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B Sperlagh, Z Mergl, Z Juranyi, ES Vizi, and GB Makara

It is now widely accepted that ATP functions as a signalling substance in the nervous system. The presence of P2 receptors mediating the action of extracellular ATP in brain regions involved in hormonal regulation raises the possibility that a similar role for ATP might also exist in the neuroendocrine system. In this study, the release from the rat isolated neurohypophysis preparation of endogenous ATP, oxytocin and vasopressin (AVP) were measured simultaneously using luciferin-luciferase and RIA techniques. After 70 min preperfusion, electrical field stimulation caused a rapid increase in the amount of ATP in the effluent and the release of AVP and oxytocin also increased stimulation-dependently. Inhibition of voltage-dependent Na+ channels by tetrodotoxin (1 microM) reduced the stimulation-evoked release of AVP and oxytocin; however, the evoked release of ATP remained unaffected. The effect of endogenous ATP on the hormone secretion was tested by suramin (300 microM), the P2 receptor antagonist. Suramin significantly increased the release of AVP, and the release of oxytocin was also enhanced. ATP, when applied to the superfusing medium, decreased the release of AVP, but not that of oxytocin, and its effect was prevented by suramin. ATP (60 nmol), added to the tissues, was readily decomposed to ADP, AMP and adenosine measured by HPLC combined with ultraviolet light detection, and the kinetic parameters of the enzymes responsible for inactivation of ATP (ectoATPase and ecto5'-nucleotidase) were also determined (Km=264+/-2.7 and 334+/-165 microM and vmax=6.7+/-1.1 and 2.54+/-0.24 nmol/min per preparation (n=3) for ectoATPase and ecto5'-nucleotidase respectively). Taken together, our data demonstrate the stimulation-dependent release, P2 receptor-mediated action and extracellular metabolism of endogenous ATP in the posterior lobe of the hypophysis and indicate its role, as a paracrine regulator, in the local control of hormone secretion.

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Z Jurányi, E Orsó, A Jánossy, K Sz Szalay, B Sperlágh, K Windisch, G P Vinson, and E S Vizi

Abstract

Both [3H]noradrenaline ([3H]NA) and ATP were released in response to supramaximal electric field stimulation in superfused rat adrenal capsule-glomerulosa preparations. The voltage-dependent potassium channel blocker 4-aminopyridine enhanced, while the ATP-sensitive potassium channel blocker glibenclamide failed to affect the stimulation-evoked release of [3H]NA. The selective α2-adrenoceptor antagonist CH-38083 enhanced the evoked release of [3H]NA while the P2 receptor agonist ATP and α,β-methylene-ATP failed to affect it. Neither the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) nor the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) influenced the stimulation-evoked [3H]NA release. The data showed that ATP was released from capsule-glomerulosa preparations in response to field stimulation together with but independently from [3H]NA, and that the local noradrenergic varicose axon terminals are not equipped with purinoceptors sensitive to ATP and/or adenosine. High concentrations of ATP also stimulated steroid hormone secretion in vitro, and thus may have a physiological role in this tissue. The presence of ecto-Ca2+-ATPases, enzymes able to terminate the effect of ATP, was demonstrated around the nerve profiles at the border of the capsule and zona glomerulosa tissue.

Journal of Endocrinology (1997) 153, 105–114