromorelin and CP464709 activated GHSR1a in transfected HEK293 cells at nanomolar concentrations. JMV2959 and YIL781 both antagonized effects in these cells, with their pA2 values at the GHSR1a receptor being 6.55 and 7.84.

CONCLUSIONS AND IMPLICATIONS

Our results indicate a novel vascular receptor or receptors whose activation by ulimorelin, capromorelin and CP464709 lowered BP. This receptor is activated by low MW GHSR1a agonists, but is not activated by ghrelin.

Effect of peripheral sympathetic nerve dysfunction on salt sensitivity of arterial pressure

1. Dysregulation of peripheral sympathetic pathways contributes to some forms of salt-dependent hypertension. However, at the present time it is not known whether salt-induced activation of sympathetic nerves or loss of normal sympathoinhibitory responses to salt-induced volume expansion contributes to neurogenic salt-dependent hypertension. The present study was performed to the test the hypothesis that loss of peripheral sympathetic nerve function results in salt-dependent hypertension. 2. The effect of three pharmacological interventions of sympathetic nerve function on the long-term salt-sensitivity of mean arterial pressure (MAP) were measured: (i) blockade of ganglionic transmission with hexamethonium (HEX; n = 5); (ii) destruction of sympathetic nerve terminals with guanethidine (GUAN; n = 7); and (iii) alpha-adrenoceptor blockade with two specific antagonists, namely prazosin (PRAZ; n = 7) and terazosin (TERAZ; n = 8). 3. Mean arterial pressure and heart rate were measured 24 h/day by radiotelemetry in conscious rats during 5 days of normal and 7 days of high (HNa) dietary sodium intake. Despite marked increases in both sodium and water intake during 7 days of the HNa diet, no statistically significant changes in MAP were observed in HEX, GUAN, PRAZ or TERAZ groups. 4. We conclude that loss of peripheral sympathetic neural pathways alone does not cause salt-dependent hypertension in the rat.

Hemodynamic effects of di-sec-butyl phenol, an anesthetic substituted phenol

Dose- and age-related hemodynamic effects were determined for an anesthetic substituted phenol, 2,6-di-sec-butyl phenol (DSB). DSB, 7.5 mg/kg, induced hypnosis in young rabbits and increased mean blood pressure to 170 +/- 14% and heart rate to 150 +/- 21% of control values. In elderly rabbits, 7.5 mg/kg DSB induced hypnosis, had no effect on blood pressure, but increased the heart rate to 130 +/- 2% of control. After ganglionic blockade with hexamethonium, 7.5 mg/kg DSB caused a decline in mean blood pressure (71 +/- 5% of control) without change in heart rate. DSB increased norepinephrine release from SH-SY5Y cells, a human neuroblastoma cell line (5.4 +/- 1.7% vs. 3.5 +/- 0.3%). DSB produced age-dependent elevation of mean blood pressure in rabbits, probably by causing release of catecholamines from the sympathetic nervous system.

Role of a Responsive Sympathetic Nervous System in the Chronic Hypotensive Effects of Losartan in Normal Rats

We have previously demonstrated the chronic hypotensive effects of the AT1 antagonist losartan in normotensive, salt-replete rats. We hypothesized that the chronic effects of losartan are mediated in part by blockade of the central sympathoexcitatory actions of angiotensin II. To test this hypothesis, we have used a novel approach to effectively "clamp" the sympathetic nervous system at a fixed level through chronic administration of the ganglionic blocking agent hexamethonium (15 mg/kg/h) and the alpha agonist phenylephrine (2.26 mg/kg/d). Two of 3 groups of rats [CON and CLAMP(NNa)] were placed on (0.1%) NaCl diets, whereas the third [CLAMP(LNa)] was placed on a low (0.002%)-sodium diet. Continuous measurements of mean arterial pressure (MAP) were made via radiotelemetry. After 9 days of hexamethonium plus phenylephrine treatment in CLAMP(NNa) and CLAMP(LNa) rats, baseline MAP was not different in all 3 groups of rats: CON (104+/-4 mm Hg), CLAMP(NNa) (104+/-4 mm Hg), and CLAMP(LNa) (106+/-2 mm Hg). After 5 days of subsequent losartan treatment, a change in MAP of only -7+/-2 mm Hg was observed in CLAMP(NNa) rats compared with -22+/-2 mm Hg in CON and CLAMP(LNa) rats. These results do not support the hypothesis that the hypotensive actions of losartan are entirely dependent on a responsive sympathetic nervous system rats.

Autonomic Mechanisms in the Age-Related Hypotensive Effect of Propofol

We hypothesized that age-related differences in cardiovascular regulatory processes play a role in the augmented vasodepressor response to anesthetic induction with propofol in older subjects. To test this hypothesis, differences in baroreceptor responsiveness (BR) were first demonstrated in young adult (6-12 mo, n=12) and aged (>42 mo, n=12) New Zealand rabbits, and then the vasodepressor effect of propofol was compared in both the absence and presence of ganglionic blockade. For each age group, half of the animals were pretreated with 20 mg/kg IV hexamethonium (HEX) with the remaining half designated as controls. BR was first assessed by plotting cardiac cycle length as a function of the decline in mean arterial blood pressure (MAP) produced by multiple IV boluses of tri-nitroglycerine. Propofol was then given as an IV bolus of 4.5, 6.4, or 8.4 mg/kg over 3 s. Each animal was studied three times, receiving a single dose in variable order with at least 7 days between injections. In control animals, marked age-related differences in BR were evident and propofol produced larger peak decreases in MAP in older rabbits at all doses. HEX pretreatment abolished BR for both young and aged rabbits. However, after HEX administration the vasodepressor response to propofol in young animals was enhanced by 150% at 4.5, 125% at 6.4, and 61% at 8.4 mg/kg, respectively, whereas the impact in aged animals was only 25%, 30%, and -10%, respectively. These data support the hypothesis that age-related enhancement of propofol-induced hypotension is largely a reflection of diminished BR.

Sympathetic mechanisms in cerebral blood flow alterations induced by spinal cord stimulation

OBJECT

Cervical spinal cord stimulation (SCS) has been found to augment cerebral blood flow (CBF) in a number of animal models, although the mechanisms underlying the cerebrovascular effects of SCS are poorly described. In this study, the authors examined the role of sympathetic tone in CBF alterations induced by SCS in rats.

METHODS

Spinal cord stimulation was performed at three intervals while CBF was monitored with laser Doppler flowmetry (LDF). Either hexamethonium (5, 10, or 20 mg/kg), prazosin (0.25, 0.5, or 1 mg/kg), idazoxan (0.5, 1, or 2 mg/kg), propranolol (1, 2, or 4 mg/kg), or vehicle was administered intravenously before the second stimulation. Changes in LDF values due to SCS were recorded as the percentage of change from baseline values and were analyzed. In vehicle-treated animals, SCS increased LDF values by 60.5 +/- 1.8% over baseline, whereas both high-dose hexamethonium and prazosin completely abolished the SCS-induced increases in LDF values. On the other hand, LDF values increased by 50.9 +/- 4% and 61.4 +/- 4% after SCS in the presence of idazoxan or propranolol, respectively. Administration of sympathetic nervous system blockers resulted in a variable degree of systemic hypotension as well. Nevertheless, induced hypotension without sympathetic blockade had only a minimal effect on SCS-induced augmentation of LDF values (48 +/- 1.4% over baseline).

CONCLUSIONS

Sympathetic tone plays a major role in SCS-induced increases in CBF. This effect seems to be mediated primarily by alpha1-adrenergic receptors. Systemic hypotension alone cannot explain the effects of sympathetic blockade on the SCS response. Clinical use of SCS in the treatment of cerebral ischemia should take alpha1-adrenergic receptor sympathetic tone into account.