Abstract

The purpose of this study was to test whether the elevated intracellular Ca⁺⁺ level ([Ca⁺⁺]_i) resulting from store-operated Ca⁺⁺ entry was associated with vascular smooth muscle contraction. Cyclopiazonic acid (CPA), a selective inhibitor of sarcoplasmic reticulum Ca⁺⁺-ATPase, concentration-dependently (1–10 μM) elevated [Ca⁺⁺]_i in rat aorta, as indicated by an increase in the fura-2 340/380 ratio. Simultaneous measurement of contraction demonstrated that 1 and 10 μM CPA induced insignificant and variable amounts of contraction, respectively. Verapamil (10 μM) had relatively little effect on the 1 and 10 μM CPA-elevated [Ca⁺⁺]_i. In contrast, Ni⁺⁺ (0.1 mM), in the presence of verapamil, abolished the 1 μM CPA-elevated [Ca⁺⁺]_i. Ni⁺⁺ (0.1 mM) also partially decreased the 10 μM CPA-elevated [Ca⁺⁺]_i and, furthermore, abolished the associated contraction. A higher Ni⁺⁺ concentration (1 mM) abolished the 10 μM CPA-elevated [Ca⁺⁺]_ithat remained after verapamil and 0.1 mM Ni⁺⁺. Phorbol dibutyrate (10 nM), a protein kinase C activator, potentiated contractions to 1 and 10 μM CPA in the presence of verapamil. Ni⁺⁺ (0.1 mM) abolished the enhanced contractions, and decreased the elevated [Ca⁺⁺]_i. These results suggest that 1) elevated [Ca⁺⁺]_i due to store-operated Ca⁺⁺ entry is dissociated from contraction; 2) the elevated [Ca⁺⁺]_i is restricted to at least two noncontractile compartments that can be differentiated by their relative sensitivities to blockade by low (0.1 mM) and higher (1 mM) Ni⁺⁺ concentrations, and 3) [Ca⁺⁺]_i elevation within the compartment sensitive to blockade by 0.1 mM Ni⁺⁺ can be coupled to contraction via protein kinase C activation.