Regulation of Ca2+ entry and Ca2+ efflux in smooth muscle cell lines

• Main Author: Broad, L. M.
• Published: University of Cambridge 1997
• Subjects: 572
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596924

In many cells, agonists that stimulate formation of inositol 1,4,5-trisphosphate (IP<SUB>3</SUB>) mobilise intracellular Ca<SUP>2+</SUP> stores and activate several Ca<SUP>2+</SUP> transport pathways in the plasma membrane. This thesis addresses the characteristics and regulation of these hormone-activated Ca<SUP>2+</SUP> transport pathways in Fura 2-loaded vascular smooth muscle cell lines. The effect of cellular differentiation on the capacitative Ca<SUP>2+</SUP> entry pathway in BC<SUB>3</SUB>Hl cells was studied using thapsigargin to empty the intracellular Ca<SUP>2+</SUP> stores. Undifferentiated BC<SUB>3</SUB>H1 cells exhibited Ca<SUP>2+</SUP> mobilisation in response to IP<SUB>3</SUB> only, whereas differentiated cells responded to IP<SUB>3</SUB>, or to caffeine and ryanodine. Although both differentiated and undifferentiated cells expressed a capacitative Ca<SUP>2+</SUP> entry pathway, the bivalent cation selectivity of the pathways differed. In undifferentiated cells, large capacitative Ca<SUP>2+</SUP> entry signals were accompanied by relatively small Mn<SUP>2+</SUP> and Ba<SUP>2+</SUP> entry signals; the converse was true of differentiated cells. The relative increase in Mn<SUP>2+</SUP> and Ba<SUP>2+</SUP> permeability in differentiated cells was not a consequence of more active Ca<SUP>2+</SUP> extrusion. The change in behaviour of the pathway after differentiation may result either from expression of a different capacitative pathway or from modification of the permeation properties of a single pathway. Stimulation of a7r5 cells with arginine-vasopressin (AVP) activated both a capacitative and non-capacitative Ca<SUP>2+</SUP> entry pathway in the plasma membrane, only the latter was permeable to Sr<SUP>2+</SUP>. AVP also stimulated Ca<SUP>2+</SUP> removal from the cytoplasm. The ability of other receptors that stimulate phosphoinositidase C to activate the non-capacitative pathway, its susceptibility to inhibition by a PIC inhibitor (U73122), and the similar sensitivities of Ca<SUP>2+</SUP> release and Sr<SUP>2+</SUP> entry to AVP suggested a close coupling between stimulation of PIC and activation of this pathway. This work has revealed that Ca<SUP>2+</SUP> mobilisation, multiple capacitative and non-capacitative Ca<SUP>2+</SUP> entry pathways and Ca<SUP>2+</SUP> efflux all contribute to the complexity of receptor-activated Ca<SUP>2+</SUP> signalling.