Thyrotropin-releasing hormone stimulates porcine melanotrope cells in primary culture.

The action of thyrotropin-releasing hormone (TRH) on melanotrope cells maintained in primary culture was studied with biochemical and electrophysiological techniques. TRH effects on polyphosphoinositide (PPI) breakdown was measured in [3H]myoinositol labelled cells maintained in suspension for 24 hours or in primary culture. TRH (50 nM) or its potent analogue (3Me-His2)-TRH increased total PPI levels by 50-125% in separate experiments after 30 min of treatment whereas corticotropin-releasing hormone (CRF) was without effect. The effect of TRH was dose-dependent (ED50 = 5 nM), the maximal effect being reached with 50 nM TRH. Using the patch-clamp technique in the cell-attached configuration spikes were recorded extracellularly. In 6 of the 13 cells tested, (3Me-His2)-TRH (10 nM) elicited an increase in the spontaneous spiking rate. Furthermore, TRH (50 nM) increased melanocyte-stimulating hormone (alpha-MSH) secretion 2-fold after 8 h of treatment. These results suggested that TRH activated phospholipase C and electrical activity in melanotrope cells; the resulting phosphoinositide breakdown and increase in intracellular free Ca2+ ultimately led to a stimulation of hormone release.