Intracellular and Extracellular Angiotensin II Enhance the L-Type Calcium Current in the Failing Heart

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Abstract
The influence of intracellular and extracellular angiotensin II (Ang II) on the L-type calcium current of cardiomyocytes isolated from cardiomyopathic hamsters was investigated. The results indicated that Ang II (10 −8 mmol/L), added to the bath, increased the peak inward calcium current (I Ca ) density by 37±3.4% ( P −8 mmol/L) also elicited an increase of peak I Ca density but enhanced the rate of I Ca inactivation, an effect not seen with extracellular Ang II. Moreover, in control animals, no change in the rate of I Ca inactivation was seen with intracellular Ang II. Thapsigargin (1 μmol/L), a potent inhibitor of sarcoplasmic reticulum (SR) ATPase, which depletes the SR, decreased the rate of I Ca inactivation elicited by intracellular Ang II, although the cytoplasmic calcium concentration was highly buffered with 10 mmol/L EGTA. These findings might indicate that intracellular Ang II releases calcium from the SR and inactivates I Ca . The effect of intracellular Ang II on peak I Ca was not altered by extracellular losartan (10 −7 mmol/L), supporting the notion that the peptide acted intracellularly. Other studies showed that intracellular Ang I administration (10 −8 mmol/L) enhanced the peak I Ca density and the rate of I Ca inactivation, an effect that was reduced by intracellular enalaprilat (10 −8 mmol/L). Moreover, intracellular enalaprilat by itself reduced the peak I Ca density. These observations might indicate that endogenous Ang II is contributing to I Ca modulation in the failing heart.