Differential effects of chronic reserpine exposure on Ca2+ sequestering mechanisms in rat submandibular gland vesicles

To determine the effects of exposure to reserpine on subcellular Ca2+ transporting systems, active Ca2+ uptake was measured with and without ruthenium red in submandibular gland vesicles obtained from rats after chronic treatment with reserpine. The properties of ruthenium red-sensitive Ca2+ uptake were similar to those measured in submandibular gland vesicles from untreated rats: it was abolished by the dye, was relatively low at 1 microM Ca2+ but increased markedly at millimolar Ca2+ levels and was positively and significantly correlated with the mitochondrial membrane marker, cytochrome-C oxidase activity, in membrane subfractions obtained by differential centrifugation (r = 0.67, p = 0.0005, n = 29). On the other hand, ruthenium red-insensitive Ca2+ uptake, though stimulated at submicromolar Ca2+ concentrations, was reduced by a mean of 54% compared to preparations from untreated animals and particulate RNA content was 18% of that found in control preparations. Moreover, the distributions of ruthenium red-insensitive Ca2+ uptake and particulate RNA (which are closely correlated in vesicles from untreated rats) were not significantly related when measured in vesicles of submandibular glands from reserpine treated rats. Other membrane markers and overall membrane protein content were not significantly altered after chronic reserpine exposure. We conclude that reserpine treatment has little effect on mitochondrial Ca2+ uptake capacity but abolishes or drastically reduces the high affinity Ca2+-sequestering activity which, in submandibular gland vesicles from untreated rats, is apparently associated with the endoplasmic reticulum.