The effect of stimulus intensity on alpha-adrenoceptor-mediated feedback control of noradrenaline release.

1 Mouse vas deferns stimulated transmurally (2.5 Hz, 2-32 V, 40-620 mA, FOR 45 s) responded with a twitch and a secondary contraction. Both responses were abolished by cinchocaine and were voltage-dependent. 2 In tissues previously incubated with (3H)-(--)-noradrenaline, stimulation also produced an increase in tritium overflow from the tissue. Phentolamine increased tritium overflow by 19% at high stimulus intensities (30 V, 600 mA) and by 130% at low stimulus intensities (11 V, 200 mA). 3 It is concluded that alpha-adrenoceptor-mediated feedback control of noradrenaline release is more marked at low stimulus intensities and that this is compatible with a role for calcium ions in this control mechanism.