REVERSIBLEFATIGUEOF STIMULUS-SECRETIONCOUPLING INTHE RAT NEUROHYPOPHYSIS

SUMMARY 1.Single ratneurointermediate lobes wereimpaled onastimulating electrode and continuously perifused withoxygenated medium. Thesecretion ofoxytocin and vasopressin intothemediumwasmeasured byspecific radio-immunoassays. 2.Thetemporal profile ofvasopressin release during a20minperiod of13Hz stimulation wascompared withthatofoxytocin. Theresults indicate thatalthough therateofsecretion ofbothoxytocin andvasopressin declines over20min,theextent andtimecourse ofthis fatigue isdifferent forthetwohormones. Thisdifference could notbeaccounted forbydifferences intherateofdiffusion ofreleased hormone from thetissue whichwassimilar totherateofwash-out of[14C]sucrose fromthe extracellular space inpre-labelled glands. 3.Inseparate experiments glands wereexposed toaprolonged period (60-70 min) of13Hzstimulation interrupted bybrief silent periods (30s-2minduration). Some recovery fromthefatigue ofvasopressin secretion wasevident after eventheshortest ofthese silent periods. 4.Infurther experiments glands werestimulated electrically for18,36,54and72s at13Hz:theorder ofpresentation oftheperiods ofstimulation wasrandomized between experiments. Thevasopressin release ratedeclined markedly andprogressively between 18and72s.Incontrast, theoxytocin release ratewasrelatively uniform throughout 72sofstimulation. Thusvasopressin secretion issubject toa relatively rapid anddramatic fatigue. 5.Theresults support thehypothesis thatthephasic discharge patterns characteristic ofvasopressin secreting neurones optimize theefficiency ofvasopressin release fromthenerveterminals intheneurohypophysis byavoiding thefatigue of stimulus-secreti on coupling thataccompanies continual stimulation.