Mitochondria buffer NCX-mediated Ca 2+-entry and limit its diffusion into vascular smooth muscle cells

Abstract

The reverse-mode of the Na +/Ca 2+-exchanger (NCX) mediates Ca 2+-entry in agonist-stimulated vascular smooth muscle (VSM) and plays a central role in salt-sensitive hypertension. We investigated buffering of Ca 2+-entry by peripheral mitochondria upon NCX reversal in rat aortic smooth muscle cells (RASMC). [Ca 2+] was measured in mitochondria ([Ca 2+] MT) and the sub-plasmalemmal space ([Ca 2+] subPM) with targeted aequorins and in the bulk cytosol ([Ca 2+] i ) with fura-2. Substitution of extracellular Na + by N-methyl- d-glucamine transiently increased [Ca 2+] MT (∼2 μM) and [Ca 2+] subPM (∼1.3 μM), which then decreased to sustained plateaus. In contrast, Na +-substitution caused a delayed and tonic increase in [Ca 2+] i (<100 nM). Inhibition of Ca 2+-uptake by the sarcoplasmic reticulum (SR) (30 μM cyclopiazonic acid) or mitochondria (2 μM FCCP or 2 μM ruthenium red) enhanced the elevation of [Ca 2+] subPM. These treatments also abolished the delay in the [Ca 2+] i response to 0Na + and increased its amplitude. Extracellular ATP (1 mM) caused a peak and plateau in [Ca 2+] i , and only the plateau was inhibited by KB-R7943 (10 μM), a selective blocker of reverse-mode NCX. Evidence for ATP-mediated NCX-reversal was also found in changes in [Na +] i . Mitochondria normally exhibited a transient elevation of [Ca 2+] in response to ATP, but inhibiting the mitochondrial NCX with CGP-37157 (10 μM) unmasked an agonist-induced increase in mitochondrial Ca 2+-flux. This flux was blocked by KB-R7943. In summary, mitochondria and the sarcoplasmic reticulum co-operate to buffer changes in [Ca 2+] i due to agonist-induced NCX reversal.