SPONTANEOUSLY HYPERTENSIVE RATS AND PLATELET CA2-ATPASES - SPECIFIC UP-REGULATION OF THE 97 KDA ISOFORM()

The use of platelets instead of smooth muscle cells (SMC) to study the abnormal Ca2+ handling found in hypertension was investigated using s pontaneously hypertensive rats (SHR). We studied the regulation of pla telet Ca2+-ATPases, as we have recently demonstrated that human platel ets, like SMC, contain the Ca2+-ATPase isoform termed SERCA2-b (sarco- endoplasmic reticulum Ca2+-ATPase). In mixed membranes isolated from p latelets of normotensive Wistar-Kyoto (WKY) rats and SHR, total Ca2+-A TPase activity was found to be 43 % higher in SHR than in WKY rats. By the use of autophosphorylation of rat platelet Ca2+-ATPases with [gam ma-P-32]ATP, followed by SDS/PAGE and Western blotting, we found that rat platelets express two distinct Ca2+-ATPases: a 100 kDa isoform, re cognized by a SERCA2-b-specific anti-peptide antibody, and a 97 kDa is oform, specifically recognized by a polyclonal anti-SERCA antibody. Co mparative analysis of platelet membrane Ca2+-ATPases from WKY rats and SHR demonstrated that the expression of the SERCA2-b isoform did not change significantly (128 +/- 22%), whereas that of the 97 kDa isoform reached 300 +/- 35% in SHR when compared with WKY rats. We concluded that the upregulation of total platelet Ca2+-ATPases in SHR is not due to the 100 kDa SERCA2+- isoform found in SMC, but is specific to the 97 kDa Ca2+-ATPase isoform which is not present in SMC. Therefore plat elets should be used with extreme caution as a surrogate model of vasc ular smooth muscle Ca2+ homoeostasis.