Uptake characteristics of the InsP3-sensitive and -insensitive Ca2+ pools in porcine aortic smooth-muscle cells: different Ca2+ sensitivity of the Ca2(+)-uptake mechanism.

We have investigated the Ca2(+)-uptake characteristics of the InsP3-sensitive and -insensitive non-mitochondrial Ca2+ pools in permeabilized cultured porcine aortic smooth-muscle cells. The InsP3-sensitive Ca2+ pool, which was also GTP sensitive, had a high Ca2+ affinity and was highly oxalate permeable. The InsP3-insensitive Ca2+ store, which was also GTP insensitive, had a much lower Ca2+ affinity and presented a low oxalate permeability. The loading of both pools decreased at high free [Ca2+], although these cells did not have a Ca2(+)-induced Ca2+ release mechanism. This decreased loading of the InsP3-sensitive Ca2+ pool at higher free [Ca2+] must be taken into consideration when investigating a possible Ca2(+)-inhibition of the InsP3-induced Ca2+ release. Part of the Ca2+ uptake into the InsP3-insensitive Ca2+ pool was not affected by the Ca2(+)-pump inhibitors vanadate, thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone.