Abstract
Capacitative calcium entry was studied in the A7r5 vascular smooth muscle cell line by measuring 45Ca2+ influx. Entry was induced by depletion of the Ca2+ pools by either the receptor agonist [Arg]8vasopressin (AVP) or the SR-Ca2+-ATPase inhibitor thapsigargin (TG). TG showed 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 influx to persist for a longer period of time. At maximally effective concentrations signals induced by AVP and TG were synergistic in the absence but not in the presence of the intracellular calcium chelator, 1,2-bis(2-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA). Depolarisation with 55 mM KCl completely inhibited 45Ca2+ influx induced by TG but only slightly the one induced by AVP, both effects being less pronounced in the presence of BAPTA. [Ca2+]c signals induced by AVP and TG were both inhibited by depolarisation.In conclusion, although 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 pathway by both agents.
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