The involvement of inositol 1,4,5-trisphosphate (IP3) formation in the voltage-dependent regulation of intracellular Ca2+ concentration ([Ca2+]i) was examined in smooth muscle cells of the porcine coronary artery. Slow ramp depolarization from -90 to 0 mV induced progressive [Ca2+]i increase. The slope was reduced or increased in the presence of Cd2+ or Bay K 8644, respectively. The decrease in [Ca2+]i via the membrane hyperpolarization induced by K+ channel openers (levcromakalim and Evans blue) under current-clamp was identical to that under voltage-clamp. The step hyperpolarization from -40 to -80 mV reduced [Ca2+]i uniformly over whole-cell area with a time constant of ~10 s. The [Ca2+]i at either potential was unaffected by heparin, an inhibitor of IP3 receptors. Alternatively, [Ca2+]i rapidly increased in the peripheral regions by depolarization from -80 to 0 mV and stayed at that level (~400 nM) during a 60-s pulse. When the pipette solution contained IP3 pathway blockers (heparin, 2-aminoethoxydiphenyl borate, xestospongin C, or U73122), the peak [Ca2+]i was unchanged but the sustained [Ca2+]i was gradually reduced by ~250 nM within 30 s. In the presence of Cd2+, a long depolarization period slightly increased the [Ca2+]i, which was lower than that in the presence of heparin alone. In coronary arterial myocytes, the sustained increase in the [Ca2+]i during depolarization was partly due to the Ca2+ release mediated by the enhanced formation of IP3. The initial [Ca2+]i elevation triggered by the Ca2+ influx though voltage-dependent Ca2+ channels may be predominantly responsible for the activation of phospholipase C for IP3 formation.
- ion channel regulation
- phospholipase C
- potassium channels
- vascular smooth muscle
- voltage-gated calcium channels
- Received March 12, 2012.
- Revision received May 12, 2012.
- Accepted May 14, 2012.
- The American Society for Pharmacology and Experimental Therapeutics