Intracellular Ca2+ silences L-type Ca2+ channels in mesenteric veins: mechanism of venous smooth muscle resistance to calcium channel blockers.

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 alpha(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.