Endocytosis inhibition protects the isolated guinea pig heart against ouabain toxicity
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H Núñez-Durán1, F Atonal, P Contreras
1Departamento de Fisiología, Facultad de Medicina, Montevideo, Uraguay.
Abstract:
We have tested whether toxicity and uptake of ouabain are linked phenomena in isolated guinea-pig heart. We perfused toxic dose of either ouabain, dihydroouabain (hydrophilic cardiac steroids), ouabagenin or digitoxin (hydrophobic steroids) in conditions of endocytosis inhibited. We used two schemes of inhibition of endocytosis. First, we cooled the heart at 0-4 degrees C and exposed it 60 min to either 1 X 10(-6) M ouabain or 3 X 10(-7)M digitoxin. Upon rewarming, the heart exposed to ouabain relaxed with a shorter time constant than those exposed to digitoxin. Second, we perfused four receptor mediated endocytosis (RME) inhibitors at the same time that the cardiac glycosides. RME inhibitors significantly delayed the cardiac arrest caused by ouabain and dihydroouabain but they did not modify the toxicity of 3 X 10(-7)M digitoxin or 5 X 10(-6)M ouabagenin. None of RME inhibitors modified the toxicity of 0.5 mM K+ or zero Na+ saline solution. We suggest that the protection against ouabain toxicity brought by endocytosis inhibition, is related to the Na-pump recycling. We infer that besides the sarcolemmal exposed Na-pumps, the intracellular Na-pumps pool may be of importance for the pharmacological effects of digitalis steroids.
Insights
Inhibiting endocytosis protected guinea-pig hearts from ouabain toxicity by affecting Na-pump recycling. This suggests intracellular Na-pumps are crucial for digitalis steroid effects.
Area of Science:
- Pharmacology
- Cardiovascular Physiology
- Cell Biology
Background:
- Cardiac glycosides, like ouabain, exert effects through Na+/K+-ATPase inhibition.
- The role of cellular uptake mechanisms, such as endocytosis, in mediating cardiac glycoside toxicity is not fully understood.
- Understanding these mechanisms is vital for developing safer therapeutic strategies.
Purpose of the Study:
- To investigate the relationship between endocytosis and the toxicity of cardiac glycosides in isolated guinea-pig hearts.
- To determine if inhibiting endocytosis affects the toxicity of different cardiac steroids (ouabain, dihydroouabain, ouabagenin, digitoxin).
- To explore the potential involvement of Na-pump recycling in the observed protective effects.
Main Methods:
- Isolated guinea-pig hearts were perfused with toxic doses of cardiac glycosides under conditions of inhibited endocytosis.
- Two endocytosis inhibition methods were employed: hypothermic conditions (0-4°C) and the use of receptor-mediated endocytosis (RME) inhibitors.
- Cardiac arrest time and relaxation kinetics were measured to assess toxicity.
Main Results:
- Inhibition of endocytosis significantly delayed cardiac arrest induced by ouabain and dihydroouabain.
- Endocytosis inhibition did not alter the toxicity of digitoxin or ouabagenin.
- Rewarming hearts exposed to ouabain under inhibited endocytosis showed faster relaxation compared to digitoxin exposure.
- RME inhibitors did not affect the toxicity of low potassium or zero sodium solutions.
Conclusions:
- Endocytosis inhibition provides protection against ouabain and dihydroouabain toxicity, suggesting a link between uptake and toxicity.
- The protective effect is likely related to Na-pump recycling, highlighting the importance of intracellular Na-pump pools.
- These findings suggest that intracellular Na-pump pools play a significant role in the pharmacological actions of digitalis steroids.