Abstract
Women are known to have a longer electrocardiographic Q-T than men, which may contribute to their being at greater risk of developing drug-induced polymorphic ventricular arrhythmias. However, little is known about the underlying mechanisms. In the present study, we evaluated potential gender differences in Q-T interval in isolated perfused rabbit hearts using the Langendorff technique and evaluated the density of outward potassium currents in single ventricular myocytes using the whole-cell patch-clamp technique. We found that female hearts demonstrated a greater Q-T lengthening (ΔQ-T%) upon an increase in cycle length (CL), resulting in a significantly longer Q-T (301 ± 4.8 ms, CL = 2.3 s) at a long CL in female hearts compared with male hearts (267 ± 4.0 ms, P < .01). Ventricular myocytes isolated from female hearts showed a smaller IKtail and peak IKl outward current density. A 50% reduction in extracellular K+ and Mg++ shifted the I-V relationship of IKl and Ito and reduced their amplitude. However, neither the I-V relationship of IKr nor the gender difference in the Q-T–CL relationship was significantly altered. We conclude that 1) female rabbit ventricular myocytes have significantly lower IKr and IKl outward current densities than do male cells, which may contribute to the gender difference in Q-T, and 2) a lower base-line IKr density may contribute to the steeper Q-T–CL relationship in female hearts.
Women are known to have a longer, CL-dependent electrocardiographic Q-T interval than men (Stramba-Badiale et al., 1997). However, little is known about the mechanism responsible for this gender difference. Prolongation of the Q-T interval on the electrocardiogram has clinical importance because it is a common feature associated with a complex form of ventricular arrhythmia known as TdP (Dessertenne, 1966). An acquired long Q-T syndrome secondary to drug administration has been associated with TdP and sudden death in patients treated with antiarrhythmic drugs (Ben-David and Zipes, 1993; Carlsson et al., 1990; Roden et al., 1986). A gender difference in Q-T duration may therefore result in a gender difference in the incidence of TdP. Indeed, recent clinical observations have indicated that the occurrence of TdP displays a gender difference with a higher-than-expected occurrence in females (Kawasaki et al., 1995; Lehmann et al., 1996; Makkar et al., 1993).
Crucial to generation of TdP is prolongation of the Q-T interval and APD that permits EADs to occur (Zeng and Rudy, 1995). Because potassium currents are major determinants of cardiac repolarization, and because shortening of the action potential suppresses EADs in isolated myocytes (Bouchard et al., 1995), the activity of one or more potassium channels may be critical in modulating EADs. In fact, most drugs that are associated with TdP clinically have also been shown to block cardiac potassium channels, especially the rapid component of IK, IKr (Ben-David and Zipes, 1993; Carlssonet al., 1990; Roden et al., 1986; Lehmannet al., 1996). Furthermore, overexpression of HERG, the gene coding for IKr, has been shown to shorten APD and suppress EAD in rabbit ventricular myocytes (Nuss et al., 1997).
In the present study, we examined the gender differences in the CL-dependent Q-T in isolated perfused rabbit hearts. In addition, we measured the density of outward potassium currents that may contribute to such a difference in single rabbit ventricular myocytes.
Materials and Methods
Langendorff preparation.
Hearts from 35 New Zealand White male and female rabbits (3–4 months old, weight 3–3.5 kg; HRP Inc., Denver, Pennsylvania) were studied using the nonrecirculating Langendorff technique as described previously (Zabel et al., 1995). The hearts were perfused with an oxygenated Tyrode’s solution (95% O2, 5% CO2), pH 7.4, containing (mmol/l) NaCl 115, KCl 4.7, CaCl2 2, MgCl2 0.7, NaH2PO4 1, NaHCO3 27.9, glucose 20, and 0.04% (w/v) purified bovine albumin. The perfusate was maintained at 37°C and delivered to the aortic inflow cannula at the constant rate of 10 ml/min by a Masterflex pump. The AV node was cauterized to slow the intrinsic HR for pacing at a fixed rate. Hearts were paced at a CL of 400 ms and twice diastolic threshold intensity via a pacing catheter positioned in contact with the right ventricular endocardium. Experiments were conducted in accordance with the guidelines of the Georgetown University Animal Care and Use Committee and the American Heart Association’s position statement on use of animals in research.
ECG recordings and Q-T measurements.
Four silver–silver chloride electrode wires were positioned in a simulated “Einthoven” configuration, with the reference and “foot” electrodes fixed beneath the heart on the walls of a tissue bath that has the approximate diameter of a rabbit thorax (Zabel et al., 1995). The signals were amplified by an ECG amplifier (Colbourn Instruments, Lehigh Valley, PA) allowing for the simultaneous recording of the three orthogonal signals designated as X, Y and Z. The ECG signals were filtered selectively at 60 Hz.
Baseline Q-T measurements: The stability of the electrophysiologic variables in our Langendorff preparation has been previously reported (Drici et al., 1996). After about 30 min of continuous drive pacing at a CL of 0.4 s, the base-line Q-T interval was recorded. To compare the gender difference in Q-T interval at a slow rate, we switched the CL to 2.3 s and measured the Q-T interval when it was stable after ≥5 min pacing at this CL. In some experiments, Q-T was measured at multiple CLs (0.4, 0.8, 1.2 and 2.3 s) after ≥5 min pacing at each CL to examine the Q-T–CL relationship. Upon completion of Q-T measurements at multiple CLs in these experiments, the CL was switched back to 0.4 s and a modified Tyrode’s solution (Liu et al., 1997) containing 50% reduced K+ and Mg++ (low K/Mg) was perfused to study the effect of low K/Mg on the Q-T–CL relationship. After a stable baseline Q-T interval was reached in the modified Tyrode’s solution (35–40 min), Q-T intervals were again measured at four CLs (0.4, 0.8, 1.2 and 2.3 s) using the same procedure as in normal Tyrode’s solution.
Data were recorded on a strip chart recorder (Gould Electronics, Cleveland, OH) at a paper speed of 100 mm/s. Four representative beats from each set of XYZ recordings were chosen for Q-T measurement. The end of the T wave was defined as the point of maximal change in the slope of the T wave as it merges with the electrical base line in any lead.
Isolation of ventricular cells.
Cells were isolated using a modified method previously described (Giles and Imaizumi, 1988). Briefly, rabbit hearts (four male and five female hearts) were removed and mounted on the Langendorff perfusion system. The following procedure was used: 1) perfusion with normal Tyrode’s solution for 10 min; 2) perfusion for approximately 20 min with a Ca++-free Tyrode’s solution; 3) perfusion for 30 to 35 min with Tyrode’s solution containing 40 U/ml of collagenase II (Worthington Biochemical, Freehold, NJ) and 50 μmol/l of CaCl2. The ventricles were then minced and gently stirred in Tyrode’s solution containing 100 μmol/l of CaCl2. After stirring for 5 to 10 min, a large number of single ventricular cells were obtained. The resulting cell suspension was then filtered through a nylon mesh. Cells were collected by centrifugation at 50 × g and then resuspended in Tyrode’s solution containing 250 μmol/l of Ca++. The cells were again collected by centrifugation and then resuspended in Tyrode’s solution containing 1 mmol/l of Ca++. After a third centrifugation, the cells were resuspended in Dulbecco’s modified Eagle’s medium supplemented with 10% bovine calf serum (HyClone Labs, Logon, UT). The myocytes were immediately seeded onto laminin-coated glass microcoverslips at a density of 104rod-shaped cells/cm2 and allowed to attach. The cells were stored in a humidified incubator in 5% CO2, 95% air at 37°C. Approximately equal numbers of cells (7–10) were studied from each heart, and all experiments were performed within 16 h after cell isolation.
Whole-cell patch clamp.
The patch-clamp technique was used to record the membrane currents in single ventricular myocytes. Command voltage pulses were generated by use of PCLAMP 6.0.2 Software (Axon Instruments, Foster City, CA) connected to an interface (Axon Instruments), an IBM-compatible Pentium computer and an Axopatch 200A amplifier. Membrane potentials and current signals were monitored on an oscilloscope (5103, Tektronix, Beaverton, OR) and stored in the lab computer. Pipettes with tip resistance of 1 to 4 MΩ were pulled from borosilicate glass (World Precision Instruments, Sarasota, FL) and filled with an intracellular solution containing (mmol/l) KCl 125, NaCl 10, CaCl2 1, Mg-ATP 5, EGTA 14, HEPES 10 and cAMP 0.1, adjusted with KOH to pH 7.2. A holding potential of −40 mV was used to inactivate fast sodium and T-type calcium currents. The external solution was Tyrode’s solution containing (mmol/l) NaCl 137, KCl 5.4, HEPES 10.0, MgCl2 1.0, CaCl2 2.0 and glucose 10.0 and was adjusted with NaOH to pH 7.4 (NaOH). In some experiments, a modified Tyrode’s solution containing 50% reduced K+and Mg++ (low K/Mg) was used to study its effect on potassium currents. Cd++ (0.2 mmol/l) was used to block thel-type calcium channel and to shift the I-V relationship of Ito and IKr to more positive potentials (Daleauet al., 1997; Agus et al., 1991). This makes possible 1) separation of IKr and the outward portion of IKl, especially at membrane potentials positive to −30 mV, and 2) marked increase in Ito availability at a holding potential of −40 mV.
Myocytes adhering to glass coverslips were placed in a small chamber mounted on the stage of an inverted microscope and superfused at room temperature (22°C–24°C) at 1.5 ml/min. Only rod-shaped single cells that were quiescent and exhibited well-defined cross-striations were studied. After establishment of whole-cell configuration and measurement of cell capacitance, series resistance was compensated by 50% to 70%. Final series resistance after compensation was less than 5 MΩ. Junction potentials under our conditions were approximately −3 mV and were not corrected. IK currents were elicited from a holding potential of −40 mV by a series of 1.5-s test pulses from −10 to +50 mV in 10-mV increments. Membrane potential was then returned to holding potential or was held at −30 mV for 2 s before return to the holding potential in order to observe IK tail currents. The I-V relationship for IK was constructed by measuring the tail currents. The voltage dependence of IK activation was fit to a Boltzmann distribution of the formIKtail/IKmax = 1/{1 + exp[(V1/2 −Vt )/k]} with a nonlinear least-squares fitting routine (Origin 4.10, Microcal Software, Northampton, MA) to estimate the half-activation potential (V1/2) and slope factor (k) for this relationship. Deactivation of IK can be well fit by a single exponential function, using an equation of the formIt = As exp(−t/τs), whereIt is the tail current at time t,As is the initial amplitude of the current and τs is the time constant of deactivation. In the presence of 0.2 mmol/l of Cd++, the same protocol for IK also activated Ito because of the marked shift of Ito activation to more positive potentials. The amplitude of Ito was estimated by measuring the peak of the transient component of the current with respect to its steady-state value.
The IKl currents were elicited from a holding potential of −40 mV by a series of 250-ms test pulses ranging from −150 to −10 mV in 10-mV increments. The amplitude of IKl at each voltage was determined by measuring the peak current relative to zero current.
Data analysis and statistics.
Q-T intervals were interpreted in a masked fashion by two investigators. Patch-clamp data were normalized for total cell capacitance to allow comparison between cells of various sizes. Student’s t test was used to assess gender differences in Q-T at a given CL and current density at a given voltage. Data were reported as mean ± S.E.M., and differences between values were considered statistically significant when P < .05.
Results
Gender difference in baseline Q-T interval.
ECG recordings were made from isolated male and female rabbit hearts to determine whether there is a gender difference in the baseline Q-T interval or the Q-T–CL relationship. After about a 30-min perfusion with Tyrode’s solution and continuous pacing at 0.4 s, the Q-T interval stabilized. Upon switching of the CL from 0.4 s to 2.3 s, Q-T interval lengthened, and it reached steady state after about 5 min. Figure 1A depicts the Q-T interval at CL 0.4 s, measured at the end of a 30-min perfusion with Tyrode’s solution and the Q-T interval at 2.3 s, measured at least 5 min after the switch of the CL in male and female rabbit hearts. At a CL of 0.4 s, baseline Q-T was only slightly longer in female hearts (232 ± 2.0 ms) than in male hearts (223 ± 3.6 ms, P > .05). However, at CL 2.3 s, the mean Q-T was significantly longer in female hearts (301 ± 4.8 ms) than in male 267 ± 4.0 ms, P < .01). Thus female hearts demonstrated greater Q-T lengthening after the increase in CL from 0.4 s to 2.3 s (29.9% ± 3.3.3% and 19.9% ± 2.7%, female vs. male, P < .05, fig. 1B).
Gender difference in IKr.
IKr is one of the major repolarizing currents and has been implicated in TdP (Carlsson et al., 1990; Lehmann et al., 1996;Roden et al., 1986). To determine whether a gender difference in IKr may contribute to the observed gender difference in Q-T, we first sought to establish the presence of IKr in our rabbit ventricular myocytes. Figure2A shows the membrane currents elicited by a 1.5-s voltage-clamp step from −40 to different test potentials ranging from −10 to +40 mV in the same cell before and after a 5-min exposure to 5 μmol/l of E-4031 (Eisai Ltd., Ibaraki, Japan). Under control conditions, a small and slowly activating outward current flowed during depolarization, followed by a large outward tail current that has been shown to represent the gradual decay of IK(Follmer and Colatsky, 1990; Sanguinetti and Jurkiewicz, 1990). The initial peak in the time-dependent outward current was due to the rapid activation and inactivation of Ito. E-4031 abolished the tail current on repolarization and also reduced the time-dependent outward current, without affecting the initial peak. The E-4031-sensitive current, obtained by digital subtraction of currents in the bottom tracings from currents in the top tracings in panel A, is shown in panel B. Compared with the tail current, the time-dependent current demonstrated marked inward rectification at very positive potentials. Superfusion of 1 to 2.5 μmol/l of dofetilide or removal of extracellular K+ also abolished the tail current (data not shown). These features (inward rectification of the time-dependent current, inhibition by E-4031, dofetilide and removal of extracellular K+) are consistent with the description of IKrin rabbit and other species (Follmer and Colatsky, 1990; Sanguinetti and Jurkiewicz, 1990; Sanguinetti et al., 1995). Similar time-dependent and tail currents were observed in nearly all the cells we studied. Only in 2 of 84 cells did we observe a large, noninactivating, time-dependent current (data not shown) that resembled the IKs described in earlier studies (Follmer and Colatsky, 1990; Salata et al., 1996; Gintant, 1996). These two cells were not included in our analysis.
To evaluate the potential gender difference in IKr, we compared the current densities, the I-V relationship and the activation and deactivation kinetics of IKr in male and female cells. Figure 3 shows the current density of IKr and its I-V relationship in male and female cells. Female cells had a significantly smaller IKr current density than male cells. However, no significant gender difference was observed in either the voltage dependence or the activation and deactivation kinetics of IKr. The half-activation potential and the slope factor of IKr in female cells were 26.6 ± 0.86 mV and 8.97 ± 0.24, respectively, whereas in male cells they were 26.9 ± 1.1 mV and 8.39 ± 0.35, respectively (P > .05). The activation potentials for IKr in both male and female cells were shifted to more positive potentials, as expected from the presence of 0.2 mmol/l of Cd++ (Daleauet al., 1997; Sanguinetti et al., 1995). The time constant (τ) of IKr decay was well fit by a single exponential function (see “Materials and Methods”). At the test potential of +50 mV, τ was 527 ± 13 ms in male and 569 ± 17 ms and female cells, respectively (P > .05). The time constant of IKr activation was obtained by studying the E-4031-sensitive current. At the test potential of +20 mV, the time-dependent IKr showed the highest amplitude (fig. 2B) and can be adequately fit by a single exponential function with time constants of 400 ± 44 ms and 392 ± 103 ms in female and male cells, respectively (P > .05).
Gender difference in IKl.
To determine whether other outward potassium currents also display a gender difference, we next examined the IKl current density in female and male ventricular myocytes. As shown in figure 4, A and B, in rabbit ventricular myocytes, the pulse protocol for IKl activated a current that is largely time-independent at membrane potentials positive to −100 mV. This current had a prominent negative slope between −50 and −10 mV. A similar voltage dependence of IKl was observed in both sexes (fig. 4B). Between −70 and −10 mV, IKlcarried an outward current that peaked at −50 mV. Because the presence of 0.2 mmol/l of Cd++ caused a positive shift of IKr activation, no IKr was activated within this membrane potential range (figs. 2 and 3). Although there was no significant difference in the inward portion of IKl between male and female cells, the outward component of IKl was significantly smaller in female cells than in male cells. Peak outward IKl at −50 mV was 1.46 ± 0.06 pA/pF in female cells and 1.67 ± 0.08 pA/pF in male cells (P < .05).
Effect of low K/Mg on IKl, Ito and IKr.
We have previously shown that a modified Tyrode’s solution containing reduced Mg++ and K+ lengthens the Q-T interval (Liu et al., 1997), which suggests an inhibition of potassium channels. To explore the possibility that different cardiac potassium channels have differential sensitivity to low K/Mg, and using low K/Mg as a tool to elucidate the differential roles of different potassium channels in repolarization, we further characterized the effect of low K/Mg on three major outward potassium currents. Figure5 shows the currents in the same cell recorded before (panel A) and after (panel B) a 5-min superfusion of low-K/Mg Tyrode’s solution. Reducing the extracellular K+and Mg++ reduced the amplitude of Ito. Also, less quasi-instantaneous inward current was observed because of the shifting of the negative slope of IKl to more negative potentials and the reduction of peak IKl amplitude. Figure5C shows the difference currents obtained by digital subtraction of the currents in panel B from those in panel A. It clearly demonstrates the inhibition of Ito during depolarization. A large IKl outward current (the time-independent outward current at −40 and −30 mV) was also inhibited at these potentials, but IKr was not affected (absence of time-dependent, activating current and deactivating tail). Figure 5, D, E and F show the I-V relationships for IKl, Ito and IKr, respectively, before and after reduction of extracellular K+ and Mg++. In the presence of low K/Mg, the I-V curve of IKl was characterized by a marked shift (about 18 mV) to more negative potentials and by a reduction in the peak amplitude (fig. 5D). A marked reduction in Ito by low K/Mg is also clearly demonstrated in figure 5E. No significant change in the I-V relationship of IKr was observed (fig. 5F). The data shown in Figure 6, D and E are pooled from both male (2–5) and female (2–11) cells. No significant gender difference was observed in the response of current to low K/Mg.
Effect of low K/Mg on the Q-T–CL relationship.
Because low K/Mg Tyrode’s solution preferentially inhibited IKl and Ito without significantly affecting IKr, we next examined the gender difference in the Q-T–CL relationship in normal and low K/Mg Tyrode’s solution. In both normal and low-K/Mg Tyrode’s solution, as the CL increased from 0.4 s to 0.8, 1.2 and 2.3 s, Q-T progressively lengthened in male and female hearts. After perfusion of Tyrode’s solution with low K/Mg, both the absolute Q-T interval and its variability increased, but a consistent, statistically significant gender difference in the absolute Q-T value was not observed (Liu et al., 1997). However, as the CL increased from 0.4 s to 0.8, 1.2 and 2.3 s, female hearts still demonstrated greater Q-T lengthening (fig. 6). As shown in figure 6, the ΔQ-T%–CL relationship was not significantly altered by perfusion of low K/Mg in either male or female hearts. In the presence of low K/Mg, as the CL increased from 0.4 s to 2.3 s, Q-T lengthened by 29.1% ± 2.7% and by 20.5% ± 2.1% in female and male hearts, respectively (P < .05)—an outcome very similar to the results obtained in normal Tyrode’s solution (29.9% ± 3.3% and 19.9% ± 2.7%, female vs. male, P < .05).
Discussion
Although the rate-corrected Q-T interval of women is known to be longer than that of men, which suggests that women have a slower repolarization process (Stramba-Badiale et al., 1997), few experiments have been performed to examine the basis for this difference or its potential consequences. The current study was designed to examine the ionic mechanisms that contribute to gender differences in repolarization.
Outward potassium channels are the major determinants of the repolarization phase of the cardiac action potential. Three major potassium currents, Ito, IK and IKl, mediate different phases of the repolarization process (Giles and Imaizumi, 1988; Litovsky and Antzelevitch, 1988; Sanguinetti and Jurkiewicz, 1990). The contribution of IK and IKl to the APD is well established, and reduction in IK or IKl causes APD and Q-T prolongation (Clayet al., 1995; Salata et al., 1995; Surawicz, 1992). On the other hand, although Ito has been shown to play an important role in determining phase 1 repolarization, there is still some uncertainty about its contribution to normal APD. Except in rats, where Ito is the major repolarizing current, it is still uncertain whether reduction of Ito leads to prolongation or shortening of the entire APD (Giles and Imaizumi, 1988;Litovsky and Antzelevitch, 1988; Kaab et al., 1996).
The present study demonstrates that female rabbit hearts have a steeper Q-T–CL relationship than male hearts, which results in a significantly longer Q-T interval at a long CL in female rabbit hearts. This is consistent with the clinical observation of a steeper Q-T–CL relationship in women (Stramba-Badiale et al., 1997). In whole-cell patch-clamp studies, we found a major gender difference in the current density of IKr and also a small, yet statistically significant gender difference in the peak IKloutward current. Both IKr and IKl outward current densities are significantly smaller in ventricular myocytes isolated from female rabbit hearts, with no apparent difference in the voltage dependence and activation/deactivation kinetics compared with male hearts.
A reduction of either IKr or IKl could contribute to the slower repolarization process and longer Q-T interval at a given CL in female hearts. Our observation of no consistent, statistically significant gender difference in the absolute Q-T value in the presence of low K/Mg, conditions that affect IKl but not IKr, suggests that IKl may play a role in causing these gender differences. However, because only IKrdemonstrates a strong time dependence during both the plateau and final phases of the action potential, it is more likely that the gender difference in the Q-T–CL relationship results from the gender difference in IKr current density, although the contribution of other currents (such as ICa) cannot be excluded. An increase in CL is followed by an increase in APD, so a greater IKr is activated at long CLs. A gender difference in baseline IKr should therefore become more pronounced at a long CL, which may in turn contribute to a more pronounced gender difference in Q-T. This interpretation is supported by the finding that, although low K/Mg preferentially reduced the contribution of IKl and Ito to APD by shifting their I-V curves and reducing the current amplitude, the gender difference in the Q-T–CL relationship was preserved (fig. 6).
IK has been reported to consist of two components, IKr and IKs, in guinea pig, dog and human ventricles (Sanguinetti and Jurkiewicz, 1990; Gintant, 1996; Liet al., 1996). In the rabbit ventricular myocytes, IK has been reported to be absent (Giles and Imaizumi, 1988), to consist of only one component (Clay et al., 1995) and to consist of two components (Salata et al., 1996). In our experiments, IK can be consistently recorded with a clear tail current with no appreciable rundown in all the cells we studied. Our results indicate that the major delayed rectifier current in normal rabbit ventricular myocytes seems to be IKr, which is evident from the complete inhibition of the tail current by 1 to 5 μmol/l of E-4031, 1 to 2.5 μmol/l of dofetilide and removal of extracellular potassium. IKs was not detected in most cells even though we included cAMP, an agent that has been shown to increase IKs without affecting IKr, in the pipette solution (Sanguinetti et al., 1995; Salata et al., 1996). It has been reported that IKs is strongly dependent on isolation methods and recording temperature (Salataet al., 1996; Walsh et al., 1989), so it is possible that our experimental conditions were not optimal for recording IKs. However, recent studies by Waldeggeret al. (1996) demonstrated that acute administration of estradiol directly inhibits the minK current expressed inXenopus oocytes, yet we observed no APD or Q-T prolongation after acute estradiol perfusion at concentrations that caused more than 50% inhibition of minK current in Waldegger’s studies (Knollmanet al., 1996). Because the minK current inXenopus oocytes results, in a manner similar to IKs, from coassembly of minK and an endogenous KvLQT1-like protein (Sanguinetti et al., 1996), these observations would certainly argue against the possibility that a gender difference in IKs could account for the gender difference in Q-T in rabbits. Further studies are needed to determine whether IKs significantly contributes to the repolarization process in the rabbit heart and, if so, whether there is any gender difference in IKs.
Finally, because of the variability of Ito amplitude in this study, we were not able to determine the gender difference in baseline Ito current density. This variability is probably caused by the marked variation in the regional distribution of Ito among the different layers of the ventricle (Litovsky and Antzelevitch, 1988). Ongoing studies in our lab will further examine potential gender differences in Ito by using cells isolated from different layers of the ventricle. However, the observation that 4-aminopyridine, at concentrations that specifically block Ito, caused similar Q-T prolongation in female and male hearts (X.K. Liu, W. Wang, S.N. Ebert, M.R. Franz and R.L. Woosley, unpublished observation, 1997) would argue against the possibility that a gender difference exists in Ito. Moreover, in the presence of low K/Mg that inhibited Ito, female hearts still demonstrated a steeper Q-T–CL relationship, which suggests that a gender difference in Ito is less likely to contribute to the gender difference in the Q-T–CL relationship.
Clinical relevance and implications.
A lower density of IKr in female hearts could contribute to the clinically observed steeper Q-T–CL relationship in women. This, together with a smaller IKl outward current, may contribute to a longer Q-T interval in women and place them at a higher risk of developing drug-induced TdP, especially at slow HRs. Interestingly, a higher incidence of TdP was found in women during complete heart block, where HRs were very slow (Kawasaki et al., 1995). In addition, because IKr current density in female hearts is already reduced compared with that in male hearts, further IKrreduction by administration of IKr blockers may cause exaggerated Q-T prolongation in female hearts. This may contribute to the greater Q-T prolongation by d-sotalol in female rabbit hearts (X.K. Liu, W. Wang and R.L. Woosley, unpublished observations, 1997) and to the higher incidence of TdP in female patients treated with d,l-sotalol (Lehmann et al., 1996). These results suggest that drugs that block IKrshould be administered with special caution in female patients.
Limitations.
In our experiments, cells were isolated from the whole ventricular mass; different channel configurations between the left and right ventricles (Verduyn et al., 1997) and among different layers of the same ventricle may have confounded our data. Also, by using Cd++ as the l-type calcium channel blocker, we were able to markedly increase the availability of Ito at a holding potential of −40 mV and separate the I-V curves of IKr and the outward portion of IKl. However, the very ability of Cd++ to shift the I-V relationship of IKr to more positive potentials also limited our interpretation of IKr kinetics (Daleau et al., 1997). Finally, there is the issue of species differences between rabbit and human, although the parallel between the CL-dependent gender difference in the Q-T interval in the rabbit and that in the human suggests that the isolated rabbit heart may be clinically relevant as an experimental model.
In conclusion, female rabbit ventricular myocytes have significantly lower IKr and outward IKl current densities than male cells, and this difference may contribute to the gender difference in Q-T interval. A lower IKr density appears to contribute to the steeper Q-T–CL relationship in female hearts.
Footnotes
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Send reprint requests to: Raymond L. Woosley, M.D., Ph.D., Department of Pharmacology, Georgetown University Medical Center, 3900 Reservoir Road, NW, Washington, DC 20007.
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↵1 This work was supported in part by a grant from the National Institutes of Health (Grant #RO1 HL54590 to R.L.W.).
- Abbreviations:
- TdP
- torsades de pointes
- APD
- action potential duration
- IKr and IKs
- rapid and slow components, respectively, of the delayed rectifier
- IKl
- inward rectifier
- Ito
- transient outward current
- CL
- cycle length
- EAD
- early afterdepolarization
- HERG
- human ether-a-go-go-related gene
- Received November 22, 1996.
- Accepted January 30, 1998.
- The American Society for Pharmacology and Experimental Therapeutics