Superficial sarcoplasmic reticulum calcium buffering of resting, voltage-dependent Ca++ influx in rat femoral arterial smooth muscle.
To determine the Ca(+2)-buffering function of sarcoplasmic reticulum (SR) in the resting state of arterial smooth muscle, the effects of cyclopiazonic acid (CPA) and thapsigargin, which inhibit the Ca(+20-ATPase of SR, on tension and cytosolic Ca+2 level ¿[Ca+2]¿ were examined in endothelium-denuded strips of rat femoral artery. In resting strips preloaded with fura-PE3, the addition of CPA (10 microM) or thapsigargin (100 nM) caused an elevation of [Ca+2]i and a contraction. These responses were inhibited by blockers of L-type voltage-dependent Ca+2 channels (verapamil, nifedipine and diltiazem). The inhibition by verapamil was much greater for contraction than for [Ca+2]i Verapamil itself decreased the resting [Ca+2]i without affecting the resting tone. In a Ca(+2)-free solution, CPA caused a small elevation of [Ca+2]i that was not accompanied by a contraction. In the presence of CPA or thapsigargin, the effects of caffeine (20 mM) on [Ca+2]i and tension were greatly inhibited. In strips where the SR Ca+2 was depleted by the addition of caffeine and a Ca(+2)-free solution, the Ca+2 influx from the extracellular space was completely taken up into the SR. This process was inhibited by CPA or thapsigargin, which resulted in a contraction. This contraction was also inhibited by verapamil. However, the basal Ca+2 influx measured by a 5-min incubation of the artery with 45Ca+2 was not increased by CPA or thapsigargin. The net Ca+2 entry measured in a 30-min incubation of the artery with 45Ca+2 was decreased by CPA or thapsigargin. The basal 45Ca+2 efflux from the artery was increased during the addition of CPA. These results suggested that the basal Ca+2 influx was completely buffered by Ca+2 uptake into the SR in the resting state of rat femoral artery and therefore the inhibition of SR Ca+2 uptake by CPA or thapsigargin caused an elevation of [Ca+2]i and a contraction. During this process, the basal Ca+2 influx via L-type voltage-dependent Ca+2 channels mainly contributed to the contraction. The present study also showed the existence of a relatively large compartment of cytosolic Ca+2 that does not contribute to contraction during the addition of CPA or thapsigargin.
Nomura
Y
Y
Department of Pharmacology, Nagoya City University Medical School, Japan.
Asano
M
M
Ito
K
K
Uyama
Y
Y
Imaizumi
Y
Y
Watanabe
M
M
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D005263:Femoral Artery / Q000378:metabolism; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology; D014661:Vasoconstriction / Q000187:drug effects
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D007211:Indoles; D019284:Thapsigargin; D002118:Calcium; C000543:cyclopiazonic acid