Cardiac Hypertrophy and Altered Cellular Electrical Activity of the Myocardium. In: Sperelakis N, editor. Physiology and Pathophysiology of the Heart. Boston: Springer US; 1989. pp. 573-93. [DOI: 10.1007/978-1-4613-0873-7_28]

Molecular basis of transient outward K+ current diversity in mouse ventricular myocytes

1. Two kinetically and pharmacologically distinct transient outward K+ currents, referred to as Ito,f and Ito,s, have been distinguished in mouse left ventricular myocytes. Ito,f is present in all left ventricular apex cells and in most left ventricular septum cells, whereas Ito,s is identified exclusively in left ventricular septum cells. 2. Electrophysiological recordings from ventricular myocytes isolated from animals with a targeted deletion of the Kv1.4 gene (Kv1.4-/- mice) reveal that Ito,s is undetectable in cells isolated from the left ventricular septum (n = 26). Ito,f density in both apex and septum cells, in contrast, is not affected by deletion of Kv1.4. 3. Neither the 4-AP-sensitive, slowly inactivating K+ current, IK,slow, nor the steady-state non-inactivating K+ current, ISS, is affected in Kv1.4-/- mouse left ventricular cells. 4. In myocytes isolated from transgenic mice expressing a dominant negative Kv4.2 alpha subunit, Kv4.2W362F, Ito,f is eliminated in both left ventricular apex and septum cells. In addition, a slowly inactivating transient outward K+ current similar to Ito,s in wild-type septum cells is evident in myocytes isolated from left ventricular apex of Kv4.2W362F-expressing transgenics. The density of Ito,s in septum cells, however, is unaffected by Kv4.2W362F expression. 5. Western blots of fractionated mouse ventricular membrane proteins reveal a significant increase in Kv1.4 protein level in Kv4.2W362F-expressing transgenic mice. The protein levels of other Kv alpha subunits, Kv1.2 and Kv2.1, in contrast, are not affected by the expression of the Kv4.2W362F transgene. 6. The results presented here demonstrate that the molecular correlates of Ito,f and Ito,s in adult mouse ventricle are distinct. Kv1.4 underlies mouse ventricular septum Ito,s, whereas Kv alpha subunits of the Kv4 subfamily underlie mouse ventricular apex and septum Ito, f. The appearance of the slow transient outward K+ current in Kv4. 2W362F-expressing left ventricular apex cells with properties indistinguishable from Ito,s in wild-type cells is accompanied by an increase in Kv1.4 protein expression, suggesting that the upregulation of Kv1.4 underlies the observed electrical remodeling in Kv4.2W362F-expressing transgenics.

Attenuation of the slow component of delayed rectification, action potential prolongation, and triggered activity in mice expressing a dominant-negative Kv2 alpha subunit

An in vivo experimental strategy, involving cardiac-specific expression of a mutant Kv 2.1 subunit that functions as a dominant negative, was exploited in studies focused on exploring the role of members of the Kv2 subfamily of pore-forming (alpha) subunits in the generation of functional voltage-gated K(+) channels in the mammalian heart. A mutant Kv2.1 alpha subunit (Kv2.1N216) was designed to produce a truncated protein containing the intracellular N terminus, the S1 membrane-spanning domain, and a portion of the S1/S2 loop. The truncated Kv2.1N216 was epitope tagged at the C terminus with the 8-amino acid FLAG peptide to generate Kv2. 1N216FLAG. No ionic currents are detected on expression of Kv2. 1N216FLAG in HEK-293 cells, although coexpression of this construct with wild-type Kv2.1 markedly reduced the amplitudes of Kv2. 1-induced currents. Using the alpha-myosin heavy chain promoter to direct cardiac specific expression of the transgene, 2 lines of Kv2. 1N216FLAG-expressing transgenic mice were generated. Electrophysiological recordings from ventricular myocytes isolated from these animals revealed that I(K, slow) is selectively reduced. The attenuation of I(K, slow) is accompanied by marked action potential prolongation, and, occasionally, spontaneous triggered activity (apparently induced by early afterdepolarizations) is observed. The time constant of inactivation of I(K, slow) in Kv2. 1N216FLAG-expressing cells (mean+/-SEM=830+/-103 ms; n=17) is accelerated compared with the time constant of I(K, slow) inactivation (mean+/-SEM=1147+/-57 ms; n=25) in nontransgenic cells. In addition, unlike I(K, slow) in wild-type cells, the component of I(K, slow) remaining in the Kv2.1N216FLAG-expressing cells is insensitive to 25 mmol/L tetraethylammonium. Taken together, these observations suggest that there are 2 distinct components of I(K, slow) in mouse ventricular myocytes and that Kv2 alpha subunits underlie the more slowly inactivating, tetraethylammonium-sensitive component of I(K, slow). In vivo telemetric recordings also reveal marked QT prolongation, consistent with a defect in ventricular repolarization, in Kv2.1N216FLAG-expressing transgenic mice.

Hemodynamic and humoral correlates in essential hypertension: relationship between patterns of LVH and myocardial ischemia

While evaluating 45 hypertensive patients with left ventricular hypertrophy (LVH) for enrollment in a clinical research protocol, we had the opportunity to compare anatomic and functional characteristics of those with LVH and ischemia on an exercise tolerance test (ETT), but without coronary artery disease by angiography (group I, n=8), versus those with a normal ETT (group II, n=37). There were no differences in age, sex, severity, and duration of hypertension between the two groups, but group I patients were significantly more overweight and had a worse lipid profile. Blood pressure at peak ETT was higher in group I despite shorter exercise duration, although resting and ambulatory pressures were similar. Group I patients had evidence of more pronounced cardiac enlargement and LVH by both ECG and echo criteria and a characteristic pattern of more pronounced thickening at the apex, but both groups had equally good systolic function and similar degrees of mild diastolic dysfunction. Analysis of 24-hour ambulatory ECG showed a significantly greater propensity to ventricular arrhythmias in group I, as shown by the presence of late potentials in 4 patients, the presence of couplets in 3, runs of ventricular tachycardia in 2 (while none of group II patients had late potentials or complex arrhythmias), and an average frequency of isolated premature ventricular contractions approximately three times higher in group I than group II patients. Our data demonstrate that hypertensives with LVH associated with myocardial ischemia at stress but with normal coronary arteriograms tend to be more overweight, attain a higher systolic blood pressure at ETT despite a shorter duration, have a higher propensity for severe arrhythmias, and have an adverse lipid profile. LVH in these subjects is more pronounced by both ECG and echo criteria and is characterized by predominantly apical hypertrophy with left atrial and ventricular dilatation rather than overall LV wall thickening.

Diminished transient outward currents in rat hypertrophied ventricular myocytes

Action potential duration is prolonged in ventricular hypertrophy induced by sustained pressure overload. Since the transient outward current (I(to)) is a major factor for determining action potential duration in rat ventricular cells, we used patch-clamp techniques to compare the characteristics of I(to) in normal and hypertrophied left ventricular cells of the rat. Left ventricular pressure overload was induced by partial ligation of the abdominal aorta for 4 to 6 weeks before study. Age-matched normal rats served as controls. Pressure overload increased the heart weight-to-body weight ratio by 47.7%. I(to) was significantly smaller in hypertrophied cells than in normal cells (20.0 +/- 1.3 versus 31.0 +/- 2.1 pA/pF, respectively, at a test potential of +60 mV; P < .001). There were no differences in the steady-state inactivation, the inactivation time course, and the time course of recovery from inactivation between normal and hypertrophied cells. At the single-channel level, there were no differences in the unitary current amplitude of the single I(to) channel between normal and hypertrophied cells, and the slope conductance was 13.7 picosiemens in normal cells and 13.4 picosiemens in hypertrophied cells. The maximum open-state probability, which was estimated from the ratio of the peak of the ensemble-averaged currents to the single-channel current amplitude, was similar for normal and hypertrophied cells (0.66 +/- 0.03 and 0.69 +/- 0.04, respectively, at a test potential of +40 mV; P = NS). We conclude that diminished I(to) contributes to action potential prolongation in hypertrophied ventricular cells from pressure-overloaded rat hearts. Reduced I(to) channel density may be responsible for the diminished whole-cell I(to).

Enhanced functional expression of transient outward current in hypertrophied feline myocytes

Cardiac hypertrophy can decrease myocardial contractility and alter the electrophysiological activity of the heart. It is well documented that action potentials recorded from hypertrophied feline ventricular cells can exhibit depressed plateau voltages and prolonged durations. Similar findings have been made by others in rabbit, rat, guinea pig, and human heart. Whole-cell patch voltage-clamp studies designed to explain these changes in the action potential suggest that the only component of the membrane current recorded from feline right ventricular (RV) myocytes found to be substantially different from normal is the 4-amino-pyridine-sensitive transient outward current (I(to)). However, it was not clear if the change in I(to) could explain the changes in the action potential of hypertrophied cardiocytes, nor was it clear if these changes reflect an alteration in the electrophysiological character of the channels underlying I(to). A kinetic comparison of I(to) elicited by hypertrophied RV myocytes with that elicited by comparable normal RV myocytes previously revealed no differences, suggesting that the increased magnitude of the peak I(to) recorded from hypertrophied myocytes arises because the current density increases and not because of any alteration in the kinetic parameters governing the current. This finding suggests that in hypertrophy additional normal channels are expressed rather than a kinetically different channel subtype emerging. Investigations designed to determine if enhancement of I(to) could explain the hypertrophy-induced changes in plateau voltage and action potential duration suggest that a change in I(to) density can indeed explain the entire effect of hypertrophy on RV action potentials. If this notion is correct, the likelihood of "sudden death" in patients with myocardial hypertrophy might be decreased by a blocker selective for cardiac I(to).

Effect of haemodynamic pressure overload of the adult ferret right ventricle on inotropic responsiveness to external calcium and rest periods

The inotropic effects of external calcium concentration ([Ca2+]o] and rest periods have been compared in papillary muscles isolated from control (n = 4) and pressure-overloaded right (n = 5) ventricles of adult ferrets. Hypertrophy was induced by pulmonary artery clipping for 30-45 days. Under control conditions (3 mM [Ca2+]o, 0.1 Hz), the isometric twitch force of hypertrophied muscles was decreased by 75%, time to peak was increased by 30% and time to half-relaxation was increased by 50% compared with non-hypertrophied preparations. The sensitivity of contraction to [Ca2+]o was decreased in hypertrophied muscles compared with control ([Ca2+] required for half-maximal contraction: 4.1 mM vs 1.7 mM) and the maximal contraction reached at high [Ca2+]o was smaller in pressure-overloaded muscles compared with control (8.3 +/- 2.0 mN mm-2 vs 19.0 +/- 2.1 mN mm-2 respectively). In both groups, rest periods longer than the steady-state interval were initially accompanied by a potentiation of the first post-rest contraction compared with steady-state. Peak potentiation occurred after a rest of 120 s in hypertrophied muscles and after a rest of 60 s in control. The maximal relative potentiation, i.e. compared with the steady-state twitch, was higher in hypertrophied muscles (+75%) than in control (+20%). After peak potentiation, the amplitude of the first post-rest contraction progressively decreased with increasing periods of rest, although at a slower rate in hypertrophy compared with control. The time constants of post-rest decay were 1203 +/- 99 s and 528 +/- 24 s respectively.(ABSTRACT TRUNCATED AT 250 WORDS)