The effects of propranolol or atenolol on the cardiovascular responses to central hypovolaemia in Europeans and Bengalees

Influences of beta-blocking agents on cardiovascular actions induced by endothelin-3 administered intracerebroventricularly in anesthetized rats

Possible involvement that the pressor and positive chronotropic effects of endothelin-3 (ET-3) administered intracerebroventricularly (i.c.v.) participate in the central beta-adrenoceptor has been studied in urethane anesthetized rats. ET-3 administered i.c.v. at dose of 50 pmol elicited the increase in blood pressure and heart rate over a long duration, but a dose of 30 pmol had a minimal effect. These cardiovascular responses to ET-3 were significantly reduced by intravenous pretreatment with pentolinium, ganglionic blocking agent, and phentolamine, alpha-blocker. Also pressor responses to ET-3 were abolished dose-dependently by i.c.v. pretreatment with propranolol at a dose of 1 mumol, nonselective beta-blocker and ICI 118551, beta 2-selective adrenoceptor blocker, but not by i.c.v. pretreatment with metoprolol, atenolol, beta 1-selective blocker, procaine and phentolamine. These results suggest that ET-3 may play a role in cardiovascular regulation through the activation of the sympathetic nervous system involved in the central beta 2-adrenoceptor system.

Oral L-arginine supplementation has no effect on cardiovascular responses to lower body negative pressure in man

L-arginine is the physiological precursor of nitric oxide, which is the major endothelium-derived relaxing factor, a potent vasodilator. This study examined the cardiovascular responses to application of incremental levels (-10, -20 and -40 mmHg) of lower body negative pressure (LBNP) in ten normal male subjects, after 1 and 7 days administration of placebo or L-arginine. It was hypothesized that L-arginine may attenuate the vasoconstrictor response elicited by application of LBNP. Placebo administration had no effect on any responses to LBNP. L-arginine administration had no significant effect on mean arterial blood pressure either after 1 day, or after 7 days. After both 1 and 7 days L-arginine ingestion, forearm blood flow (FBF) and forearm vascular conductance (FVC) were significantly lower than before L-arginine ingestion (p < 0.01), but were not significantly different from after placebo ingestion. The magnitude of the changes in FBF and FVC, induced by application of LBNP, were not different under any conditions. These findings suggest that L-arginine supplementation in healthy subjects, either acutely or chronically, has no significant effect on peripheral haemodynamics, or on reflex responses to LBNP.

Cardiac beta 2-adrenoceptors and the inotropic response to exercise in man

To find out what role, if any, beta 2-adrenoceptors play in cardiac contractility, the heart rate, stroke volume and cardiac output of twelve healthy male and female volunteers (aged 18-28 years) were studied at rest (standing) and during two stages of treadmill exercise, 2 h after ingestion of propranolol (100 mg) or atenolol (100 mg) or a placebo, on different occasions, in a double-blind crossover manner. Cardiac output was measured by a carbon dioxide-rebreathing method. Atenolol and propranolol caused equal reductions in heart rate at rest, and in heart rate and cardiac output during exercise (p less than 0.001, two-way analysis of variance). Neither atenolol nor propranolol had any significant effect on resting cardiac output, resting stroke volume or stroke volume during exercise. Since atenolol (100 mg) has been shown to be beta 1-adrenoceptor-selective, we conclude that cardiac inotropic function during exercise is largely beta 1-adrenoceptor-mediated with little or no beta 2-adrenoceptor involvement.

Haemodynamic responses to nonhypotensive central hypovolaemia induced by lower body negative pressure in men and women

Haemodynamic responses to low levels of lower body negative pressure (LBNP) were investigated in two groups of healthy, normotensive volunteers (8 men and 8 women) during two repeated experimental runs on two occasions, the latter determined by the different phases of the menstrual cycle in the women. The data consisted of systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean blood pressure (MBP), pulse rate (fc), forearm blood flow (FBF) and forearm vascular conductance (FC). The resting cardiovascular status was similar in men and women, except that women had a significantly higher fc than men. LBNP (1.3, 2.7 and 4 kPa) had no significant effect on any BP variable or on fc. However, FBF and FC were reduced at all levels of LBNP. Significant overshoots in FBF and FC were seen in all subjects following the release of LBNP of 2.7 and 4 kPa and, in most cases, after release of LBNP of 1.3 kPa. There were no significant gender differences in any of the responses to LBNP. Furthermore, none of the cardiovascular variables measured showed significant differences between the follicular and luteal phases of the menstrual cycle in women, either at rest or during exposure to LBNP, and the responses in the men on the two occasions were not different. These findings indicate that gender differences in responses to LBNP hypothesized previously are not apparent during and after exposure to low levels of LBNP.

Ethnic differences in the pharmacokinetics of oral nifedipine

1. The pharmacokinetics of a single oral dose of 20 mg nifedipine, given as capsules, has been compared in five South Asian volunteers with data for 27 Caucasian volunteers. 2. The area under the plasma concentration-time curve (AUC) of nifedipine was three fold higher in South Asians (989 +/- 166 ng ml-1 h) than in Caucasians (323 +/- 116 ng ml-1 h). 3. The ratio of the AUC of nifedipine to that of the nitropyridine analogue, which is formed largely as a first pass metabolite, was significantly higher in South Asians (4.6 +/- 1.9) than in Caucasians (2.3 +/- 1.1) indicating a lower first pass metabolism in South Asians. 4. The terminal half-lives of nifedipine and the nitropyridine metabolite were significantly greater in South Asians than in Caucasians. 5. Consumption of a spicy curry diet for 3 days by Caucasians did not significantly affect the pharmacokinetics of a single oral dose of nifedipine. 6. The treatment of patients of South Asian origin with nifedipine should be initiated with lower doses than would be given to Caucasians.