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Endocrine and metabolic effects of labetalol in man
Insights
Intravenous labetalol significantly lowered blood pressure and heart rate in hypertensive patients. It also increased plasma glucose and prolactin levels, with unclear mechanisms for these metabolic effects.
Area of Science:
- Pharmacology
- Endocrinology
- Cardiovascular Medicine
Background:
- Labetalol is an alpha- and beta-adrenoceptor blocking drug used for hypertension.
- Understanding its acute effects on cardiovascular and metabolic parameters is crucial.
Purpose of the Study:
- To evaluate the effects of intravenous labetalol on heart rate, blood pressure, and endocrine/metabolic variables in hypertensive patients.
- To investigate the adrenoceptor activity and potential mechanisms behind observed changes.
Main Methods:
- Intravenous infusion of labetalol (100 mg over 10 min) in 12 hypertensive patients.
- Comparison with a saline infusion control study.
- Measurement of heart rate, blood pressure, plasma glucose, free fatty acids, insulin, C-peptide, growth hormone, and serum prolactin.
Main Results:
- Significant reductions in heart rate, systolic, and diastolic blood pressure.
- Significant increase in plasma glucose levels.
- Significant increase in serum prolactin, more pronounced in females.
- No significant changes in free fatty acids, insulin, C-peptide, or growth hormone.
Conclusions:
- Acute blood pressure lowering suggests predominant alpha-adrenoceptor activity, while heart rate reduction indicates beta-adrenoceptor blockade.
- The mechanism for increased plasma glucose may involve elevated norepinephrine levels.
- The prolactin increase is not clearly linked to adrenergic receptors, possibly involving antidopaminergic activity.
Abstract:
The effects of intravenous infusion of the alpha- and beta-adrenoceptor blocking drug labetalol (100 mg over 10 min) on heart rate, blood pressure, and several endocrine and metabolic variables have been evaluated in 12 hypertensive patients (6 men and 6 women). Drug administration was associated with significant lowering of heart rate and systolic and diastolic blood pressure in comparison with a control study performed with saline infusion. Plasma glucose was significantly increased, while no changes were observed in serum-free fatty acids, insulin, C-peptide, and growth hormone levels. Serum prolactin concentration was significantly increased in the whole group, a marked rise occurred in females, while only a trend upward was observed in males. The acute lowering of blood pressure suggests a predominant activity on alpha-adrenoceptors following intravenous labetalol infusion, although the reduction in heart rate is consistent with the beta-adrenoceptor blocking effect of the drug. Although the reason for the increase in plasma glucose is not apparent, it might depend on the rise in norepinephrine levels observed after labetalol. Stimulation of prolactin release by intravenous labetalol is not easily attributable to interference with adrenergic receptors. The mechanism of this action of the drug is presently obscure, although a possible antidopaminergic activity of labetalol might be involved.