(6) THE MITOCHONDRIAL REDOX POTENTIAL IS PARTIALLY REGULATED BY Ca2+ DURING INCREASED WORK IN INTACT CARDIAC TRABECULAE

Rolf Brandes
Loyola University, Chicago, IL

The oxidative phosphorylation (ox.phos.) rate, in isolated mitochondria, is stimulated by increased [ADP], resulting in decreased mitochondrial [NADH] ([NADH]m). In vivo, however, increased mechanical work has not generally demonstrated increased [ADP], and increased [NADH]m has instead been suggested to stimulate the ox.phos. rate. Increased [NADH]m, in turn, has been suggested to be a consequence of increased cytosolic [Ca2+] ([Ca2+]c), resulting in rising mitochondrial [Ca2+] ([Ca2+]m) which stimulate mitochondrial dehydrogenases to increase the NADH production rate.

Using fluorescence spectroscopy, we have monitored [NADH]m, [Ca2+]c and [Ca2+]m using Ca2+ sensitive dyes (Indo-1 and Rhod-2/AM) in isolated rat trabeculae. Work, average Force and average [Ca2+]c were simultaneously increased by increased pacing frequency.

Immediately following a work jump (0.25 to 2 Hz), [Ca2+]c rose while [Ca2+]m was still low and [NADH]m fell; suggesting a Ca2+ independent rise of the ox.phos. rate. Following continued stimulation, [Ca2+]c remained high while [Ca2+]m and [NADH]m slowly increased; consistent with slow mitochondrial Ca2+ uptake and a Ca2+ dependent stimulation of the NADH production rate. Immediately following decreased work (2 to 0.25 Hz), [Ca2+]c fell while [Ca2+]m remained high and [NADH]m rose; consistent with a continued stimulation of the NADH production rate while the ox.phos. rate was reduced. As [Ca2+]m slowly fell, [NADH]m also fell and returned to control value.

Following increased work, the [Ca2+]m signal increased by 11 ± 2 % with a time constant, Tau(Ca, up) = 28 ± 8 sec, and the [NADH]m signal recovered by 18 ± 2 % with a time constant, Tau(NADH, up) = 23 ± 5 sec. When work was reduced, the [Ca2+]m and [NADH]m signals decreased back to control levels with time constants; Tau(Ca, down) = 78 ± 12 sec and Tau(NADH, down) = 53 ± 6 sec.