Simultaneous exposure to ATP and phenylephrine induces a sustained elevation in the intracellular calcium concentration in supraoptic neurons.
Vasopressin (VP) release from the hypothalamo-neurohypophyseal system (HNS) is stimulated by ATP activation of P2X purinergic receptors and by activation of 1-adrenergic receptors by phenylephrine (PE). These responses are potentiated by simultaneous exposure to ATP+PE. Potentiation was blocked by depleting intracellular calcium stores with thapsigargin. To test the hypothesis that the synergistic response to ATP+PE reflects alterations in the intracellular calcium concentration ([Ca2+]i), [Ca2+]i was monitored in supraoptic neurons in HNS explants loaded with fura 2-AM. Both ATP and PE induced rapid, but transient, elevations in [Ca2+]i. In 0.3 mM Ca2+, the peak response to ATP was greater than to PE but did not differ from the peak response to ATP+PE. A sustained elevation in [Ca2+]i was induced by ATP+PE, that was greater than ATP or PE alone. In 2 mM Ca2+, the peak response to ATP+PE was significantly greater than to either ATP or PE alone, and the sustained response to ATP+PE was greater than to either agent alone. Responses were comparable in the presence of TTX. The sustained elevation in [Ca2+]i was also observed when ATP+PE was removed after 1 min, but it was eliminated by either thapsigargin or removing external calcium, indicating that both calcium influx and calcium release from internal stores are required. Some cells were vasopressinergic based on a VP-induced increase in [Ca2+]i. These observations support the hypothesis that simultaneous exposure to ATP+PE induces a different pattern of [Ca2+]i than either agent alone that may initiate events leading to synergistic stimulation of VP release.
Song
Zhilin
Z
Department of Physiology and Biophysics, University of Colorado at Denver and Health Sciences Center, Aurora, Colorado 80045, USA. zhilin.song@uchsc.edu
Vijayaraghavan
Sukumar
S
Sladek
Celia D
CD
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D008297:Male; D009474:Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism*; D010656:Phenylephrine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013495:Supraoptic Nucleus / Q000166:cytology*; D013779:Tetrodotoxin; D019284:Thapsigargin
D010656:Phenylephrine; D013779:Tetrodotoxin; D019284:Thapsigargin; D000255:Adenosine Triphosphate; D002118:Calcium