VASOPRESSIN STIMULATION OF CA2+ MOBILIZATION, 2 BIVALENT CATION ENTRY PATHWAYS AND CA2+ EFFLUX IN A7R5 RAT SMOOTH-MUSCLE CELLS

1. Arg(8)-vasopressin (AVP)-regulated Ca2+ transport pathways were investigated in fura-2-loaded A7r5 cells using both single cell and population measurements. 2. AVP evokes an initial concentration-dependent rise in cytosolic free Ca2+ concentration ([Ca2+](i)) to a peak which is independent of extracellular Ca2+, and a sustained Ca2+ signal that results from a balance between stimulation of Ca2+ entry and efflux. 3. Depletion of intracellular Ca2+ stores with thapsigargin, ionomycin, or prior treatment with AVP in Ca2+-free medium activates 'capacitative' entry of Ca2+, Ba2+ or Mn2+. Capacitative Mn2+ entry is inhibited by refilling stores with Ca2+; neither Sr2+ nor Ba2+ substitute for Ca2+ to give this effect. 4. In cells with empty stores, AVP stimulates further bivalent cation entry, and the effect persists when extracellular Na+ is replaced by N-methyl-D-glucamine or under depolarizing conditions (extracellular KCl concentration ([KCl](o)), 135 mM). This effect of AVP is not therefore merely a consequence of AVP causing membrane hyperpolarization or stimulation of Na+-Ca2+ exchange, but results from opening of a bivalent cation influx pathway. 5. Several lines of evidence indicate that AVP-stimulated bivalent cation entry is not a consequence of more complete emptying of the intracellular stores and consequent further activation of the capacitative pathway. BVP stimulates Ba2+ entry when the intracellular Ca2+ stores have been both emptied by ionomycin and prevented from refilling by thapsigargin. Mn2+ permeates the capacitative pathway, but AVP does not further increase Mn2+ entry, confirming that AVP does not further activate the capacitative pathway and that the two pathways differ in their permeability to Mn2+. When the extracellular [Sr2+] is low, empty stores do not stimulate detectable Sr2+ entry but addition of AVP causes substantial Sr2+ entry. 6. A decrease in [Ca2+](i) occurs when 50 nM AVP is added during a sustained elevation of [Ca2+](i) evoked by thapsigargin. Since AVP does not inhibit the capacitative pathway, this result suggests that AVP stimulates Ca2+ extrusion. 7. We conclude that stimulation of Ca2+ mobilization, two modes of bivalent cation entry, and Ca2+ efflux all contribute to the complex concentration-dependent effects of AVP in A7r5 smooth muscle cells.