Multiple pathways for high voltage-activated ca(2+) influx in anterior pituitary lactotrophs and somatotrophs

Regulation of the Ca2+ channel α2δ-1 subunit expression by epidermal growth factor via the ERK/ELK-1 signaling pathway

Voltage-gated Ca²⁺ (Ca_V) channels are expressed in endocrine cells where they contribute to hormone secretion. Diverse chemical messengers, including epidermal growth factor (EGF), are known to affect the expression of Ca_V channels. Previous studies have shown that EGF increases Ca²⁺ currents in GH3 pituitary cells by increasing the number of high voltage-activated (HVA) Ca_V channels at the cell membrane, which results in enhanced prolactin (PRL) secretion. However, little is known regarding the mechanisms underlying this regulation. Here, we show that EGF actually increases the expression of the Ca_Vα₂δ-1 subunit, a key molecular component of HVA channels. The analysis of the gene promoter encoding Ca_Vα₂δ-1 (CACNA2D1) revealed binding sites for transcription factors activated by the Ras/Raf/MEK/ERK signaling cascade. Chromatin immunoprecipitation and site-directed mutagenesis showed that ELK-1 is crucial for the transcriptional regulation of CACNA2D1 in response to EGF. Furthermore, we found that EGF increases the membrane expression of Ca_Vα₂δ-1 and that ELK-1 overexpression increases HVA current density, whereas ELK-1 knockdown decreases the functional expression of the channels. Hormone release assays revealed that Ca_Vα₂δ-1 overexpression increases PRL secretion. These results suggest a mechanism for how EGF, by activating the Ras/Raf/MEK/ERK/ELK-1 pathway, may influence the expression of HVA channels and the secretory behavior of pituitary cells.

Ca2+ Channels in Anterior Pituitary Somatotrophs: A Therapeutic Perspective

Ca2+ influx through voltage-gated Ca2+ channels (VGCCs) plays a key role in GH secretion. In this review, we summarize the current state of knowledge regarding the physiology and molecular machinery of VGCCs in pituitary somatotrophs. We next discuss the possible involvement of Ca2+ channelopathies in pituitary disease and the potential use of Ca2+ channel blockers to treat pituitary disease. Various types of VGCCs exist in pituitary cells. However, because L-type Ca2+ channels (LTCCs) contribute the major component to Ca2+ influx in somatotrophs, lactotrophs, and corticotrophs, we focused on these channels. An increasing number of studies in recent years have linked genetic missense mutations in LTCCs to diseases of the human cardiovascular, nervous, and endocrine systems. These disease-associated genetic mutations occur at homologous functional positions (activation gates) in LTCCs. Thus, it is plausible that similar homologous missense mutations in pituitary LTCCs can cause abnormal hormone secretion and underlying pituitary disorders. The existence of LTCCs in pituitary cells opens questions about their sensitivity to dihydropyridines, a group of selective LTCC blockers. The dihydropyridine sensitivity of pituitary cells, as with any other excitable cell, depends primarily on two parameters: the pattern of their electrical activity and the dihydropyridine sensitivity of their LTCC isoforms. These two parameters are discussed in detail in relation to somatotrophs. These discussions are also relevant to lactotrophs and corticotrophs. High dihydropyridine sensitivity may facilitate their use as drugs to treat pituitary oversecretion disorders such as acromegaly, hyperprolactinemia, and Cushing disease.

Multiple Ca2+ channel-dependent components in growth hormone secretion from rat anterior pituitary somatotrophs

The involvement of L-type Ca(2+) channels in both 'basal' and 'stimulated' growth hormone (GH) secretion is well established; however, knowledge regarding the involvement of non-L-type Ca(2+) channels is lacking. We investigated whether non-L-type Ca(2+) channels regulate GH secretion from anterior pituitary (AP) cells. To this end, GH secretion was monitored from dissociated AP cells, which were incubated for 15 min with 2 mm K(+) ('basal' secretion) or 60 mm K(+) ('stimulated' secretion). The role of non-L-type Ca(2+) influx was investigated using specific channel blockers, including ω-agatoxin-IVA, ω-conotoxin GVIA or SNX-482, to block P/Q-, N- or R-type Ca(2+) channels, respectively. Our results demonstrate that P/Q-, N- and R-type Ca(2+) channels contributed 21.2 ± 1.9%, 20.2 ± 7.6% and 11.4 ± 1.8%, respectively, to 'basal' GH secretion and 18.3 ± 1.0%, 24.4 ± 5.4% and 14.2 ± 4.8%, respectively, to 'stimulated' GH secretion. After treatment with a 'cocktail' that comprised the previously described non-L-type blockers, non-L-type Ca(2+) channels contributed 50.9 ± 0.4% and 45.5 ± 2.0% to 'basal' and 'stimulated' GH secretion, respectively. Similarly, based on the effects of nifedipine (10 μM), L-type Ca(2+) channels contributed 34.2 ± 3.7% and 54.7 ± 4.1% to 'basal' and 'stimulated' GH secretion, respectively. Interestingly, the relative contributions of L-type/non-L-type Ca(2+) channels to 'stimulated' GH secretion were well correlated with the relative contributions of L-type/non-L-type Ca(2+) channels to voltage-gated Ca(2+) influx in AP cells. Finally, we demonstrated that compartmentalisation of Ca(2+) channels is important for GH secretion. Lipid raft disruption (methyl-β-cyclodextrin, 10 mm) abrogated the compartmentalisation of Ca(2+) channels and substantially reduced 'basal' and 'stimulated' GH secretion by 43.2 ± 3.4% and 58.4 ± 4.0%, respectively. In summary, we have demonstrated that multiple Ca(2+) channel-dependent pathways regulate GH secretion. The proper function of these pathways depends on their compartmentalisation within AP cell membranes.