Functional properties of K⁺ currents in adult mouse ventricular myocytes

Abstract

Although the K + currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time-and voltage-dependent outward K + currents which modulate action potential repolarization. A Ca 2+ -independent transient outward K + current, I to , can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K + current, I Kur , can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K + current, I ss , can be obtained by combining these two procedures: (i) inactivating I to and (ii) eliminating I Kur by application of low concentration of 4-AP. I ss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of I to is more than 20-fold faster than that of I Kur . These results demonstrate that the outward K + currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32• C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K + current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K + currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K + channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of I Kur , I to and I ss during action potential repolarization. The extensive efforts to develop transgenic mouse models of cardiovascular diseases have resulted in strong interest in defining the functional properties and molecular basis of the K + currents in mouse heart. Accordingly, a number of detailed studies focusing on K + currents in adult mouse ventricular myocytes have appeared (Benndorf et al. 1987;Wang & Duff, 1997;Barry et al. 1998;Babij et al. 1998;Zhou et al. 1998Zhou et al. , 2003 London et al. 1998a,b;Guo et al. 1999Guo et al. , 2000Wickenden et al. 1999; Xu et al. 1999a,b; DuBell et al. 2000;Jeron et al. 2000;Zaritsky et al. 2001;Kuo et al. 2001;Brunet et al. 2004;Bondarenko et al. 2004). In combination, these results provide a reasonably Judith Brouillette and Robert B. Clark contributed equally to this study consistent and quite detailed account of the number and type of time-and voltage-dependent K + currents expressed in adult mouse ventricle. Although a number of different strategies have been developed for separating the total K + current into individual K + conductance components, none of these approaches...

Mechanism of shortened action potential duration in Na+-Ca2+ exchanger knockout mice

“…Especially in mouse myocytes, I to dominates both the early and late phases of the AP (6,21). Accordingly, we see a strong acceleration of AP repolarization when modeling the increase of I to that we observe in NCX KO myocytes (Fig.…”

“…In mouse cardiomyocytes, AP repolarization is extremely rapid with only a brief plateau phase at very negative potentials (6,7,35). The plateau-maintaining currents I NCX and I Ca contribute only mildly to AP repolarization in computer simulations (Fig.…”

“…I to consists of fast and slow components (6,20) that can be dissected by exponential analysis (34). Double exponential curves were fitted to the I to tracings.…”

Reduction in Kv Current Enhances the Temporal Dispersion of the Action Potential in Diabetic Myocytes: Insights From a Novel Repolarization Algorithm

“…In the attempt to define the role of diminished outward Kv currents on the prolongation of AP and enhanced beat‐to‐beat variability observed in diabetic myocytes, outward K + currents in control mouse myocytes were inhibited with 4‐AP (0.1 mmol/L) 32, 33, 34, 35. This pharmacological intervention led to a prolongation of AP comprising 95% of the whole repolarization, with a maximum of 5.3‐fold increase in duration at 15.6% of the repolarization (Figure 9A and 9B).…”

“…To test the effects of inhibition of Kv currents, APs were continuously recorded for the same cells before and after exposure to 4‐aminopyridine (4‐AP; 0.1 mmol/L) 30, 32, 33, 34, 35. At this concentration level, 4‐AP has been previously reported to abolish the large, slowly inactivating phase of the Kv‐based I K,slow 32, 34, 35 leading to prolongation of AP 33. To test the acute effects of high glucose, APs were continuously recorded for the same cells before and after exposure to a Tyrode solution containing 20 mmol/L of glucose.…”