Fransen Paul, Van Hove Cor E, van Langen Johanna, Schrijvers Dorien M, Martinet Wim, De Meyer Guido R Y, Bult Hidde
Laboratory of Physiopharmacology, University of Antwerp, Universiteitsplein 1 Building T, 2.18, Wilrijk B-2610, Belgium.
BMC Physiol. 2012 Sep 3;12:9. doi: 10.1186/1472-6793-12-9.
Electrophysiological studies of L-type Ca2+ channels in isolated vascular smooth muscle cells revealed that depolarization of these cells evoked a transient and a time-independent Ca2+ current. The sustained, non-inactivating current occurred at voltages where voltage-dependent activation and inactivation overlapped (voltage window) and its contribution to basal tone or active tension in larger multicellular blood vessel preparations is unknown at present. This study investigated whether window Ca2+ influx affects isometric contraction of multicellular C57Bl6 mouse aortic segments.
Intracellular Ca2+ (Cai2+, Fura-2), membrane potential and isometric force were measured in aortic segments, which were clamped at fixed membrane potentials by increasing extracellular K+ concentrations. K+ above 20 mM evoked biphasic contractions, which were not affected by inhibition of IP3- or Ca2+ induced Ca2+ release with 2-aminoethoxydiphenyl borate or ryanodine, respectively, ruling out the contribution of intracellular Ca2+ release. The fast force component paralleled Cai2+ increase, but the slow contraction coincided with Cai2+ decrease. In the absence of extracellular Ca2+, basal tension and Cai2+ declined, and depolarization failed to evoke Cai2+ signals or contraction. Subsequent re-introduction of external Ca2+ elicited only slow contractions, which were now matched by Cai2+ increase. After Cai2+ attained steady-state, isometric force kept increasing due to Ca2+- sensitization of the contractile elements. The slow force responses displayed a bell-shaped voltage-dependence, were suppressed by hyperpolarization with levcromakalim, and enhanced by an agonist of L-type Ca2+ channels (BAY K8644).
The isometric response of mouse aortic segments to depolarization consists of a fast, transient contraction paralleled by a transient Ca2+ influx via Ca2+ channels which completely inactivate. Ca2+ channels, which did not completely inactivate during the depolarization, initiated a second, sustained phase of contraction, which was matched by a sustained non-inactivating window Ca2+ influx. Together with sensitization, this window L-type Ca2+ influx is a major determinant of basal and active tension of mouse aortic smooth muscle.
对分离的血管平滑肌细胞中L型钙通道的电生理研究表明，这些细胞的去极化会诱发一个瞬时的和一个与时间无关的钙电流。持续的、非失活电流出现在电压依赖性激活和失活重叠的电压处（电压窗口），目前尚不清楚其对较大的多细胞血管标本中基础张力或主动张力的贡献。本研究调查了窗口钙内流是否影响多细胞C57Bl6小鼠主动脉段的等长收缩。
在主动脉段中测量细胞内钙（Cai2+，Fura-2）、膜电位和等长力，通过增加细胞外钾浓度将其钳制在固定膜电位。高于20 mM的钾诱发双相收缩，分别用2-氨基乙氧基二苯硼酸或ryanodine抑制IP3或钙诱导的钙释放均不影响该收缩，排除了细胞内钙释放的作用。快速力成分与Cai2+增加平行，但缓慢收缩与Cai2+减少同时发生。在无细胞外钙的情况下，基础张力和Cai2+下降，去极化未能诱发Cai2+信号或收缩。随后重新引入细胞外钙仅诱发缓慢收缩，此时与Cai2+增加相匹配。Cai2+达到稳态后，由于收缩元件的钙敏化，等长力持续增加。缓慢的力反应呈现钟形电压依赖性，可被左卡尼汀超极化抑制，并被L型钙通道激动剂（BAY K8644）增强。
小鼠主动脉段对去极化的等长反应包括一个快速的、瞬时的收缩，与通过完全失活的钙通道的瞬时钙内流平行。在去极化过程中未完全失活的钙通道引发了第二个持续的收缩阶段，与持续的非失活窗口钙内流相匹配。与敏化一起，这种窗口L型钙内流是小鼠主动脉平滑肌基础张力和主动张力的主要决定因素。
