Sodium-pump injury and arrhythmogenic transient depolarizations in catecholamine-induced cardiac hypertrophy

Cardiac hypertrophy induced by sustained β-adrenoreceptor activation: pathophysiological aspects

Cardiac hypertrophy is promoted by adrenergic over-activation and represents an independent risk factor for cardiovascular morbidity and mortality. The basic knowledge about mechanisms by which sustained adrenergic activation promotes myocardial growth, as well as understanding how structural changes in hypertrophied myocardium could affect myocardial function has been acquired from studies using an animal model of chronic systemic beta-adrenoreceptor agonist administration. Sustained beta-adrenoreceptor activation was shown to enhance the synthesis of myocardial proteins, an effect mediated via stimulation of myocardial growth factors, up-regulation of nuclear proto-oncogenes, induction of cardiac oxidative stress, as well as activation of mitogen-activated protein kinases and phosphatidylinositol 3-kinase. Sustained beta-adrenoreceptor activation contributes to impaired cardiac autonomic regulation as evidenced by blunted parasympathetically-mediated cardiovascular reflexes as well as abnormal storage of myocardial catecholamines. Catecholamine-induced cardiac hypertrophy is associated with reduced contractile responses to adrenergic agonists, an effect attributed to downregulation of myocardial beta-adrenoreceptors, uncoupling of beta-adrenoreceptors and adenylate cyclase, as well as modifications of downstream cAMP-mediated signaling. In compensated cardiac hypertrophy, these changes are associated with preserved or even enhanced basal ventricular systolic function due to increased sarcoplasmic reticulum Ca(2+) content and Ca(2+)-induced sarcoplasmic reticulum Ca(2+) release. The increased availability of Ca(2+) to maintain cardiomyocyte contraction is attributed to prolongation of the action potential due to inhibition of the transient outward potassium current as well as stimulation of the reverse mode of the Na(+)-Ca(2+) exchange. Further progression of cardiac hypertrophy towards heart failure is due to abnormalities in Ca(2+) handling, necrotic myocardial injury, and increased myocardial stiffness due to interstitial fibrosis.

Mechanisms underlying delayed afterdepolarizations in hypertrophied left ventricular myocytes of rats

Cardiac hypertrophy was induced in rats by daily injection of isoproterenol (5 mg/kg ip) for 7 days. Membrane voltage and currents were recorded using the whole cell patch-clamp technique in left ventricular myocytes from control and hypertrophied hearts. Ryanodine-sensitive delayed afterdepolarizations (DADs) and transient inward current (I(ti)) appeared in hypertrophied cells more often and were of larger amplitude than in control cells. DADs and I(ti) are carried principally by Na/Ca exchange with smaller contributions from a nonselective cation channel and from a Cl- channel. The latter is expressed only in hypertrophied myocytes. In hypertrophy, the density of caffeine-induced Na/Ca exchange current (I(Na/Ca)) was increased by 26%, sarcoplasmic reticulum (SR) Ca2+ content as assessed from the integral of I(Na/Ca) was increased by 30%, the density of Na-pump current (I(pump)) was reduced by 40%, and the intracellular Na+ content, measured by Na+-selective microelectrodes was increased by 55%. The results indicate that DADs and I(ti) are generated by spontaneous Ca2+ release from an overloaded SR caused by a downregulated Na pump and an upregulated Na/Ca exchange. These findings may explain the propensity for arrhythmias seen in this model of hypertrophy.

Force-frequency response in isoproterenol-induced hypertrophied rat heart

Rate-dependent force production was investigated using small trabecular muscle from control and hypertrophied rat cardiac muscle. Cardiac hypertrophy was induced by daily subcutaneous injection of isoproterenol (0.3 mg/kg body weight) for 12 days. The force-frequency relationship, for the control rat myocardium, is clearly biphasic. A stepped increase in stimulation frequency from 0.1 to 0.5 Hz results in a decrease in contractile force (negative phase). However, at higher stimulation frequency above 0.5 Hz, an increased contractile force is revealed (positive phase). Membrane action potential duration (APD50) was used to reflect sarcolemmal Ca2+ influx. The frequency-dependent increase in APD50 and the ability of nifedipine, a sarcolemmal L-type Ca2+ channel blocker, to eliminate the positive-force frequency response, indicate that sarcolemmal Ca2+ influx is important for force development at high stimulation frequency. Relative Ca2+ content of sarcoplasmic reticulum is estimated from rapid cooling contractures. The parallel change of rapid cooling contractures and twitch force suggests that the sarcoplasmic reticulum Ca2+ content alters with varying frequencies of stimulation. Isoproterenol-induced hypertrophied muscle shows a greater contractile force, increased nifedipine-sensitive force development and prolonged APD50 compared to controls. These data suggest a greater availability of intracellular Ca2+ to activate contraction in hypertrophied muscle, possibly by amplified Ca2+ influx via L-type channel.

Isoform-specific regulation of myocardial Na,K-ATPase alpha-subunit in congestive heart failure. Role of norepinephrine

BACKGROUND

Myocardial ouabain-binding sites and Na,K-ATPase activity are reduced in congestive heart failure (CHF), but the mechanisms by which CHF reduces the Na,K-ATPase remain unknown. We proposed to investigate whether the changes are accompanied by isoform-specific reductions of the Na,K-ATPase alpha-subunit proteins in CHF and whether similar changes could be produced by exogenous norepinephrine administration.

METHODS AND RESULTS

CHF was induced in dogs by rapid ventricular pacing at a rate of 225 beats per minute for 8 weeks (protocol 1). A second group of dogs were paced at 100 beats per minute and served as controls. In protocol 2, norepinephrine was infused in normal dogs using a subcutaneous osmotic minipump for 8 weeks. The control dogs received normal saline through the pump. Animals were studied after 8 weeks of pacing or norepinephrine infusion. After the baseline hemodynamics and interstitial norepinephrine concentration had been obtained, the hearts were removed for measuring [3H]ouabain-binding sites and Na,K-ATPase alpha-subunit proteins using isoform-specific monoclonal antibodies.

RESULTS

Myocardial [3H]ouabain-binding sites were reduced in dogs with CHF and chronic norepinephrine infusion. The Western blot analysis showed that adult canine hearts possess both alpha 1 and alpha 3 isoforms of the Na,K-ATPase alpha-subunit but not the alpha 2 isoform protein. CHF and NE infusion had no effect on the Na,K-ATPase alpha 1-subunit protein but did reduce the alpha 3 isoform protein significantly. In addition, there was a significant inverse correlation between the amount of myocardial alpha 3 isoform protein and interstitial norepinephrine content in the dogs. In contrast, the specific activity of the sarcolemmal marker 5'-nucleotidase did not differ among the groups of animals.

CONCLUSIONS

The reduction of myocardial Na,K-ATPase in CHF is limited to the alpha 3 isoform. Furthermore, because similar changes in myocardial ouabain-binding sites and Na,K-ATPase alpha 3 isoform were produced by chronic norepinephrine infusion, the decrease in the Na,K-ATPase in CHF is most likely mediated via excess sympathetic stimulation.

Catecholamine-induced cardiac hypertrophy uncouples beta-adrenoceptors from slow calcium channels

Changes in the parameters of Ca(2+)-dependent slow action potentials (APs) and in their sensitivity to noradrenaline, forskolin, dibutyryl-cAMP and extracellular Ca2+ concentration were studied and compared in left ventricular trabeculae from normal control rats and rats with cardiac hypertrophy. Cytochemical studies were also carried out to determine changes in the activity of membrane-bound adenylate cyclase. Hypertrophy was induced by administration of 5 mg/kg isoproterenol once daily for 7 days. In hypertrophied cardiac muscle, the overshoot of the slow APs was increased by 75%, the maximum rate of rise (Vmax) increased by 76% and the AP duration at 50% repolarization (APD50) prolonged by 56%. The Vmax, an indicator of the slow inward Ca2+ current, increased, in a dose-dependent manner, in response to the beta-adrenoceptor agonist noradrenaline, the adenylate cyclase activator forskolin, the protein kinase activator cAMP and elevated Ca2+ concentration in normal control preparations, whereas in hypertrophied myocardium, the beta-agonist noradrenaline and the adenylate cyclase activator forskolin had no effect. In cytochemical studies with ATP as substrate, adenylate cyclase activity was localized in the sarcolemma, and significantly fewer reaction products appeared on the outer side of the cell membrane in hypertrophied myocytes than in control myocytes. The results suggest that catecholamine-induced cardiac hypertrophy damages the catalytic subunit of membrane-bound adenylate cyclase, thus uncoupling beta-adrenoceptors from slow Ca2+ channels in the transmembrane signalling process.