Na+/Ca2+ exchange current (I(Na/Ca)) and sarcoplasmic reticulum Ca2+ release in catecholamine-induced cardiac hypertrophy.
OBJECTIVE
Catecholamines that accompany acute physiological stress are also involved in mediating the development of hypertrophy and failure. However, the cellular mechanisms involved in catecholamine-induced cardiac hypertrophy, particularly Ca2+ handling, are largely unknown. We therefore investigated the effects of cardiac hypertrophy, produced by isoprenaline, on I(Na/Ca) and sarcoplasmic reticulum (SR) function in isolated myocytes.


METHODS
I(Na/Ca) was studied in myocytes from Wistar rats, using descending (+80 to -110 mV) voltage ramps under steady state conditions. Myocytes were also loaded with fura-2 and either field stimulated or voltage clamped to assess [Ca2+]i and SR Ca2+ content.


RESULTS
Ca2+-dependent, steady state I(Na/Ca) density was increased in hypertrophied myocytes (P<0.05). Ca2+ release from the SR was also increased, whereas resting [Ca2+]i and the rate of decline of [Ca2+]i to control levels were unchanged. SR Ca2+ content, estimated by using 10.0 mmol/l caffeine, was also significantly increased in hypertrophied myocytes, but only when myocytes were held and stimulated from their normal resting potential (-80 mV) but not from -40 mV. However, the rate of decline of caffeine-induced Ca2+ transients or I(Na/Ca) was not significantly different between control and hypertrophied myocytes. Ca2+-dependence of I(Na/Ca), examined by comparing the slope of the descending phase of the hysteresis plots of I(Na/Ca) vs. [Ca2+]i, was also similar in the two groups of cells.


CONCLUSIONS
Data show that SR Ca2+ release and SR Ca2+ content were increased in hypertrophied myocytes, despite an increase in the steady state I(Na/Ca) density. The observation that increased SR function occurred only when myocytes were stimulated from -80 mV suggests that Na+ influx may play a role in altering Ca2+ homeostasis in hypertrophied cardiac muscle, possibly through increased reverse Na+/Ca2+ exchange, particularly at low stimulation frequencies.
Chorvatova
Alzbeta
A
Department of Medicine, University Clinical Departments, UCD Daulby Street, The Duncan Building, Liverpool L69 3GA, UK.
Hart
George
G
Hussain
Munir
M
D000200:Action Potentials; D000818:Animals; D001145:Arrhythmias, Cardiac / Q000378:metabolism / Q000503:physiopathology; D020013:Calcium Signaling; D006332:Cardiomegaly / Q000378:metabolism* / Q000503:physiopathology; D002395:Catecholamines; D008297:Male; D032383:Myocytes, Cardiac / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*
D002395:Catecholamines; D019831:Sodium-Calcium Exchanger