VASOPRESSIN STIMULATES CA2-MUSCLE CELLS VIA ACTIVATION OF PHOSPHOLIPASE A(2)( SPIKING ACTIVITY IN A7R5 VASCULAR SMOOTH)

[Arg(8)]-vasopressin (AVP) is both a potent vasoconstrictor and a mito gen for vascular smooth muscle cells. AVP binds to a single class of r eceptors (V(1)a) in the A7r5 rat aortic smooth muscle cell line (K-d a pproximate to-2 nmol/L). Stimulation of these cells with AVP results i n an increase in cytoplasmic free Ca2+ concentration ([Ca2+](i)) by re leasing intracellular Ca2+ stores and increasing Ca2+ influx; the EC(5 0) for these effects is approximate to 5 nmol/L. AVP has recently been reported to stimulate arachidonic acid release in primary cultures of rat aortic smooth muscle over a much lower concentration range (EC(50 ), approximate to 0.05 nmol/L). The present study examined the effects of varying concentrations of AVP on spontaneous Ca2- spiking activity in fura 2-loaded A7r5 cells. Frequency of Ca2+ spiking increased with increasing [AVP] in the range of 10 to 500 pmol/L. Higher concentrati ons of AVP inhibited spiking but elicited the characteristic [Ca2+](i) changes ascribed to the release of Ca2+ stores and increased Ca2+ ent ry. The effects of both low and high concentrations of AVP were inhibi ted by -(beta-mercapto-beta,beta,-pentamethylenepropionic acid),2-O me thyltyrosine]arginine vasopressin, a selective V-1a vasopressin antago nist. Nimodipine (50 nmol/L), a blocker of L-type voltage-sensitive Ca 2+ channels, abolished the Ca2+-spiking activity without inhibiting a maximal [Ca2+](i) response to AVP (1 mu mol/L). AVP-stimulated Ca2+ sp iking, but not release of intracellular Ca2+ stores, was also abolishe d by ONO-RS-082 (1 mu mol/L), an inhibitor of phospholipase A(2). Thes e results suggest that occupation of a small fraction of V-1a vasopres sin receptors by AVP results in stimulation of phospholipase A(2) and leads to increased Ca2+-spiking activity. This effect may be important for fine tuning of vascular tone, whereas maximal stimulation by AVP (full receptor occupancy) may be required for more vigorous or sustain ed vasoconstriction or mitogenesis.