Article date: August 2000
By: K H Yuill, M K Convery, P C Dooley, S A Doggrell, J C Hancox in Volume 130, Issue 8, pages 1753-1766
BDF 9198 (a congener of DPI 201–106 and BDF 9148) was found to be a positive inotrope on guinea‐pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea‐pig isolated ventricular myocytes were studied using the whole cell patch clamp method.
In normal external solution, at 37°C, action potential duration at 50% repolarization (APD50) was 167.4±8.36 ms (n=37). BDF 9198 produced a concentration‐dependent increase in APD50 (no significant increase at 1×10−10 M; and APD50 values of 273.03±35.8 ms at 1×10−9 M; n=6, P<0.01 and 694.7±86.3 ms at 1×10−7 M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after‐depolarizations.
Qualitative measurements of INa with physiological [Na]o showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations.
Quantitative recording conditions for INa were established using low external [Na] and by making measurements at room temperature. The current–voltage relation, activation parameters and time‐course of INa were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 μM), despite a 2 fold difference in current amplitude. This suggests that voltage‐clamp during flow of INa was adequately maintained under our conditions.
Selective measurements of INa at room temperature showed that BDF 9198 induced a concentration‐dependent, sustained component of INa (ILate) and caused a slight left‐ward shift in the current–voltage relation for peak current. The drug‐induced ILate showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and ILate were abolished by 30 μM TTX and were sensitive to external [Na].
Inactivation of control INa during a 200 ms test pulse to −30 mV followed a bi‐exponential time‐course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time‐constant unchanged, but increased the magnitude of the slow inactivation time‐constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, INa inactivation may be comprised of more than two phases.
No significant effects of 1×10−6 M BDF 9198 were observed on the L‐type calcium current, or delayed and inward rectifying potassium currents measured at 37°C.
It is concluded that the prolongation of APD50 by BDF 9198 resulted from selective modulation of INa. Reduced current inactivation induced a persistent INa, increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for ‘QT prolongation’ of the ECG. The observation of after‐depolarizations suggests a potential for proarrhythmia at some drug concentrations.
BDF 9198 (a congener of DPI 201–106 and BDF 9148) was found to be a positive inotrope on guinea‐pig isolated ventricular muscle strips. The effects of BDF 9198 on action potentials and ionic currents from guinea‐pig isolated ventricular myocytes were studied using the whole cell patch clamp method.
In normal external solution, at 37°C, action potential duration at 50% repolarization (APD50) was 167.4±8.36 ms (n=37). BDF 9198 produced a concentration‐dependent increase in APD50 (no significant increase at 1×10−10 M; and APD50 values of 273.03±35.8 ms at 1×10−9 M; n=6, P<0.01 and 694.7±86.3 ms at 1×10−7 M; P<0.001, n=7). At higher concentrations in the range tested, BDF 9198 also induced early and delayed and after‐depolarizations.
Qualitative measurements of INa with physiological [Na]o showed prolongation of the current by BDF 9198, and the appearance of transient oscillatory inward currents at high concentrations.
Quantitative recording conditions for INa were established using low external [Na] and by making measurements at room temperature. The current–voltage relation, activation parameters and time‐course of INa were similar before and after a partial blocking dose of Tetrodotoxin (TTX, 1 μM), despite a 2 fold difference in current amplitude. This suggests that voltage‐clamp during flow of INa was adequately maintained under our conditions.
Selective measurements of INa at room temperature showed that BDF 9198 induced a concentration‐dependent, sustained component of INa (ILate) and caused a slight left‐ward shift in the current–voltage relation for peak current. The drug‐induced ILate showed a similar voltage dependence to peak current in the presence of BDF 9198. Both peak current and ILate were abolished by 30 μM TTX and were sensitive to external [Na].
Inactivation of control INa during a 200 ms test pulse to −30 mV followed a bi‐exponential time‐course. In addition to inducing a sustained current component, BDF 9198 left the magnitude of the fast inactivation time‐constant unchanged, but increased the magnitude of the slow inactivation time‐constant. Additional experiments with a longer pulse (1 s) raised the possibility that in the presence of BDF 9198, INa inactivation may be comprised of more than two phases.
No significant effects of 1×10−6 M BDF 9198 were observed on the L‐type calcium current, or delayed and inward rectifying potassium currents measured at 37°C.
It is concluded that the prolongation of APD50 by BDF 9198 resulted from selective modulation of INa. Reduced current inactivation induced a persistent INa, increasing the net depolarizing current during the action potential. This action of the drug indicates a potential for ‘QT prolongation’ of the ECG. The observation of after‐depolarizations suggests a potential for proarrhythmia at some drug concentrations.
British Journal of Pharmacology (2000) 130, 1753–1766; doi:10.1038/sj.bjp.0703476
DOI: 10.1038/sj.bjp.0703476
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