BRADYKININ ACTIVATES R-TYPE, T-TYPE, AND L-TYPE CA2-MUSCLE CELLS( CHANNELS AND INDUCES A SUSTAINED INCREASE OF NUCLEAR CA2+ IN AORTIC VASCULAR SMOOTH)

The mechanism(s) of Ca2+ entry stimulated by bradykinin (BK) and the r eceptor subtype responsible for this effect were examined in human and rabbit aortic vascular smooth muscle cells (VSMCs). Using the whole-c ell voltage clamp technique, BK(10(-6) M) significantly (p < 0.05) inc reased both T- and L-type Ca2+ currents (I-Ca) in rabbit aortic VSMCs. Using the fura-2 total intracellular Ca2+ ([Ca](i)) measurement techn ique, BK (10(-6) M) induced a transient increase of [Ca](i) followed b y a sustained component. Pretreatment of rabbit VSMCs with sarcoplasmi c reticulum (SR) Ca2+ releaser caffeine (1-5 mM) significantly decreas ed 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) i nduced a nonsignificant sustained increase of [Ca](c) but significant (p < 0.05) sustained increase of [Ca](n) that was reversed but not pre vented by the specific B-1 receptor antagonist R126 (10(-6) M) as well as by the B-2 receptor antagonist R817 (10(-6) M). In both VSMC prepa rations, the specific B-1 agonist R211 (10(-9) to 10(-7) M) rapidly in duced a nonsignificant increase of [Ca](c) but a significant (p < 0.05 ) sustained increase of [Ca]n that was prevented but not reversed by t he B-1 selective antagonist R126 (10(-6) M). The sustained increase of [Ca](c) and [Ca](n) induced by BK and B-1 receptor agonist was blocke d by extracellular application of EGTA. These results strongly suggest that B-1 and probably B-2 receptors are functional in human and rabbi t aortic VSMCs. BK-induced transient increase of [Ca](i) is mainly due to the stimulation of T- and L-type I-Ca as well as to Ca2+ release f rom caffeine-and ryanodine-sensitive Ca2+ pools. The sustained compone nt induced by the hormone or the B-1 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.