SUBCELLULAR MECHANISMS OF ANGIOTENSIN-II AND ARGININE-VASOPRESSIN ACTIVATION OF AREA POSTREMA NEURONS

Angiotensin II (ANG II) and arginine vasopressin (AVP) act on area pos trema (AP) neurons to modulate the baroreflex. Because activation of A P neurons by either ANG II or AVP increases intracellular free Ca2+ co ncentrations ([Ca2+](i)), the goal of this study was to analyze the fa ctors affecting the [Ca2+](i) responses to ANG II and AVP. Neurons wer e recovered from 14- to 16-day old rats and studied after 8-14 days in culture by use of the microscopic digital image analysis for fura 2-l oaded cells. The effects of ANG II (100 nM) and AVP (100 nM) on [Ca2+] (i) were determined in normal (2 mM) and low (<10 nM) extracellular Ca 2+ concentrations. In 143 of 240 neurons, ANG II increased [Ca2+](i) 4 .65-fold after 20 s, and a similar response was observed in the absenc e of extracellular Ca2+ (3.65-fold after 20 s). After 60 s of observat ion, steady-state levels of increased [Ca2+](i) were still present und er both conditions. Pretreatment-with AT(1) antagonist or pertussis to xin abolished the response to ANG II. AVP also increased [Ca2+](i) (3. 6-fold at peak, 20 s) in normal and low extracellular Ca2+. Pretreatme nt with AVP V-1 antagonist or pertussis toxin abolished the response t o AVP. This study indicates that ANG II-induced increases in [Ca2+](i) are independent of extracellular Ca2+ concentrations and involve the activation of AT(1) receptors and a pertussis toxin-sensitive G protei n. Although AVP affects a fewer number of AP neurons, the mechanisms o f activation are also independent of extracellular Ca2+ concentration and are mediated by a pertussis toxin-sensitive G protein.