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1 Department of Medicine, University of Liverpool, Liverpool L69 3GA, United Kingdom; and 2 Department of Physiology and Pharmacology, Tel-Aviv University, Ramat-Aviv 69978, Israel
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 (Iti)
appeared in hypertrophied cells more often and were of larger amplitude
than in control cells. DADs and Iti 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
(INa/Ca) was increased by 26%, sarcoplasmic
reticulum (SR) Ca2+ content as assessed from the integral
of INa/Ca was increased by 30%, the density of
Na-pump current (Ipump) was reduced by 40%, and
the intracellular Na+ content, measured by
Na+-selective microelectrodes was increased by 55%. The
results indicate that DADs and Iti 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.
transient inward current; sodium/calcium exchange current; chloride current; nonselective cation current; sodium-pump current; arrhythmia
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