Mitochondria can act as a Ca²⁺ uptake compartment due to the large electrochemical gradient in favour of Ca²⁺ accumulation. Although a role for mitochondria in the control of Ca signalling under physiological conditions had been largely dismissed, recent data indicate that in some types of smooth muscle cells, mitochondria may play an important role, by affecting the spatio-temporal aspects (McGeown et al. 1996; Drummond & Tuft, 1999; McCarron & Muir, 1999). In the present work, therefore, we have examined the role of mitochondria in control of Ca²⁺ signalling in rat ureter smooth muscle cells.

Single cells were produced by enzymatic digestion, following humane killing of the rats. The cells were loaded with the Ca-sensitive indicator fura-2, voltage-clamped and the mitochondrial Ca²⁺ uptake inhibitor-carbonyl cyanide m-chlorophenyhydrazone (CCCP), with and without oligomycin B Ð a blocker of the mitochondrial ATP synthase, applied.

CCCP in voltage-clamped rat ureteric cells produced a significant (P < 0.05, t test), elevation of the resting baseline level of intracellular Ca²⁺ (n = 27). This elevation was associated with the generation of inward current. There was also a significant decrease in the rate of restoration of the Ca²⁺ transients induced by depolarising voltage steps or application of 10 µM carbachol, by 1.42 ± 0.25 (n = 27) and 1.53 ± 0.34 times (n = 4), respectively.

Clearly alteration of mitochondrial Ca²⁺ homeostasis has profound effects on cytosolic Ca²⁺ handling and excitability, in these cells. Since 3 mM ATP and 11 mM Hepes were present in the pipette solution an inhibition of ATP production or decrease in pH seem to be unlikely mechanisms to contribute to the effect of CCCP on Ca²⁺ signalling in rat ureteric cells. Interaction between the mitochondrial and SR Ca²⁺ store are suggested by preliminary data showing that the frequency of Ca²⁺ puffs and STOCs were transiently increased in the presence of CCCP. These data suggest that mitochondria can modulate the characteristics of both elementary and global Ca²⁺ events in rat ureteric cells.

We are grateful to the Novartis Foundation for supporting L. Borisova.

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