Ca mobilization from the intracellular Ca store in spontaneously hypertensive rats

To evaluate the size of the intracellular Ca store in spontaneously hypertensive rats (SHR), caffeine-induced contractions were investigated in mesenteric artery preparation. Twelve-week-old SHR and age-matched Wistar-Kyoto (WKY) rats were used. The isolated mesenteric artery was prepared by the method of McGregor. Caffeine (2 mg)- and norepinephrine (2 micrograms)-induced pressor responses were measured in both normal (Ca, 2 mM) and Ca-free solution (1 mM EGTA solution). In the presence of Ca, pressor responses to exogenous norepinephrine in SHR were higher than those in WKY rats. In Ca-free solution, pressor responses to norepinephrine were markedly reduced in SHR compared to WKY rats and caffeine-induced contractions in SHR were lower than those in WKY rats. These results suggest that pressor responses to norepinephrine in SHR may be more dependent on extracellular Ca than in WKY rats and the caffeine-sensitive intracellular Ca store might be less in SHR.