Bradykinin activates R-, T-, and L-type Ca<SUP>2+</SUP> channels and induces a sustained increase of nuclear Ca<SUP>2+</SUP> in aortic vascular smooth muscle cells

Abstract

The mechanism(s) fo Ca2+ entry stimulated by bradykinin (BK) and the receptor subtype responsible for this effect were examined in human and rabbit aortic vascular smooth muscle cells (VSMCs). Using the whole-cell voltage clamp technique, BK (10(-6)M) significantly (p < 0.05) increased both T- and L-type Ca2+ currents (ICa) in rabbit aortic VSMCs. Using the fura-2 total intracellular Ca2+ ([Ca]i) measurement technique, BK (10(-6) M) induced a transient increase of [Ca]i followed by a sustained component. Pretreatment of rabbit VSMCs with sarcoplasmic reticulum (SR) Ca2+ releaser caffeine (1-5 mM) significantly decreased the BK-induced transient increase of [Ca]i without affecting the sustained component induced by this hormone. This sustained phase was blocked by extracellular application of the Ca2+ chelator EGTA. Using the fluo-3 confocal microscopy Ca2+ measurement technique to localize cytosolic ([Ca]c) and nuclear ([Ca]n) free Ca2+ distribution, the resting sustained concentration of Ca2+ in the cytoplasm of rabbit and human aortic VSMCs was less than that in the nucleus. BK (10(-7) M) induced a nonsignificant sustained increase of [Ca]c but significant (p < 0.05) sustained increase of [Ca]n that was reversed but not prevented by the specific B1 receptor antagonist R126 (10(-6) M) as well as by the B2 receptor antagonist R817 (10(-6) M). In both VSMC preparations, the specific B1 agonist R211 (10(-9) to 10(-7) M) rapidly induced a nonsignificant increase of [Ca]c but a significant (p < 0.05) sustained increase of [Ca]n that was prevented but not reversed by the B1 selective antagonist R126 (10(-6) M). The sustained increase of [Ca]c and [Ca]n induced by BK and B1 receptor agonist was blocked by extracellular application of EGTA. These results strongly suggest that B1 and probably B2 receptors are functional in human and rabbit aortic VSMCs. BK-induced transient increase of [Ca]i is mainly due to the stimulation of T- and L-type Ica as well as to Ca2+ release from caffeine- and ryanodine-sensitive Ca2+ pools. The sustained component induced by the hormone or the B1 agonist is mainly nuclear and is due to the stimulation of Ca2+ influx through the R-type Ca2+ channels that are present at the sarcolemma and the nuclear membranes.