Temporal patterns of electrical remodeling in canine ventricular hypertrophy: Focus on IKs downregulation and blunted β-adrenergic activation

Abstract

Objectives: Electrical remodeling in cardiac hypertrophy often involves the downregulation of K + currents, including β-adrenergic (β-A)-sensitive I Ks . Temporal patterns of ion-channel downregulation are poorly resolved. In dogs with complete atrioventricular block (AVB), we examined (1) the time course of molecular alterations underlying I Ks downregulation from acute to chronic hypertrophy; and (2) concomitant changing responses of repolarization to β-adrenergic receptor (β-AR) stimulation. Methods and Results: Serial left-ventricular (LV) biopsies were collected from anesthetized dogs during sinus rhythm (SR; control) and at 3, 7 and 30 days of AVB. KCNQ1 mRNA and protein decreased within 3 days (protein expression 58 ± 10% of control), remaining low thereafter. β1-AR mRNA and protein decreased more gradually to 53 ± 8% at 7 days. In chronic-AVB LV myocytes, I Ks -tail density was reduced: 1.4 ± 0.3 pA/pF versus 2.6 ± 0.4 pA/pF in controls. β-A enhancement of I Ks was reduced. Isoproterenol shortened action-potential duration in control cells, while causing heterogeneous repolarization responses in chronic AVB. β-A early afterdepolarizations were induced in 4 of 13 chronic-AVB cells, but not in controls. In intact conscious dogs, isoproterenol shortened QT c at SR (by −8 ± 3% from 295 ms), left it unaltered at 3 days AVB (+1 ± 3% from 325 ms) and prolonged QT c at 30 days (+ 6 ± 3% from 365 ms). Conclusions: Profound decrease of KCNQ1 occurs within days after AVB induction and is followed by a more gradual decrease of β1-AR expression. Downregulation and blunted β-A activation of I Ks contribute to the loss of β-A-induced shortening of ventricular repolarization, favoring proarrhythmia. Provocation testing with isoproterenol identifies repolarization instability based on acquired channelopathy.

Serial left-ventricular biopsy sampling using a minimally invasive trans-thoracic approach in adult dogs

“…All analysed mechanical parameters were compatible with our earlier work [4,5]. Also, under control conditions, repeated biopsy sampling from the apicolateral LV wall did not significantly influence the expression of cardiomyocyte-specific proteins that have been shown to be altered in remodelling processes early after the induction of AVB [16]. These findings support the minimally invasive character of the technique and are in line with the macroscopic finding of only slight local fibrosis at barely visible puncture sites.…”

“…6). To control for influences of repeated biopsy sampling itself on protein expression, we studied cardiomyocytespecific proteins known to change after AVB, such as the K + -channel sub-unit KCNQ1 and the β-1 adrenergic receptor [16]. The expression level of both proteins remained unaltered in repeated biopsies of three control dogs (Fig.…”

“…3). Our serial approach allows to discern even mildly fluctuating expression patterns in longterm studies because individual dogs serve as their own controls [4,16]. These subtle changes are likely missed in group comparisons due to inherent inter-individual differences.…”

“…For further processing, biopsies were either immediately frozen in isopentane, pre-cooled with liquid nitrogen and stored at −80°C or fixed in 3% glutaraldehyde and stored at 4°C. Light and electron microscopy, PCR and Western blotting were performed according to standard protocols [4,16].…”

Basal and IGF-I-dependent regulation of potassium channels by MAP kinases and PI3-kinase during eccentric cardiac hypertrophy

“…Interestingly, Brooksby et al (5) have shown that the electrical remodeling related to cardiac hypertrophy was restricted to a decrease in I K1 current density with no difference in the resting potential level of cardiomyocytes from control and spontaneously hypertensive rats. The fewer electrophysiological studies available in the literature related to volume-overload induced cardiac hypertrophy showed a decrease in I K (50,56) but an increase in I K1 without any change in I to (56).…”