STUDIES ON CAPACITATIVE CALCIUM-ENTRY IN VASCULAR SMOOTH-MUSCLE CELLSBY MEASURING CA-45(2+) INFLUX

Capacitative calcium entry was studied in the A7r5 vascular smooth mus cle cell line by measuring Ca-45(2+) influx. Entry was induced by depl etion of the Ca2+ pools by either the receptor agonist [Arg](8)vasopre ssin (AVP) or the SR-Ca2+-ATPase inhibitor thapsigargin (TG). TG showe d a higher efficacy for calcium influx than AVP. This is probably due to a larger Ca2+ release from the pools induced by TG compared to AVP and the irreversible inhibition of the SR-Ca2+-ATPase by TG causing in flux to persist for a longer period of time. At maximally effective co ncentrations signals induced by AVP and TG were synergistic in the abs ence but not in the presence of the intracellular calcium chelator, 1, 2-bis(2-aminophenoxy) ethane-N,N,N,N'-tetraacetic acid (BAPTA). Depola risation with 55 mM KCl completely inhibited Ca-45(2+) influx induced by TG but only slightly the one induced by AVP, both effects being les s pronounced in the presence of BAPTA. [Ca2+](c) signals induced by AV P and TG were both inhibited by depolarisation. In conclusion, althoug h our results show differences between AVP- and TG-induced Ca2+ influx , they can be explained by their different mechanism of action and are in accordance with an activation of the same capacitative entry pathw ay by both agents.