Abstract 2633: Privileged Calcium Communication and Bioenergetic Feedback Between the Sarcoplasmic Reticulum and Mitochondria in Cardiac Myocytes
In cardiac myocytes, Ca influx via L-type Ca channels (ICa,L) triggers Ca release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyRs), creating a microdomain of high cytosolic [Ca] ([Ca]c). Mitochondria take up Ca via a Ca-uniporter (mCU) with a relatively low affinity for Ca (Kd~10–30 μM). Thus, efficient mitochondrial Ca uptake may require close proximity of the mCU to RyRs. In heart failure, the contribution of reverse mode Na/Ca-exchanger (INCX) to cytosolic Ca transients is increased. Since INCX-mediated Ca influx is slower than ICa,L-triggered SR Ca release and originates from a more remote source, we speculated that mitochondrial Ca uptake and subsequent stimulation of NADH production by the Krebs cycle is less efficient under these conditions. To monitor mitochondrial [Ca] ([Ca]m) together with [Ca]c, guinea-pig myocytes were loaded with rhod-2 (locating to mitochondria), patch-clamped and dialyzed with indo-1 to report [Ca]c. Voltage steps from −45 to +10 or +80 mV elicited ICa,L- or INCX-mediated Ca-influx, respectively. Before INCX-mediated Ca influx, SR Ca load was depleted by thapsigargin. As expected, the upstroke of [Ca]c was faster after ICa,L-induced SR Ca release than after INCX-mediated Ca influx (32±7 vs. 354±16 ms; p<.001). To achieve a comparable increase of [Ca]m, the amplitude (Δ) of [Ca]c had to be >2-fold higher after INCX-mediated Ca influx than after ICa,L-induced SR Ca release (0.5±0.1 vs 0.2±0.1 ΔF/F0*Δ μM−1; p<.01). To monitor the bioenergetic response, autofluorescence of NADH and flavoproteins was recorded. At comparable [Ca]c, caffeine-induced SR Ca release increased NADH and reduced flavoproteins, whereas INCX-mediated Ca influx led to no change. A similar result was obtained also in non-patched cells briefly superfused with either caffeine (10 mM) or 0 mM extracellular [Na] (to induce SR- and INCX-related Ca-transients, respectively), despite comparable increases in [Ca]c and twitch amplitudes. We conclude that a privileged Ca communication exists between the SR and mitochondria, while INCX-mediated Ca influx is less efficient for mitochondrial Ca uptake and thus, eliciting a bioenergetic response. This may have negative implications for energy supply-and-demand matching in heart failure.