Effects of age and hypertension on cardiac responses to the alpha1-agonist phenylephrine in humans

Effects of phenylephrine on systemic and cerebral circulations in humans: a systematic review with mechanistic explanations

We conducted a systematic review of the literature reporting phenylephrine-induced changes in blood pressure, cardiac output, cerebral blood flow and cerebral tissue oxygen saturation as measured by near-infrared spectroscopy in humans. We used the proportion change of the group mean values reported by the original studies in our analysis. Phenylephrine elevates blood pressure whilst concurrently inducing a reduction in cardiac output. Furthermore, despite increasing cerebral blood flow, it decreases cerebral tissue oxygen saturation. The extent of phenylephrine's influence on cardiac output (r = -0.54 and p = 0.09 in awake humans; r = -0.55 and p = 0.007 in anaesthetised humans), cerebral blood flow (r = 0.65 and p = 0.002 in awake humans; r = 0.80 and p = 0.003 in anaesthetised humans) and cerebral tissue oxygen saturation (r = -0.72 and p = 0.03 in awake humans; r = -0.24 and p = 0.48 in anaesthetised humans) appears closely linked to the magnitude of phenylephrine-induced blood pressure changes. When comparing the effects of phenylephrine in awake and anaesthetised humans, we found no evidence of a significant difference in cardiac output, cerebral blood flow or cerebral tissue oxygen saturation. There was also no evidence of a significant difference in effect on systemic and cerebral circulations whether phenylephrine was given by bolus or infusion. We explore the underlying mechanisms driving the phenylephrine-induced cardiac output reduction, cerebral blood flow increase and cerebral tissue oxygen saturation decrease. Individualised treatment approaches, close monitoring and consideration of potential risks and benefits remain vital to the safe and effective use of phenylephrine in acute care.

Acute Cannabigerol Administration Lowers Blood Pressure in Mice

Cannabigerol is a cannabinoid compound synthesized by Cannabis sativa, which in its acid form acts as the substrate for both Δ⁹-tetraydrocannabinol and cannabidiol formation. Given its lack of psychoactive effects, emerging research has focused on cannabigerol as a potential therapeutic for health conditions including algesia, epilepsy, anxiety, and cancer. While cannabigerol can bind to classical cannabinoid receptors, it is also an agonist at α2-adrenoreceptors (α2AR) which, when activated, inhibit presynaptic norepinephrine release. This raises the possibility that cannabigerol could activate α2AR to reduce norepinephrine release to cardiovascular end organs to lower blood pressure. Despite this possibility, there are no reports examining cannabigerol cardiovascular effects. In this study, we tested the hypothesis that acute cannabigerol administration lowers blood pressure. Blood pressure was assessed via radiotelemetry at baseline and following intraperitoneal injection of cannabigerol (3.3 and 10 mg/kg) or vehicle administered in a randomized crossover design in male C57BL/6J mice. Acute cannabigerol significantly lowered mean blood pressure (-28 ± 2 mmHg with 10 mg/kg versus -12 ± 5 mmHg vehicle, respectively; p = 0.018), with no apparent dose responsiveness (-22 ± 2 mmHg with 3.3 mg/kg). The depressor effect of cannabigerol was lower in magnitude than the α2AR agonist guanfacine and was prevented by pretreatment with the α2AR antagonist atipamezole. These findings suggest that acute cannabigerol lowers blood pressure in phenotypically normal mice likely via an α2AR mechanism, which may be an important consideration for therapeutic cannabigerol administration.

Mn(II) Catalyzed Oxidation of Atenolol by Cerium(IV) in Aqueous Sulfuric Acid Medium: A Spectrophotometer Aided Kinetic, Mechanistic and Thermodynamic Study

The kinetics and mechanism of manganese(II) catalyzed oxidation of atenolol by cerium(IV) sulfate in aqueous H2SO4 at a constant ionic strength of 0.50 mol dm-3 was studied spectrophotometrically. The reaction showed first order kinetics in cerium(IV) whereas fractional order in both manganese(II) and atenolol. Addition of products showed no effect on the rate of the reaction. The main product, 2-(4-(2-hydroxy-3-oxopropoxy)phenyl)acetamide, was identified with the aid of IR and mass spectral data. Stoichiometry with respect to the drug substrate and reagent was established as 2:1. Added H2SO4, SO4 2− and HSO4 − showed negligible effect on the rate of the reaction. HCe(SO4)3 − was found to be the predominant reactive species under the specified experimental conditions. The rate constants (k), catalytic constant (kc ) and equilibrium constant (K 6) for the proposed mechanism were determined. The kinetic and thermodynamic activation parameters were computed for both the slow rate determining step and complex forming equilibrium step.

Activation of α1A -adrenoceptors desensitizes the rat aorta response to phenylephrine through a neuronal NOS pathway, a mechanism lost with ageing

BACKGROUND AND PURPOSE

A NO-mediated desensitization of vasoconstrictor responses evoked by stimulation of α₁ -adrenoceptors has been reported in different vessels. We investigated the involvement of each α₁ -adrenoceptor subtype and constitutive NOS isoforms and the influence of ageing and hypertension on this process.

EXPERIMENTAL APPROACH

Wistar and spontaneously hypertensive rats (SHR), 16, 32, 52 and 72 weeks-old, were used to evaluate the desensitization process. Expression of α₁ -adrenoceptor subtypes, endothelial NOS (eNOS) and neuronal NOS (nNOS) were determined in rat aorta and left ventricle (LV). Expression levels were also evaluated in LV of a group of heart failure patients with a wide age range.

KEY RESULTS

Repeated application of phenylephrine decreased subsequent α₁ -adrenoceptor-mediated vasoconstriction by increasing nNOS protein expression in aorta, but not in tail or mesenteric resistance arteries, where mRNA levels of nNOS were undetectable. This desensitization process disappeared in the absence of endothelium or in the presence of L-NAME (100 μM), nNOS inhibitors, SMTC (1 μM) and TRIM (100 μM), and 5-methylurapidil (100 nM, α_1A -antagonist), but not BMY7378 (10 nM, α_1D -antagonist). The α_1A /nNOS-mediated desensitization was absent in aged SHR and Wistar animals, where the expression of α_1A -adrenoceptors was reduced in aorta and LV. In human LV, a negative correlation was found between age and α_1A -adrenoceptor expression.

CONCLUSIONS AND IMPLICATIONS

The α_1A -adrenoceptor subtype, through endothelial nNOS-derived NO, may act as a physiological 'brake' against the detrimental effects of excessive α₁ -adrenoceptor-mediated vasoconstriction. Reduced α_1A -adrenoceptor- and nNOS-mediated desensitization in aged patients could be involved in the age-dependent elevation of adrenergic activity.

Impact of Restless Legs Syndrome on Cardiovascular Autonomic Control

STUDY OBJECTIVES

To examine whether patients with restless legs syndrome demonstrate specific alterations in cardiovascular autonomic control.

METHODS

Patients with moderate-severe restless legs syndrome (n = 20, 80% female) and controls (n = 20) matched for age, sex, body mass index, and free of hypertension and cardiovascular disease were enrolled. We assessed cardiovagal baroreflex gain via the modified Oxford technique, sympathetically mediated vascular responses to isometric exercise to fatigue, bradycardiac response to Valsalva maneuver, and respiratory sinus arrhythmia during paced breathing. Standard electrocardiography, beat-by-beat arterial pressure, respiration, and popliteal blood flow velocity were recorded continuously.

RESULTS

Resting blood pressure and heart rate were similar between groups. However, baroreflex gain averaged 14.3 ± 1.4 msec/mm Hg in restless legs syndrome and was lower than in controls (22.6 ± 3.5 msec/mm Hg, P = 0.04). Hemodynamic responses to isometric exercise were similar between groups, though participants with restless legs syndrome had lower leg blood flow (P < 0.001), with greater leg vascular resistance (P < 0.0001), before and during isometric exercise. Respiratory sinus arrhythmia and Valsalva ratios were similar between groups. Neither baroreflex gain nor vascular resistance was correlated with sleep duration, sleep quality, or symptom duration.

CONCLUSION

Patients with restless legs syndrome demonstrate compromised cardiovagal control, specific to the arterial baroreflex, with greater peripheral vascular resistance, potentially due to heightened sympathetic outflow. These autonomic alterations may directly relate to the higher prevalence of cardiovascular disease in restless legs syndrome.

Autonomic control of the heart is altered in Sprague-Dawley rats with spontaneous hydronephrosis

The renal medulla plays an important role in cardiovascular regulation, through interactions with the autonomic nervous system. Hydronephrosis is characterized by substantial loss of renal medullary tissue. However, whether alterations in autonomic control of the heart are observed in this condition is unknown. Thus we assessed resting hemodynamics and baroreflex sensitivity (BRS) for control of heart rate in urethane/chloralose-anesthetized Sprague-Dawley rats with normal or hydronephrotic kidneys. While resting arterial pressure was similar, heart rate was higher in rats with hydronephrosis (290 ± 12 normal vs. 344 ± 11 mild/moderate vs. 355 ± 13 beats/min severe; P < 0.05). The evoked BRS to increases, but not decreases, in pressure was lower in hydronephrotic rats (1.06 ± 0.06 normal vs. 0.72 ± 0.10 mild/moderate vs. 0.63 ± 0.07 ms/mmHg severe; P < 0.05). Spectral analysis methods confirmed reduced parasympathetic function in hydronephrosis, with no differences in measures of indirect sympathetic activity among conditions. As a secondary aim, we investigated whether autonomic dysfunction in hydronephrosis is associated with activation of the renin-angiotensin system (RAS). There were no differences in circulating angiotensin peptides among conditions, suggesting that the impaired autonomic function in hydronephrosis is independent of peripheral RAS activation. A possible site for angiotensin II-mediated BRS impairment is the solitary tract nucleus (NTS). In normal and mild/moderate hydronephrotic rats, NTS administration of the angiotensin II type 1 receptor antagonist candesartan significantly improved the BRS, suggesting that angiotensin II provides tonic suppression to the baroreflex. In contrast, angiotensin II blockade produced no significant effect in severe hydronephrosis, indicating that at least within the NTS baroreflex suppression in these animals is independent of angiotensin II.

Effects of hypertension on cardiovascular responses to epinephrine in humans

Cardiac beta-receptor responsiveness is diminished by both aging and hypertension. However, concomitant decreases in the activity of counterregulatory mechanisms, such as the arterial baroreflex and neuronal catecholamine uptake, influence the ultimate cardiac responses to adrenergic agents in vivo. In the present study, we evaluated by echocardiography cardiac responses to intravenous infusion of epinephrine in 14 young and 18 older normotensive men and women and in 10 young and 17 older hypertensive men and women. To assess the relative contribution of intrinsic cardiac and counterregulatory components to the overall response, infusions were repeated combined with a ganglionic blocker in the young groups. Epinephrine-induced increases in heart rate were similar in the four groups. Increases in stroke volume, ejection fraction, and cardiac index were similar in the two hypertensive and two young normotensive groups. In contrast, they were attenuated in the older normotensive group, resulting in higher left ventricular responses in older hypertensive than in normotensive subjects. Heart rate and left ventricular responses to epinephrine in the presence of ganglionic blockade did not differ between the two young groups. Increases in plasma norepinephrine due to epinephrine infusion were larger in hypertensive than in normotensive subjects. One may conclude that compared with young normotensive subjects, in hypertensive subjects mechanisms increasing versus decreasing cardiac responses to epinephrine may remain in balance, and, compared with older normotensive subjects, older hypertensive subjects exhibit enhanced cardiac responses to sympathetic stimulation.

Impact of blood pressure on the Doppler echocardiographic assessment of severity of aortic stenosis

OBJECTIVE

To investigate the impact of blood pressure (BP) on the Doppler echocardiographic (Doppler-echo) evaluation of severity of aortic stenosis (AS).

METHODS

Handgrip exercise or phenylephrine infusion was used to increase BP in 22 patients with AS. Indices of AS severity (mean pressure gradient (DeltaP(mean)), aortic valve area (AVA), valve resistance, percentage left ventricular stroke work loss (% LVSW loss) and the energy loss coefficient (ELCo)) were measured at baseline, peak BP intervention and recovery.

RESULTS

From baseline to peak intervention, mean (SD) BP increased (99 (8) vs 121 (10) mm Hg, p<0.001), systemic vascular resistance (SVR) increased (1294 (264) vs 1552 (372) dynexs/cm(5), p<0.001) and mean (SD) transvalvular flow rate (Q(mean)) decreased (323 (67) vs 306 (66) ml/s, p = 0.02). There was no change in DeltaP(mean) (36 (13) vs 36 (14) mm Hg, p = NS). However, there was a decrease in AVA (1.15 (0.32) vs 1.09 (0.33) cm(2), p = 0.02) and ELCo (1.32 (0.40) vs 1.24 (0.42) cm(2), p = 0.04), and an increase in valve resistance (153 (63) vs 164 (74) dynexs/cm(5), p = 0.02), suggesting a more severe valve stenosis. In contrast, % LVSW loss decreased (19.8 (6) vs 16.5 (6)%, p<0.001), suggesting a less severe valve stenosis. There was an inverse relationship between the change in mean BP and AVA (r = -0.34, p = 0.02); however, only the change in Q(mean) was an independent predictor of the change in AVA (r = 0.81, p<0.001).

CONCLUSIONS

Acute BP elevation due to increased SVR can affect the Doppler-echo evaluation of AS severity. However, the impact of BP on the assessment of AS severity depends primarily on the associated change in Q(mean), rather than on an independent effect of SVR or arterial compliance, and can result in a valve appearing either more or less stenotic depending on the direction and magnitude of the change in Q(mean).

Cardiac autonomic modulation in hypertensive patients with Chagas' disease

BACKGROUND

Arterial hypertension and Chagas' disease are prevalent pathologies in Latin America. It has been demonstrated that each one of them may cause cardiac autonomic dysfunction. This study aimed to investigate the pattern of cardiac autonomic modulation in chagasic-hypertensive patients.

METHODS

Subjects (n=120) without left ventricular dysfunction were distributed in four groups: healthy control (n=30); hypertensive (n=30); chagasic (n=30) and chagasic-hypertensive (n=30). Patients were evaluated by autoregressive spectral analysis of heart rate variability in three different conditions: baseline, cold face and passive tilt tests. Power spectral densities in low (0.04-0.15 Hz) and high (0.15-0.50 Hz) frequency bands were estimated in both absolute and normalized units.

RESULTS

Baseline median values (percentile 25 to percentile 75) of mean arterial pressure (in mmHg) were 93.3 (85.0-96.7), 116.7 (*, #) (110.0-129.2), 86.7 (83.3-92.5) and 106.7 (*, #) (106.7-110.0) for healthy control, hypertensive, chagasic and chagasic-hypertensive patients, respectively (*p<0.05 versus healthy control, #p<0.05 against chagasic group). Heart rate at rest did not differ among groups. Regarding to spectral parameters in baseline conditions, the absolute power of high frequency component of heart rate variability of the chagasic-hypertensive group was significantly lower than that found in healthy control and hypertensive patients. There were no differences in spectral parameters responses during cold face test. After passive tilt test, however, decreases in high frequency oscillations and increases in sympathovagal balance (low and high frequency ratio) were significantly lower in hypertensive, chagasic and chagasic-hypertensive patients as compared with healthy control.

CONCLUSIONS

These data indicate that chagasic-hypertensive patients presented an impairment of cardiac parasympathetic modulation at baseline conditions as well as in response to passive orthostatic stress.

Sympatho-excitatory responses to once-daily dihydropyridines in young versus older hypertensive patients: amlodipine versus felodipine extended release

BACKGROUND

Once-daily dihydropyridines exert both indirect sympatho-excitatory and direct central sympatho-inhibitory effects. Age may affect this balance by influencing blood pressure (BP) or renin responses.

METHODS

We evaluated BP, sympathetic and cardiac responses after the first dose and after 8 weeks of treatment with placebo, amlodipine 5 mg/day or felodipine extended release (ER) 5 mg/day in 29 young (22-50 years) versus 37 older (60-77 years) hypertensive patients, using a double-blind, parallel group design.

RESULTS

In the young group, neither dihydropyridine dose decreased BP after the first dose and both caused decreases by 5-10 mmHg after chronic treatment. In the older group, felodipine ER decreased BP rapidly and amlodipine more gradually, and after chronic treatment, systolic BP decreased by 20-25 mmHg. Felodipine ER increased the heart rate by 5-10 bpm after the first dose in both age groups and caused persistent increases in the cardiac index (by 0.2 l/min per square metre) and the ejection fraction only in the older group. Amlodipine did not affect cardiac function in the young, and with chronic dosing decreased the heart rate by 3-5 bpm and the cardiac index by 0.2 l/min per square metre in the older group. In the young hypertensive patients, both dihydropyridines increased plasma norepinephrine (NE) after chronic dosing, with little effect after the first dose. In contrast, in the older group felodipine ER increased plasma NE after the first dose but not with chronic dosing, whereas amlodipine had no effect after the first dose, and after chronic dosing tended to decrease plasma NE.

CONCLUSION

We conclude that age is a major determinant not only of the BP but also of the cardiac and sympathetic responses to once-daily dihydropyridines.

Evaluation of methods for improving precision of blood pressure measurements in phase I clinical trials

Small sample sizes are typically incorporated in early Phase I clinical studies, which may lead to insignificant changes in safety parameters such as blood pressure. Therefore, it is paramount to identify an optimal, noninvasive method of accurately measuring blood pressure and an appropriate analysis strategy yielding the smallest variability. The goals of this study were (1) to compare the variability between automated and manual blood pressure measurements, (2) to determine whether triplicate blood pressure measurements were independent of one another, and (3) to assess how the number of blood pressure readings affects variability and study sample size. Twenty healthy volunteers were enrolled in this randomized, two-way crossover study. Each subject received three incremental infusions of phenylephrine or normal saline on separate days to simulate blood pressure variability. The mean systolic blood pressure readings with the automated device were consistently higher than the manual device by 3 to 5 mmHg. Conversely, the mean diastolic blood pressure readings with the automated device were consistently 3 to 5 mmHg lower than the manual device. However, the variability and absolute change in blood pressure were essentially identical with manual and automated methods. No systematic order effects such as the first blood pressure reading always being higher were detected, suggesting that the triplicate readings were independent of one another and that an interval of 2 minutes between readings is adequate. Compared to a single measurement, collecting blood pressure in triplicate results in a 40% lower sample size needed to detect a 5-mmHg difference in systolic blood pressure.

Autonomic control of the cardiovascular system during acclimatization to high altitude: effects of sildenafil

Both acute hypoxia and sildenafil may influence autonomic control through transient cardiovascular effects. In a double-blind study, we investigated whether sildenalfil (Sil) could interfere with cardiovascular effects of hypoxia. Twelve healthy men [placebo (Pla) n = 6; Sil, n = 6] were exposed to an altitude of 4,350 m during 6 days. Treatment was continuously administered from 6 to 8 h after arrival at altitude (3 × 40 mg/day). The autonomic control on the heart was assessed by heart rate variability (HRV) during sleep at sea level (SL) and between day 1–2 and day 5–6 in hypoxia. Arterial pressure (AP) and total peripheral resistances (TPR) were obtained during daytime. There was no statistical difference between groups in HRV, AP, and TPR throughout the study. Hypoxia induced a decrease in R-R interval and an increase in AP in both groups. Low frequency-to-high frequency ratio increased at day 1–2 (Pla, P = 0.04; Sil, P = 0.02) and day 5–6 (Pla and Sil, P = 0.04) vs. SL, whereas normalized high-frequency power decreased only in Pla ( P = 0.04, day 1–2 vs. SL). Normalized low-frequency power increased at high altitude (Pla and Sil, P = 0.04, day 5–6 vs. SL). TPR decreased at day 2 in Pla ( P = 0.02) and tended to normalize at day 6 ( P = 0.07, day 6 vs. day 2). Acute hypoxia induced a decrease in parasympathetic and increase in sympathetic tone, which tended to be reversed with acclimatization. Sil had no deleterious effects on the cardiovascular response to high-altitude exposure and its control by the autonomic nervous system.

The effect of aging on phenylephrine response in normal subjects

INTRODUCTION

With aging, cardiac responses to β-adrenergic stimulation decline but the responses to α1-stimulation are less clear. Moreover, whether aging, in the absence of disease, influences the left ventricular response to an increase in afterload is unclear. This study examined the effect of aging on heart rate (HR), blood pressure (BP), cardiac index (CI) and several left ventricular contractility measurements during α 1-stimulation with a phenylephrine infusion.

METHODS

Subjects were rigorously screened to be normal by history, physical, blood tests, ECG, ETT and echocardiogram. Twelve young (mean 26 years, all male) and 15 aged (69 years, 11 males) subjects were studied during 10 minute infusions of phenylephrine at 0.5 and 1.0 mcg/ kg/min. HR, BP and radionuclide ventriculographic cardiac volumes were measured.

RESULTS

Systolic BP increased more in the aged than in the young (22 vs. 13%, p=0.003), while heart rate (16 vs. 21%, p=0.05) fell less. Contractile responses to phenylephrine, including EF, stroke volume index (SVI), stroke work index and left ventricular contractility index were not altered with aging. Systemic vascular resistance (SVR) was higher at baseline and at each infusion rate, but there was no age-associate change in the response to PE.

CONCLUSIONS

In a healthy normal aged population, a preserved SVI response in the setting of a higher baseline SVR results in an increased SBP response to α1-stimulation. Contractile responses to increased afterload are not altered with aging. Age-associated differences in the response to α1-stimulation are small and are explained by altered baroreflex sensitivity and a stiffer vasculature.