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Mechanism of direct cardiostimulating actions of hydralazine
Insights
Hydralazine enhances heart muscle contraction through increased calcium (Ca2+) inflow. This effect involves beta-adrenoceptors and cyclic AMP, but also another independent mechanism.
Area of Science:
- Cardiovascular Pharmacology
- Cardiac Electrophysiology
Background:
- Hydralazine, a vasodilator, may possess direct positive inotropic effects on the myocardium.
- Understanding the precise mechanism of hydralazine's inotropic action is crucial for cardiovascular research.
Purpose of the Study:
- To investigate the mechanism behind hydralazine's positive inotropic effect on ventricular myocardium.
- To elucidate the role of calcium (Ca2+) inflow and signaling pathways in hydralazine's action.
Main Methods:
- Utilized isolated perfused chick hearts to study myocardial contractility.
- Induced slow action potentials by voltage-inactivating fast Na+ channels using elevated potassium (25 mM).
- Administered hydralazine and propranolol to assess their effects on contractile force, heart rate, and cyclic AMP levels.
Main Results:
- Hydralazine (10(-3) M) increased contractile force, heart rate, and myocardial cyclic AMP.
- Hydralazine (10(-4) M) rapidly induced slow action potentials and contractions in electrically stimulated, K+-depolarized hearts.
- Propranolol only partially inhibited these hydralazine-induced effects.
Conclusions:
- Hydralazine enhances myocardial contractility, at least partly, by directly increasing Ca2+ inflow into cardiomyocytes.
- The positive inotropic effect is mediated by both beta-adrenoceptor activation (leading to elevated cyclic AMP) and an additional, distinct mechanism.
Abstract:
The vasodilator, hydralazine, was reported to also exert a direct positive inotropic effect on the myocardium at high concentrations. In the present study we investigated the mechanism of this positive inotropic action by using the ventricular myocardium of isolated perfused chick hearts. Hydralazine (10(-3) M) enhanced contractile force and heart rate, and elevated the myocardial cyclic AMP level. To study the Ca2+-dependent slow action potentials, the fast N+ channels were voltage-inactivated with elevated K+ (25 mM), resulting in a loss of electrical excitability. Hydralazine (10(-4) M) rapidly (less than 3 min) allowed the generation of slow action potentials and accompanying contractions by electrical stimulation. These effects of hydralazine were only partially prevented by propranolol. The results suggest that the increase of myocardial contractility produced by hydralazine is the result, at least in part, of a direct effect on the myocardium to increase Ca2+ inflow. The increased Ca2+ influx and inward slow current is due partly to activation of beta-adrenoceptors, with resultant elevation of cyclic AMP, and partly to another mechanism.