Agonist-induced [Ca2+]i Waves and Ca(2+)-induced Ca2+ Release in Mammalian Vascular Smooth Muscle Cells

Focal application of vasopressin to cultured vascular smooth muscle cells (A7r5 cells) elicits first a localized increase of intracellular Ca2+ concentration ([Ca2+]i) and then a wave of elevated [Ca2+]i that propagates at constant velocity throughout the cell. The cellular mechanisms of such complex spatiotemporal patterns of [Ca2+]i are of interest because they are involved fundamentally in cellular signal transduction in many types of cells. Vasopressin evoked a [Ca2+]i transient even in the absence of extracellular Ca2+, and intracellular perfusion with heparin completely blocked the response to vasopressin stimulation. Therefore the initial response to vasopressin reflects release of Ca2+ from an intracellular myo-inositol-1,4,5-trisphosphate (IP3)-sensitive Ca2+ store. We tested four hypotheses on how a localized increase in [Ca2+]i propagates as a [Ca2+]i wave throughout the entire cell: the hypotheses distinguished 1) whether IP3 or Ca2+ is the primary intracellular messenger that diffuses, and 2) whether positive feedback on the release of intracellular Ca2+ (Ca2+i) is involved (further release of Ca2+ through activation of phospholipase C by Ca2+ and increased production of IP3 or by Ca(2+)-induced Ca2+ release). The results of various experimental interventions, which included probing Ca2+i stores (heparin, caffeine, and ryanodine), were compared with predictions from mathematical models for intracellular diffusion, release, and uptake of Ca2+. We conclude that in A7r5 smooth muscle cells, which have been stimulated focally with vasopressin, Ca2+ is released initially by IP3. The localized increase in [Ca2+]i then propagates throughout the cell as a [Ca2+]i wave. Ca2+ activates its own release, through Ca(2+)-induced release of Ca2+, by diffusing to distant Ca(2+)-release sites.