Vasopressin-induced intracellular Ca2+ increase in isolated rat supraoptic cells

1. The intracellular Ca2+ concentration ([Ca2+](i)) was monitored in single magnocellular neurones freshly isolated from rat supraoptic nucleus. Application of 100 nM vasopressin increased [Ca2+](i). Two types of [Ca2+](i) responses were observed: (i) a transient response, displayed by 86% of the vasopressin-sensitive neurones, and (ii) a sustained response displayed by 14% of the vasopressin-sensitive neurones. 2. Among responding neurones, 52% were vasopressin sensitive, 44% were oxytocin sensitive and 4% were sensitive to both peptides. 3. Responses to vasopressin were dose dependent, showed a progressive desensitization after successive applications, were specifically blocked by the V-1a vasopressin receptor antagonist, SR 49059, and were unaffected by the oxytocin receptor antagonist, d(CH2)(5)OVT. 4. Vasopressin responses were completely suppressed by the removal of external Ca2+. 5. The intracellular Ca2+ mobilizers, caffeine and tBuBHQ, did not affect resting or vasopressin-induced [Ca2+](i) changes. Thapsigargin (200 nM) on its own evoked an increase in [Ca2+](i), and reduced the [Ca2+](i) increase evoked by vasopressin by 52%, suggesting that thapsigargin-sensitive Ca2+ stores are partially involved in the vasopressin response. 6. Immunocytochemical identification revealed that vasopressin-responding neurones synthesize vasopressin whereas oxytocin-responding neurones synthesize oxytocin. 7. In conclusion, vasopressin- (partially external Ca2+ dependent) and oxytocin (totally external Ca2+ independent)-induced [Ca2+](i) changes are mediated by specific receptors. In addition, vasopressin and oxytocin neurones are specifically autoregulated by their own peptides.