Store-operated Ca entry and Ca responses to hypothalamic releasing hormones in anterior pituitary cells from Orai1-/- and heptaTRPC knockout mice.

Store operated Ca entry (SOCE) is the most important Ca entry pathway in non-excitable cells. However, SOCE can also play a pivotal role in excitable cells such as anterior pituitary (AP) cells. The AP gland contains five different cell types that release six major AP hormones controlling most of the entire endocrine system. AP hormone release is modulated by Ca signals induced by different hypothalamic releasing hormones (HRHs) acting on specific receptors in AP cells. TRH and LHRH both induce Ca release and Ca entry in responsive cells while GHRH and CRH only induce Ca entry. SOCE has been shown to contribute to Ca responses induced by TRH and LHRH but no molecular evidence has been provided. Accordingly, we used AP cells isolated from mice devoid of Orai1 channels (noted as Orai1-/- or Orai1 KO mice) and mice lacking expression of all seven canonical TRP channels (TRPC) from TRPC1 to TRPC7 (noted as heptaTRPC KO mice) to investigate contribution of these putative channel proteins to SOCE and intracellular Ca responses induced by HRHs. We found that thapsigargin-evoked SOCE is lost in AP cells from Orai1-/- mice but unaffected in cells from heptaTRPC KO mice. Conversely, while spontaneous intracellular Ca-oscillations related to electrical activity were not affected in the Orai1-/- mice, these responses were significantly reduced in heptaTRPC KO mice. We also found that Ca entry induced by TRH and LHRH is decreased in AP cells isolated from Orai1-/-. In addition, Ca responses to several HRHs, particularly TRH and GHRH, are decreased in the heptaTRPC KO mice. These results indicate that expression of Orai1, and not TRPC channel proteins, is necessary for thapsigargin-evoked SOCE and is required to support Ca entry induced by TRH and LHRH in mouse AP cells. In contrast, TRPC channel proteins appear to contribute to spontaneous Ca-oscillations and Ca responses induced by TRH and GHRH. We conclude that expression of Orai1 and TRPC channels proteins may play differential and significant roles in AP physiology and endocrine control.