Sarcoplasmic reticulum and endothelium independently regulate venous smooth muscle [Ca(2+)](i) and contraction

In rings of rabbit facial vein (RFV), depletion of sarcoplasmic reticulum (SR) Ca(2+) by caffeine abolished the subsequent isometric contraction to 25 mM K(+) physiological salt solution (25K-PSS). However, the associated steady-state increase of smooth muscle intracellular free Ca(2+) concentration ([Ca(2+)](i)), measured using fura PE3 and cuvette photometry, was not altered. Treatment with the specific SR Ca(2+) pump inhibitor cyclopiazonic acid (30 microM) after caffeine-induced SR Ca(2+) depletion restored and greatly augmented the 25K-PSS-induced contraction. This suggests that SR Ca(2+) depletion leads to a dissociation of K(+)-induced [Ca(2+)](i) increase from contraction that was dependent on Ca(2+) pump-mediated SR Ca(2+) uptake. Endothelium removal augmented the 25K-PSS-induced [Ca(2+)](i) increase after caffeine-induced SR Ca(2+) depletion. However, this was associated with only a small and transient contraction. Exposure of endothelium-denuded RFV to cyclopiazonic acid after caffeine-induced SR Ca(2+) depletion further amplified the 25K-PSS-induced [Ca(2+)](i) increase, which was associated with a large and sustained contraction. However, the latter [Ca(2+)](i) increase was still higher than in endothelium-intact RFV. This suggests that the endothelium dampens the [Ca(2+)](i) rise associated with K(+)-induced Ca(2+) influx, but independently of Ca(2+) pump-mediated SR Ca(2+) uptake.