Calcium signaling in endothelin- and platelet-derived growth factor-stimulated chondrocytes.
In bovine articular chondrocytes, endothelin (ET) and platelet-derived growth factor (PDGF) receptors mediate agonist-induced increases in inositol phosphate hydrolysis, cytoplasmic calcium concentration ([Ca2+]i), and mitogenesis. In most cells, ET stimulated nonoscillatory [Ca2+]i elevations with dose-dependent increases in both spike and plateau amplitudes. However, about 15% of cells showed oscillatory Ca2+ responses with a constant frequency and variable shape and duration of spiking. ET-1 and ET-2 were more potent than ET-3 in stimulating [Ca2+]i responses in inhibiting the specific binding of 125I-ET-1 and 125I-ET-3 and in promoting internalization of the receptor-ligand complex, consistent with actions through endothelin ETA receptors. Similar nonoscillatory and oscillatory patterns of Ca2+ responses were observed in PDGF-stimulated cells. In cells showing nonoscillatory Ca2+ responses to ET-1, subsequent stimulation with PDGF was frequently followed by the development of an oscillatory Ca2+ response. Nonoscillatory responses to both agonists were only slightly reduced in Ca(2+)-deficient medium, but the oscillatory responses were critically dependent on Ca2+ entry. Ca2+ spiking was not altered in the presence of the voltage-sensitive Ca2+ channel blocker, nifedipine; also, depolarization of chondrocytes by high K+ did not induce [Ca2+]i responses, confirming that voltage-sensitive calcium channels are not expressed in these cells. At high agonist concentrations, ET- but not PDGF-stimulated cells underwent rapid desensitization. Activation of ETA and PDGF receptors was associated with differential stimulation of thymidine incorporation; ET-1 induced a low-amplitude bell-shaped dose-response curve; PDGF induced a sustained sigmoidal and dose-dependent rise. These data indicate that two distinct types of Ca(2+)-mobilizing receptors initiate similar patterns of [Ca2+]i responses but have different capacities to maintain and reinitiate the Ca2+ signaling, as well as to promote mitogenesis.
Stojilkovic
S S
SS
Endocrinology and Reproduction Research Branch, National Institute of Child Health and Human Development, Bethesda, Maryland.
Vukicevic
S
S
Luyten
F P
FP
D000375:Aging / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D002358:Cartilage, Articular / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D002417:Cattle; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D003593:Cytoplasm / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D016232:Endothelins / Q000378:metabolism / Q000494:pharmacology*; D007295:Inositol Phosphates / Q000378:metabolism; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology*; D017466:Receptors, Endothelin / Q000378:metabolism; D017479:Receptors, Platelet-Derived Growth Factor / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects*; D013936:Thymidine / Q000378:metabolism
D016232:Endothelins; D007295:Inositol Phosphates; D010982:Platelet-Derived Growth Factor; D017466:Receptors, Endothelin; D017479:Receptors, Platelet-Derived Growth Factor; D002118:Calcium; D013936:Thymidine