Absence of muscarinic modulation of vasopressin release from the isolated rat neurophypophysis.

1. Isolated rat neurohypophyses were incubated in Locke solution at 37 degress C and the vasopressin output into the medium determined by bioassay. 2. Potassium chloride 60 mM caused a 9-fold increase in the rate of vasopressin release that was abolished when calcium chloride was omitted from the Locke solution. 3. Acetylcholine 5.5x10-4M neither alone nor in the presence of atropine 2.9x10-6M changed the "resting" release of vasopressin. 4. Neither acetylcholine 5.5x10-4M oxotremorine 10-4 and k3x10-4M altered the vasopressin release evoked by potassium chloride 60 mM. 5. In contrast to the peripheral adrenergic nerve fibres, the secretory terminal fibres of the neurohypophysis do not appear to contain muscarinic inhibitory or nicotinic excitatory receptors.