It is not clear how mitochondrial energy production is regulated in intact tissue when energy consumption suddenly changes. Whereas mitochondrial [NADH] ([NADH]m) may regulate cellular respiration rate and energetic state, it is not clear how [NADH]m itself is controlled during increased work in vivo. We have varied work and [Ca2+] in intact cardiac muscle while assessing [NADH]m using fluorescence spectroscopy. When increased work was accompanied by increasing average [Ca2+]c (by increasing [Ca2+]c or pacing frequency), [NADH]m initially fell and subsequently recovered to a new steady state level. Upon reduction of work, [NADH]m overshot and then returned to control levels. In contrast, when work was increased without increasing average [Ca2+]o (by increasing sarcomere length), [NADH]m fell similarly, but no recovery or overshoot was observed. This Ca(2+)-dependent recovery and overshoot may be attributed to Ca(2+)-dependent stimulation of mitochondrial dehydrogenases. We conclude that the immediate initial increase in respiration rate upon elevation of work is not activated by increased [NADH]m (since [NADH]m rapidly fell) or by [Ca2+]o (since work could also be increased at constant [Ca2+]c). However, during sustained high work, a Ca(2+)-dependent mechanism causes slow recovery of [NADH]m toward control values. This demonstrates a Ca(2+)-dependent feed-forward control mechanism of cellular energetics in cardiac muscle during increased work.

The main goal of this study is to investigate the role of mitochondrial [Ca(2+)], [Ca(2+)](m), in the possible up-regulation of the NADH production rate during increased workload. Such up-regulation is necessary to support increased flux through the electron transport chain and increased ATP synthesis rates. Intact cardiac trabeculae were loaded with Rhod-2(AM), and [Ca(2+)](m) and mitochondrial [NADH] ([NADH](m)) were simultaneously measured during increased pacing frequency. It was found that 53% of Rhod-2 was localized in mitochondria. Increased pacing frequency caused a fast, followed by a slow rise of the Rhod-2 signal, which could be attributed to an abrupt increase in resting cytosolic [Ca(2+)], and a more gradual rise of [Ca(2+)](m), respectively. When the pacing frequency was increased from 0.25 to 2 Hz, the slow Rhod-2 component and the NADH signal increased by 18 and 11%, respectively. Based on a new calibration method, the 18% increase of the Rhod-2 signal was calculated to correspond to a 43% increase of [Ca(2+)](m). There was also a close temporal relationship between the rise (time constant approximately 25 s) and fall (time constant approximately 65 s) of [Ca(2+)](m) and [NADH](m) when the pacing frequency was increased and decreased, respectively, suggesting that increased workload and [Ca(2+)](c) cause increased [Ca(2+)](m) and consequently up-regulation of the NADH production rate.

The oxidative phosphorylation rate in isolated mitochondria is stimulated by increased [ADP], resulting in decreased [NADH]. In intact hearts, however, increased mechanical work has generally not been shown to cause an increase in [ADP]. Therefore, increased [NADH] has been suggested as an alternative for stimulating the phosphorylation rate. Such a rise in [NADH] could result from stimulation of various substrate dehydrogenases by increased intracellular [Ca2+] (e.g., during increased pacing frequency). We have monitored mitochondrial [NADH] in isolated rat ventricular trabeculae, using a novel fluorescence spectroscopy method where a native fluorescence signal was used to correct for motion artifacts. Work was controlled by increased pacing frequency and assessed using time-averaged force. At low-pacing rates (approximately 0.1 Hz), [NADH] immediately decreased during contraction and then slowly recovered (approximately 5 s) before the next contraction. At higher rates, [NADH] initially decreased by an amount related to pacing rate (i.e., work). However, during prolonged stimulation, [NADH] slowly (approximately 60 s) recovered to a new steady-state level below the initial level. We conclude that 1) during increased work, oxidative phosphorylation is not initially stimulated by increased mitochondrial [NADH]; and 2) increased pacing frequency slowly causes stimulation of NADH production.

Sonicated calf thymus DNA with an average length of approximately 100 base pairs has been found to form a cholesteric liquid crystal at a concentration of approximately 250 mg of DNA/mL of solution. Immediately after preparation, small ordered domains of a few micrometers are formed, resulting in an opaque solution. This liquid crystal can readily be oriented in the magnetic field of an NMR magnet, resulting in a clear birefringent phase. The DNA molecules align with their helix axes perpendicular to the field so that the cholesteric pitch axis was parallel with the field. A pitch length of approximately 2.5 microns for the cholesteric phase was determined both from optical measurements (optical light rotation) and from NMR measurements (solvent diffusion). The observation that DNA molecules can be magnetically oriented opens up new possibilities for studying the structure and dynamics of the aligned DNA molecules.

The fluorescent indicator indo-1 is widely used to monitor intracellular calcium concentration. However, quantitation is limited by uncertain effects of the intracellular environment on indicator properties. The goal of this study was to determine the effects of protein and acidosis on the fluorescence spectra and calcium dissociation constant (Kd) of indo-1. With 350 nm excitation light, the ratio of indo-1 fluorescence in the absence versus the presence of saturating Ca2+ at wavelength lambda (S lambda) and Kd increased with [protein]. At pH 7.3, Kd, S400, and S470, which were 210 nM, 0.033, and 1.433 in the absence of protein, increased to 808 nM, 0.161, and 2.641, respectively, by adding proteins from frog muscle and to 638 nM, 0.304, and 3.039, respectively, by adding proteins from rat heart. Effects of protein on indo-1 fluorescence were reduced at higher [indo-1]. Acidosis (pH 6.3) had separate effects, which were additive to those of protein: in the absence of protein, acidosis increased Kd to 640 nM; frog muscle proteins further increased Kd to 1700 nM. Acidosis also changed S lambda slightly. In summary, interaction with protein or protons alters indo-1 calcium-binding and fluorescence. These findings are consistent with several previous studies and suggest that indo-1 calibration constants need to be derived in the presence of appropriate types of protein, ratio of [indo-1]/[protein], and pH.