Expression of the hyperpolarization-activated current, I(f), in cultured adult rat ventricular cardiomyocytes and its modulation by hypertrophic factors

The hyperpolarization-activated, cyclic nucleotide-gated (HCN) current, I(f), is typically overexpressed in myocytes from hypertrophied and failing hearts, where it may play an arrhythmogenic role. Signaling pathways activated by agonists such as angiotensin-II, endothelin-1 and phenylephrine, via G protein-coupled receptors (GPCR), promote myocardial hypertrophy, but their effect on cellular electrophysiological remodeling, particularly I(f) expression is largely unknown. Thus, I(f) expression was measured by patch-clamp and quantitative RT-PCR measurement in cultured adult rat ventricular cardiomyocytes (VCM) exposed to different culture conditions, that is, in the absence or presence of: fetal bovine serum (FBS, 5%), 0.1 microM angiotensin-II, 0.1 microM endothelin-1 or 20 microM phenylephrine. Membrane capacitance (C(m)) was used to estimate cell size and current density in patch-clamped VCM. At 8 days of culture, about 60% of VCM showed I(f). In serum-free medium, I(f) density was increased by phenylephrine (2.28+/-0.51 vs. 0.84+/-0.30 pA/pF in CTR, p<0.05) and endothelin-1 (2.20+/-0.38 vs. 1.03+/-0.34 pA/pF in control, p<0.05), not by angiotensin-II (1.60+/-0.50 pA/pF, not significant vs. control). Similarly, in cells cultured with 5% FBS, phenylephrine and endothelin-1 significantly increased I(f) density by 159.3% and 59.5% (p<0.05 vs. untreated cells), while angiotensin-II did not modify it. The effect of endothelin-1 was abolished by using the selective endothelin receptor type A (ET(1A)) antagonist BQ-123 (1 microM). mRNA levels for HCN2 and HCN4 were significantly increased during in vitro culture; exposure to endothelin-1 increased HCN2 mRNA. A similar pattern of I(f) overexpression was detected in hypertrophied left ventricular cardiomyocytes from old hypertensive rats. Thus, adult VCM in primary culture undergo changes in I(f) expression reminiscent of in vivo hypertrophy; endothelin-1 (but no angiotensin-II) seems to play a role in ionic remodeling.