INTROBLJCTIONElectrophysiological recordings from GH, cells using conventional high resistance micro-electrodes have revealed that somatosrarin (SS) depresses [l] and TRH enhances cell excitability, The effect of TRH is quite complex, consisting of Rf) initial phase of transient hyperpolarization 12-S] followed by a second phase in wlrich mcmbranc conductance is decreased while both the rate of production and the length of APs are increased (2,3,5-71. This second phase causes a plateau of elevated intracellular Ca 2'* due to influx of extracellular Ca'+ ([&lo] for review see [I 13) and coincides with a sustained increase in secretion [9,12-161. The importance of Ca* + in this sustained secretion is controversial [9,11,12], and it has been suggested that this cation is only necessary at low TRH concentrations when the increases in diacylglycerol are submaximal

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