[Ca ²⁺ ](i) and pH(in) homeostasis in Kaposi sarcoma cells

Changes in intracellular pH (pH(in)) and intracellular calcium concentration [Ca ²⁺ ](i) play a major role in signal transduction leading to cell growth, differentiation and transformation. In some tumor cell lines, vacuolar (V-type) H ⁺ -adenosine triphosphatases (ATPases) are important in pH(in) regulation. To clarify the neoplastic nature and endothelial origin of Kaposi sarcoma (KS), pH(in) and [Ca ²⁺ ](i) and the functional expression of V-type H ⁺ -ATPases were evaluated in cultured endothelial marker-positive KS cells derived from AIDS-KS skin lesions as compared to human umbilical vein endothelial cells (HUVEC). Human skin fibroblasts (HSF) were also examined. Cells were examined using fluorescence spectroscopy with the pH(in) indicator SNARF-1 and the [Ca ²⁺ ](i) indicator Fura-2. We found that whereas pH(in) recovery from acid loading occurred in the absence of Na ⁺ and HCO ₃ ^- in HUVEC and HSF, KS cells did not recover. Moreover, removal of extracellular Na ⁺ had no effect on [Ca ²⁺ ](i) in HUVEC, but transiently increased [Ca ²⁺ ](i) in KS cells and HSF. This [Ca ²⁺ ](i) spike was unaffected by Ca ²⁺ -free medium, suggesting that it is not due to Na ⁺ /Ca ²⁺ exchange. In addition, use of K ⁺ -containing and K ⁺ -free medium to mimic depolarization or hyperpolarization, which may occur during Na ⁺ removal, did not cause [Ca ²⁺ ](i) changes. The [Ca ²⁺ ](i) levels were also not sensitive to intracellular acidification but were specifically sensitive to [Na ⁺ ]. Thus, KS cells differ from normal endothelial cells in the kinetics of pH(in) recovery to acid loads, and in the presence of a specific [Na ⁺ ]-sensitive intracellular Ca ²⁺ pool. These differences in ion homeostasis indicate that these cell types are not developmentally related or that alterations in ion transport are a part of the etiology of the KS lesion.