Endocrine Abstracts

Calcium mobilization from intracellular stores represents an important cell signalling process that is regulated, in mammalian cells, by inositol-1,4,5-triphosphate (IP₃), cyclic ADP ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Intracellular calcium is important for mobilization of secretory granules to the plasma membrane in preparation for exocytosis. NAADP mobilizes calcium from lysosome-related acidic compartments and it has been shown that two-pore channels (TPCs) comprise a family of NAADP receptors, with TPC1 expressed on endosomal membranes. Western blot analysis and RT-PCR have revealed TPC1 expression in anterior pituitary and double-labelling immunofluorescence labelling for growth hormone GH and TPC1 demonstrated colocalisation in somatotrophs. Here we test the hypothesis that GH secretion in somatotrophs from TPC1 null mice would be impaired and excess storage of secretory granules would result. Anterior pituitary sections from male and female wild-type (WT) and TPC1 null mice (n=4 of each) were immunogold labelled for GH and examined by quantitative electron microscopy to determine somatotroph size, secretory granule characteristics and distribution. In female TPC1 null mice there was a significant (P<0.01 vs WT) increase in cell and cytoplasmic area, and a significant increase (P<0.01) in granule density suggesting increased synthesis and storage of GH. Furthermore, there was a decrease in the percentage of secretory granules within a 300 nm margin of the plasma membrane indicating that fewer granules were distributed adjacent to sites of secretion (P<0.01). In male TPC1 null mice there was no significant difference measured in cell and cytoplasmic area, granule distribution or size but a significant (P<0.05 vs WT) increase in granule density was measured. In conclusion, the data are consistent with increased GH storagel in the absence of TPC1.