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.
Laporte
R
R
Division of Maternal-Fetal Medicine, Department of Obstetrics and Gynecology, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA.
Laher
I
I
D000818:Animals; D002118:Calcium / Q000172:deficiency / Q000378:metabolism*; D004730:Endothelium, Vascular / Q000502:physiology*; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D009994:Osmolar Concentration; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D014661:Vasoconstriction / Q000502:physiology*; D014680:Veins / Q000378:metabolism*
D002118:Calcium