Effect of caffeine on azide-sensitive Mg2+, ATP-dependent increase of tetracycline fluorescent response in modeling of Ca ions accumulation in mitochondria

The influence of caffeine on the Mg2+, ATP-dependent Ca(2+)-uptake was investigated in the experiments, conducted on mitochondria isolated from myometrium of nonpregnant estrogenized rats. NaN3-sensitive CTC fluorescence increasing (lambda f = = 520 nm) was used as a test for active Ca2+ transport. Kinetics of NaN3-sensitive Mg2+, ATP-dependent component of CTC fluorescence change fits to the pattern of the first-order reaction either in the absence or in the presence of caffeine (20 mM). Caffeine (0-20 mM) inhibited both the stationary level (settled on the 2-3d min. of incubation) and the initial rate V0, and rate constant k of CTC fluorescence change. Magnitude of the apparent inhibition constant I0.5 for caffeine is 10.41 +/- 1.81 mM, inhibition process has weak positive cooperativity--the value of apparent Hill coefficient for caffeine is equal to 1.2 +/- 0.3. Data obtained suggest that caffeine inhibits both stationary Ca2+ capacity of mitochondria and the rate of NaN3-sensitive Mg2+, ATP-dependent Ca(2+)-accumulation in case of myometrium. These data could be useful for further investigation of molecular and membrane mechanisms of caffeine action on the intracellular Ca2+ homeostasis in uterus smooth muscle and its contractive activity.