Mode of mitochondrial Ca super(2+) clearance and its influence on secretory responses in stimulated chromaffin cells

• Published in: Cell calcium (Edinburgh) Vol. 39; no. 1; pp. 35 - 46
• Main Author: Warashina, Akira
• Format: Journal Article
• Language: English
• Published: 01.01.2006
• ISSN: 0143-4160
• DOI: 10.1016/j.ceca.2005.09.001

To study the role of mitochondrial Ca super(2+) clearance in stimulated cells, changes in free Ca super(2+) concentration in the cytosol, [Ca super(2+)] sub(c) and that in mitochondria, [Ca super(2+)] sub(m) along with secretory responses were observed using chromaffin cells co-loaded with Fura-2 and Rhod-2 in the perfused rat adrenal medulla. When the cells were stimulated with 40 mM K super(+) in the perfusate, the duration of [Ca super(2+)] sub(m) response markedly increased with prolongation of the stimulation period, exhibiting a mean half-decay time of 21 min with 30 s stimulation, whereas its amplitude was not altered with stimulations of 10-30 s. A computer simulation analysis showed that such a mode of [Ca super(2+)] sub(m) response can be produced if excess Ca super(2+) taken up by mitochondria precipitates as calcium phosphate (Pi) salt. In the presence of 5 mu M rotenone plus 10 mu M oligomycin, a decrease in the duration of [Ca super(2+)] sub(m) response and a slight but significant increase (24%) in the secretory response to 30 s stimulation with 40 mM K super(+) were observed. Simulation analyses suggested that this effect of rotenone may be due to reduction in mitochondrial Ca super(2+) uptake induced by rotenone-elicited partial depolarization of the mitochondrial membrane potential. In chromaffin cells transsynaptically stimulated through the splanchnic nerve, the intensity of NAD(P)H autofluorescence changed with time courses similar to those of [Ca super(2+)] sub(m) responses. The temporal profiles of those two responses were prolonged in a similar manner by application of an inhibitor of mitochondrial Na super(+)/Ca super(2+) exchanger, CGP37157. Thus, due to the unique Ca super(2+) buffering mechanism, [Ca super(2+)] sub(m) responses associated with massive mitochondrial Ca super(2+) uptake may occur within a limited concentration range in which Ca super(2+)-sensitive dehydrogenases are activated to control the mitochondrial redox state in stimulated chromaffin cells.