Binding and degradation of vasoactive intestinal peptide in prolactin‐producing cultured rat pituitary tumour cells (GH4C1)

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Abstract
Vasoactive intestinal peptide (VIP) stimulates prolactin (PRL) secretion from cultured rat pituitary cells (GH cells) (Bjøro et al. 1984). This study demonstrates the presence of specific receptors for 125I-VIP on the GH4C1 cells. Specific binding was rapid and biphasic giving a transient plateau lasting from 7 to 30 min. Thereafter specific binding declined to about one-third after 90 min. This coincided with enhanced degradation of 125I-VIP. The degradation was mainly cell-mediated and only partly receptor dependent. Trichloroacetic acid precipitation and absorption chromatography indicated that the degradation products were either 125I- and/or small labelled peptide fragments. Bioassay, RIA and rebinding studies also demonstrated degradation of VIP. Pretreatment of GH4C1 cells with trypsin decreased the rate of degradation of 125I-VIP, but also reduced the amount of specific binding. Scatchard analysis of binding data indicated the existence of two independent classes of receptors, one with Kd = 2.2 nM and Bmax = 15 fmol per 10(6) cells and another with Kd = 180 nM and Bmax = 550 fmol per 10(6) cells. The IC50 for VIP, PHI and secretin were 4, 5 and 500 nM, respectively. We conclude that the high affinity receptor is the most probable mediator of VIP on PRL secretion. The effect of VIP and PHI on PRL secretion in GH4C1 cells is mediated through one common receptor.

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