Functional coupling between the caffeine/ryanodine-sensitive Ca2+ store and mitochondria in rat aortic smooth muscle cells.

We investigated the role of mitochondria in the agonist-induced and/or caffeine-induced Ca 2+ transients in rat aortic smooth muscle cells. We explored the possibility that proliferation modulates the coupling between mitochondria and endoplasmic reticulum. Ca 2+ transients induced by either ATP or caffeine were measured in presence or absence of drugs interfering with mitochondrial activity in freshly dissociated cells (day 1) and in subconfluent primary culture (day 12). We found that the mitochondrial inhibitors, rotenone or carbonyl cyanide m -chlorophenylhydrazone, as well as the permeability transition pore inhibitor, cyclosporin A, had no effect on the ATP-induced Ca 2+ transient at either day 1 or day 12, but prevented caffeine-induced cytosolic Ca 2+ increase at day 12 but not at day 1. Close connections between ryanodine receptors and mitochondria were observed at both day 1 and 12. Thapsigargin (TG) prevented ATP- and caffeine-induced Ca 2+ transients at day 1. At day 12, where only 50% of the cells were sensitive to caffeine, TG did not prevent the caffeine-induced Ca 2+ transient, and prevented ATP-induced Ca 2+ transient in only half of the cells. Together, these data demonstrate that rat aortic smooth muscle cells at day 1have an ATP- and caffeine-sensitive pool, which is functionally independent but physically closely linked to mitochondria and totally inhibited by TG. At day 12, we propose the existence of two cell populations: half contains IP 3 receptors and TG-sensitive Ca 2+ pumps only; the other half contains, in addition to the IP 3 -sensitive pool independent from mitochondria, a caffeine-sensitive pool. This latter pool is linked to mitochondria through the permeability transition pore and is refilled by both TG-sensitive and insensitive mechanisms.