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Identification of Ikr kinetics and drug binding in native myocytes.

Zhou Q, Zygmunt AC, Cordeiro JM, Siso-Nadal F, Miller RE, Buzzard GT, Fox JJ - Ann Biomed Eng (2009)

Bottom Line: Determining the effect of a compound on I (Kr) is a standard screen for drug safety.Often the effect is described using a single IC(50) value, which is unable to capture complex effects of a drug.Although the method was developed for I (Kr), the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified.

View Article: PubMed Central - PubMed

Affiliation: Gene Network Sciences, 58 Charles Street, Cambridge, MA 02141, USA. qzhou@gnsbiotech.com

ABSTRACT
Determining the effect of a compound on I (Kr) is a standard screen for drug safety. Often the effect is described using a single IC(50) value, which is unable to capture complex effects of a drug. Using verapamil as an example, we present a method for using recordings from native myocytes at several drug doses along with qualitative features of I (Kr) from published studies of HERG current to estimate parameters in a mathematical model of the drug effect on I (Kr). I (Kr) was recorded from canine left ventricular myocytes using ruptured patch techniques. A voltage command protocol was used to record tail currents at voltages from -70 to -20 mV, following activating pulses over a wide range of voltages and pulse durations. Model equations were taken from a published I (Kr) Markov model and the drug was modeled as binding to the open state. Parameters were estimated using a combined global and local optimization algorithm based on collected data with two additional constraints on I (Kr) I-V relation and I (Kr) inactivation. The method produced models that quantitatively reproduce both the control I (Kr) kinetics and dose dependent changes in the current. In addition, the model exhibited use and rate dependence. The results suggest that: (1) the technique proposed here has the practical potential to develop data-driven models that quantitatively reproduce channel behavior in native myocytes; (2) the method can capture important drug effects that cannot be reproduced by the IC(50) method. Although the method was developed for I (Kr), the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified.

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Time scale and state occupation vs. voltage. (a) Time constants at 0.2 uM; (b) State occupation at 0.2 uM; (c) Time constants at 2 uM; (d) State occupation at 2 uM. Left panel: model from the averaged data. Right panel: model from averaging the parameters
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Fig10: Time scale and state occupation vs. voltage. (a) Time constants at 0.2 uM; (b) State occupation at 0.2 uM; (c) Time constants at 2 uM; (d) State occupation at 2 uM. Left panel: model from the averaged data. Right panel: model from averaging the parameters

Mentions: Finally, we take advantage of the form of the Markov model to relate the observed time scale of the onset of drug block from the use-dependence simulations to time scales that can be calculated directly from the model. For a fixed value of voltage, the equations of the model are linear. Thus, we can find the steady-state solution as well as the eigenvalues (and therefore time constants) of the system as a function of voltage. The system has five degrees of freedom, and therefore five time constants. Figure 10 shows the two largest time constants as a function of voltage for two doses (a and b: 0.2 uM, c and d: 2 uM) and two models (left: model from averaged data, right: model from averaged parameters). The other three time constants are very fast compared to the two largest and therefore not shown. The dashed line represents the time scale associated with the action of the drug. This time scale is faster at those voltages where the drug bound state of the channel is maximal. Comparing Figs. 10a to 10c, the time scale over this voltage range decreases with increasing drug concentration. Furthermore, the model in Fig. 10, left, has smaller drug bound state than that shown in Fig. 10, right, which corresponds to the weaker effect of drug block in Fig. 9b as compared to Fig. 9c.Figure 10


Identification of Ikr kinetics and drug binding in native myocytes.

Zhou Q, Zygmunt AC, Cordeiro JM, Siso-Nadal F, Miller RE, Buzzard GT, Fox JJ - Ann Biomed Eng (2009)

Time scale and state occupation vs. voltage. (a) Time constants at 0.2 uM; (b) State occupation at 0.2 uM; (c) Time constants at 2 uM; (d) State occupation at 2 uM. Left panel: model from the averaged data. Right panel: model from averaging the parameters
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2690829&req=5

Fig10: Time scale and state occupation vs. voltage. (a) Time constants at 0.2 uM; (b) State occupation at 0.2 uM; (c) Time constants at 2 uM; (d) State occupation at 2 uM. Left panel: model from the averaged data. Right panel: model from averaging the parameters
Mentions: Finally, we take advantage of the form of the Markov model to relate the observed time scale of the onset of drug block from the use-dependence simulations to time scales that can be calculated directly from the model. For a fixed value of voltage, the equations of the model are linear. Thus, we can find the steady-state solution as well as the eigenvalues (and therefore time constants) of the system as a function of voltage. The system has five degrees of freedom, and therefore five time constants. Figure 10 shows the two largest time constants as a function of voltage for two doses (a and b: 0.2 uM, c and d: 2 uM) and two models (left: model from averaged data, right: model from averaged parameters). The other three time constants are very fast compared to the two largest and therefore not shown. The dashed line represents the time scale associated with the action of the drug. This time scale is faster at those voltages where the drug bound state of the channel is maximal. Comparing Figs. 10a to 10c, the time scale over this voltage range decreases with increasing drug concentration. Furthermore, the model in Fig. 10, left, has smaller drug bound state than that shown in Fig. 10, right, which corresponds to the weaker effect of drug block in Fig. 9b as compared to Fig. 9c.Figure 10

Bottom Line: Determining the effect of a compound on I (Kr) is a standard screen for drug safety.Often the effect is described using a single IC(50) value, which is unable to capture complex effects of a drug.Although the method was developed for I (Kr), the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified.

View Article: PubMed Central - PubMed

Affiliation: Gene Network Sciences, 58 Charles Street, Cambridge, MA 02141, USA. qzhou@gnsbiotech.com

ABSTRACT
Determining the effect of a compound on I (Kr) is a standard screen for drug safety. Often the effect is described using a single IC(50) value, which is unable to capture complex effects of a drug. Using verapamil as an example, we present a method for using recordings from native myocytes at several drug doses along with qualitative features of I (Kr) from published studies of HERG current to estimate parameters in a mathematical model of the drug effect on I (Kr). I (Kr) was recorded from canine left ventricular myocytes using ruptured patch techniques. A voltage command protocol was used to record tail currents at voltages from -70 to -20 mV, following activating pulses over a wide range of voltages and pulse durations. Model equations were taken from a published I (Kr) Markov model and the drug was modeled as binding to the open state. Parameters were estimated using a combined global and local optimization algorithm based on collected data with two additional constraints on I (Kr) I-V relation and I (Kr) inactivation. The method produced models that quantitatively reproduce both the control I (Kr) kinetics and dose dependent changes in the current. In addition, the model exhibited use and rate dependence. The results suggest that: (1) the technique proposed here has the practical potential to develop data-driven models that quantitatively reproduce channel behavior in native myocytes; (2) the method can capture important drug effects that cannot be reproduced by the IC(50) method. Although the method was developed for I (Kr), the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified.

Show MeSH
Related in: MedlinePlus