The properties of intracellular Ca2+ stores of intact- and of saponin-skinned A7r5 (an established cell line from embryonic rat aorta) smooth muscle cells were studied by measuring 45Ca2+ and 54Mn2+ fluxes. 2. Application of 5 microM-vasopressin to intact cells increased the fractional loss of 45Ca2+ in Ca2(+)-free solution by a factor of 5.2. This effect was not influenced by a pre-incubation with 10 microM-ryanodine. Caffeine (25 mM) did not stimulate the fractional loss of 45Ca2+ from intact cells. 3. In skinned cells 10 microM-IP3 (inositol 1,4,5-trisphosphate) and 5 microM-A23187 (a calcium ionophore) released the same amount of 45Ca2+. This release did not require GTP and was not affected by a pre-incubation with 10 microM-ryanodine. Caffeine (25 mM) did not release stored Ca2+. 4. NaF (1 mM) plus 10 microM-AlCl3 inhibited by 72% the 45Ca2+ uptake by the IP3-sensitive store of skinned cells at 0.15 microM-Ca2+. Cyclic AMP-dependent protein kinase did not stimulate this ATP-dependent 45Ca2+ uptake, nor could the presence of phospholamban be demonstrated immunologically. 5. The 45Ca2+ uptake by cells which had been depleted of Ca2+ with 5 microM-vasopressin was 69% higher than the uptake obtained without such proceeding depletion. This enhanced 45Ca2+ uptake did not occur through voltage-operated Ca2+ channels, because blockade of these channels with verapamil, or depolarization of the plasma membrane by increasing [K+] from 5.9 to 59 mM in the presence of verapamil, did not modify this uptake. 6. A similar increase of the 54Mn2+ uptake occurred in intact cells with a depleted Ca2+ store. If, however, the cells were first skinned and subsequently exposed to 54Mn2+, the ATP-dependent 54Mn2+ uptake amounted to less than 6% of the ATP-dependent 45Ca2+ uptake. 7. If intact cells were first exposed to a 45Ca2(+)- or 54Mn2(+)-containing solution, and subsequently skinned in a non-radioactive intracellular solution, the addition of 10 microM-A23187 to these cells released stored Ca2+ or Mn2+. The amount of released Ca2+ was only slightly larger than the amount of released Mn2+. If the intracellular store was depleted before loading, the amount of Ca2+ or Mn2+ released by the ionophore increased by 68 and 28%, respectively. 8. It is concluded that A7r5 smooth muscle cells do not express a Ca2(+)-induced Ca2+ release mechanism, but do contain an IP3-induced Ca2+ release mechanism which can release approximately all intracellularly accumulated 45Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)
