Intracellular Ca 2+ Silences L-Type Ca 2+ Channels in Mesenteric Veins

Abstract

Rationale : Calcium channel blockers (CCBs) exert their antihypertensive effect by reducing cardiac afterload but not preload, suggesting that Ca 2+ influx through L-type Ca 2+ channels (LTCC) mediates arterial but not venous tone. Objective : The object of this study was to resolve the mechanism of venous resistance to CCBs. Methods and Results : We compared the sensitivity of depolarization (KCl)-induced constriction of rat small mesenteric arteries (MAs) and veins (MVs) to the dilator effect of CCBs. Initial findings confirmed that nifedipine progressively dilated depolarization-induced constrictions in MAs but not MVs. However, Western blots showed a similar expression of the α 1C pore-forming subunit of the LTCC in both vessels. Patch-clamp studies revealed a similar density of whole-cell Ca 2+ channel current between single smooth muscle cells (SMCs) of MAs and MVs. Based on these findings, we hypothesized that LTCCs are expressed but “silenced” by intracellular Ca 2+ in venous SMCs. After depletion of intracellular Ca 2+ stores by the SERCA pump inhibitor thapsigargin, depolarization-induced constrictions in MVs were blocked 80% by nifedipine suggesting restoration of Ca 2+ influx through LTCCs. Similarly, KCl-induced constrictions were sensitive to block by nifedipine after depletion of intracellular Ca 2+ stores by caffeine, ryanodine, or 2-aminoethoxydiphenyl borate. Cell-attached patch recordings of unitary LTCC currents confirmed rare channel openings during depolarization of venous compared to arterial SMCs, but chelating intracellular Ca 2+ significantly increased the open-state probability of venous LTCCs. Conclusions : We report that intracellular Ca 2+ inactivates LTCCs in venous SMCs to confer venous resistance to CCB-induced dilation, a fundamental drug property that was previously unexplained.