COUPLING OF STORE-OPERATED CA++ ENTRY TO CONTRACTION IN RAT AORTA

The purpose of this study was to test whether the elevated intracellul ar Ca++ level ([Ca++](i)) resulting from store-operated Ca++ entry was associated with vascular smooth muscle contraction. Cyclopiazonic aci d (CPA), a selective inhibitor of sarcoplasmic reticulum Ca++-ATPase, concentration-dependently (1-10 mu M) elevated [Ca++], in rat aorta, a s indicated by an increase in the fura-2 340/380 ratio. Simultaneous m easurement of contraction demonstrated that 1 and 10 mu M CPA induced insignificant and variable amounts of contraction, respectively. Verap amil (10 mu M) had relatively little effect on the 1 and 10 mu M CPA-e levated [Ca++](i). In contrast, Ni++ (0.1 mM), in the presence of vera pamil, abolished the 1 mu M CPA-elevated [Ca++](i). Ni++ (0.1 mM) also partially decreased the 10 mu M CPA-elevated [Ca++](i) and, furthermo re, abolished the associated contraction. A higher Ni++ concentration (1 mM) abolished the 10 mu M CPA-elevated [Ca++](i) that remained afte r verapamil and 0.1 mM Ni++. Phorbol dibutyrate (10 nM), a protein kin ase C activator, potentiated contractions to 1 and 10 mu M CPA in the presence of verapamil. Ni++ (0.1 mM) abolished the enhanced contractio ns, and decreased the elevated [Ca++](i). These results suggest that 1 ) elevated [Ca++](i) due to store-operated Ca++ entry is dissociated f rom contraction; 2) the elevated [Ca++](i) is restricted to at least t wo noncontractile compartments that can be differentiated by their rel ative sensitivities to blockade by low (0.1 mM) and higher(1 mM) Ni co ncentrations, and 3) [Ca++](i) elevation within the compartment sensit ive to blockade by 0.1 mM Ni++ can be coupled to contraction via prote in kinase C activation.