Regulation of mitochondrial [NADH] by cytosolic [Ca2+] and work in trabeculae from hypertrophic and normal rat hearts

Pressure overload hypertrophy has previously been shown to reduce contractility but paradoxically to increase O-2 consumption rates at a given force. Because Oz consumption rates are related to mitochondrial [NADH] ([NADH](m)), we tested the hypothesis that with hypertrophy, control of [NADH](m) may be altered. Left ventricular trabeculae were isolated from banded and control rat hearts, and fluorescence spectroscopy was used to monitor [NADH](m) and cytosolic [Ca2+] ([Ca2+](c)). The hearts from banded rats developed hypertrophy (heart-to-body weight ratio increased from 4.1+/-0.1 to 4.9+/-0.1 mg/g) and hypertension (systolic arterial pressure increased from 117+/-4 to 175+/-5 mm Hg). Muscle workload was increased by stepwise increases in pacing frequency (up to 2 Hz). After increased work, [NADH](m) fell and then slowly recovered toward control levels, When work was decreased, [NADH](m) overshot control values and then slowly returned. The Ca2+-independent initial fall was larger for trabeculae from rats with hypertrophied hearts than from control rats (eg, 17+/-2% versus 11+/-1% when work was increased by increasing the frequency from 0.25 to 1 Hz), At 1 Hz, average [Ca2+](c) was approximate to 280 nmol/L, and the Ca2+-dependent [NADH](m) recovery was larger for trabeculae from rats with hypertrophied hearts (17+/-4% versus 10+/-2%) despite similar average [Ca2+](c). At steady state after Ca2+-dependent recovery, there was no difference in [NADH](m) (fall of 1+/-2% versus 1+/-1%). Furthermore, the Ca2+-dependent overshoot was larger for trabeculae from hypertrophied than from control hearts (increase of 14+/-2% versus 9+/-2% when frequency was decreased from 1 to 0.25 Hz), We conclude that (1) there is initially a larger imbalance in NADH production versus consumption rate in hypertrophy (because NADH fell more) and (2) the Ca2+-dependent recovery mechanism is enhanced in hypertrophy (because NADH recovered and overshot more), thus compensating for the larger imbalance.