Regional and mechanistic differences in platelet-derived growth factor-isoform-induced alterations in cytosolic free calcium in porcine vascular smooth muscle cells

Three specific platelet-derived growth factor (PDGF) isoforms are thought to bind with differing affinities to two distinct PDGF receptors which undergo activation following dimerization. Recent evidence has been presented that marked differences exist between the ability of PDGF-AA versus PDGF-AB and PDGF-BB to stimulate alterations in second messengers in cultures of vascular smooth muscle cells (VSMC), a result which was thought to be due to low numbers of the A-type receptor in this cell type (Sachinidis, A., Locker, R., Vetter, W., Tatje, D., and Hoppe, J. (1990) J. Biol. Chem. 265, 10238-10243, 1990). In particular, PDGF-BB and PDGF-AB but not PDGF-AA could elicit alterations in cytosolic free calcium (Ca²⁺_i). However, because these studies were performed on large cell populations using biochemical assays of PDGF activity, a minor PDGF-AA-Ca²⁺-responsive population of cells might go undetected. To test this possibility, VSMC were isolated from either thoracic or abdominal pig aorta, and alterations in Ca²⁺_i were monitored using Multiparameter Digitized Video Microscopy following stimulation with PDGF isoforms alone, or either before or after exposure of VSMC to 5 mM EGTA. PDGF-AA-responsive cells were found to exist only in cultures of thoracic VSMC, caused oscillations in Ca²⁺_i, represented 20% of the PDGF-BB-responsive cells, and were subsequently responsive to PDGF-BB. PDGF-BB elicited monophasic alterations in Ca²⁺_i in both thoracic and abdominal VSMC. Prior addition of EGTA inhibited PDGF-AA but not PDGF-BB-induced alterations in Ca²⁺_i. Addition of EGTA during PDGF-AA-induced Ca²⁺_i oscillations inhibited subsequent oscillations in Ca²⁺_i, while addition of EGTA at the peak of the PDGF-BB Ca²⁺ response resulted in a more rapid return of Ca²⁺_i to prestimulation levels. These data suggest that regional differences in the distribution of PDGF-A- and B-type receptor exists in vivo, and that activation of the A-and B-type PDGF receptors results in distinct alterations in Ca²⁺_i.