Contribution of sarcoplasmic reticulum Ca2+ to the activation of Ca2+ -activated K+ channels in the resting state of arteries from spontaneously hypertensive rats.
OBJECTIVE
Localized release of Ca2+ from the sarcoplasmic reticulum (SR) toward the plasmalemma, sometimes visualized as Ca2+ sparks, can activate Ca2+-activated K+ (KCa) channels. We have already reported that the addition of charybdotoxin (ChTX), a blocker of KCa channels, to the resting state of arteries from spontaneously hypertensive rats (SHR) caused a powerful contraction, suggesting that KCa channels were active in the resting state. This study aimed to determine whether the Ca2+ responsible for activity of KCa channels was derived from SR.


METHODS
Possible mechanisms underlying the ChTX-induced contractions were examined in endothelium-denuded strips of femoral, mesenteric, small mesenteric and carotid arteries from 13-week-old SHR and normotensive Wistar-Kyoto (WKY) rats by using selective inhibitors of the Ca2+ spark process.


RESULTS
ChTX (100 nmol/l) induced a contraction in the SHR arteries. The ChTX-induced contractions were increased by a moderate membrane depolarization by 15.9 mmol/l K+ and were abolished by nifedipine (100 nmol/l). When SR Ca2+ was depleted by treatment of the strips with ryanodine (10 mumol/l) plus caffeine (20 mmol/l) or with thapsigargin (100 nmol/l), the ChTX-induced contraction was decreased in femoral, mesenteric and small mesenteric arteries and was almost abolished in the carotid artery. A similar phenomenon can be observed in arteries from WKY rats after a moderate membrane depolarization. In both SHR and WKY rats, SR Ca2+-dependent ChTX-induced contraction always represents 20-30% of the maximal K+-induced contraction.


CONCLUSIONS
We conclude that activation of KCa channels depended upon influx of Ca2+ through L-type Ca2+ channels and release of Ca2+ from the SR, suggesting that recycling of entering Ca2+ from the superficial SR toward the plasmalemma sufficiently elevated Ca2+ near these channels to activate them.
Asano
Masahisa
M
Department of Pharmacology, Nagoya City University Medical School, Nagoya, Japan. masano@med-nagoya-cu.ac.jp
Nomura
Yukiko
Y
D000818:Animals; D001158:Arteries / Q000187:drug effects / Q000503:physiopathology*; D002118:Calcium / Q000502:physiology*; D002339:Carotid Arteries / Q000187:drug effects / Q000503:physiopathology; D018999:Charybdotoxin / Q000494:pharmacology; D006973:Hypertension / Q000503:physiopathology*; D066298:In Vitro Techniques; D008638:Mesenteric Arteries / Q000187:drug effects / Q000503:physiopathology; D010455:Peptides / Q000494:pharmacology; D024681:Potassium Channels, Calcium-Activated / Q000378:metabolism*; D051381:Rats; D011918:Rats, Inbred SHR / Q000502:physiology*; D011921:Rats, Inbred WKY; D012016:Reference Values; D012146:Rest; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology; D014661:Vasoconstriction / Q000187:drug effects
D010455:Peptides; D024681:Potassium Channels, Calcium-Activated; D018999:Charybdotoxin; D012433:Ryanodine; D019284:Thapsigargin; C064719:iberiotoxin; D002118:Calcium