Multiple splice variants of the pituitary adenylate cyclase-activating polypeptide type 1 receptor detected by RT-PCR in single rat pituitary cells

Alternative splicing of the rat type 1 pituitary adenylate cyclase-activating polypeptide (PACAP) receptor (PVR1) produces variants that couple either to both adenylyl cyclase (AC) and phospholipase C (PLC) (PVR1 short, PVR1 hop, PVR1 hiphop), or to AC alone (PVR1 hip). We have previously shown that populations of clonal alphaT3-1 gonadotrophs express PVR1 hop and PVR1 short mRNAs, whereas clonal GH4C1 somatotrophs do not. Here we have used the single cell RT-PCR technique to investigate whether normal rat gonadotrophs and somatotrophs express PVR1 mRNA, whether a single cell co-expresses multiple splice variant forms, and whether differential PVR1 mRNA expression correlates with differences in PACAP-stimulated Ca2+ signalling. We found that individual rat gonadotrophs expressed mRNA either for PVR1 hop, for PVR1 short, or co-expressed the two forms. Although we found no differences between the splice variant(s) expressed and the characteristics of PACAP-stimulated Ca2+ responses, the expression of PVR1 mRNA is consistent with the known PACAP stimulation of the PLC system in gonadotrophs. Individual rat somatotrophs also expressed PVR1 hop or PVR1 short (but not PVR1 hip) mRNAs although these forms were never co-expressed. The expression of PVR1 mRNA in somatotrophs can explain in part the activation by PACAP of the AC system in such cells. In conclusion, the single cell RT-PCR technique was used to demonstrate expression of multiple PVR1 splice variants in single identified pituitary cells. These findings open up important questions on the role of alternative splicing in cell biology.

Stimulation of Ca2+ influx in alpha T3-1 gonadotrophs via the cAMP/PKA signaling system

To investigate the regulation of free cytosolic calcium concentration ([Ca2+]i) by the adenosine 3',5'-cyclic monophosphate (cAMP) signaling system in clonal gonadotrophs, microfluorimetric recordings were made in single indo 1-loaded alpha T3-1 cells. Forskolin, 8-bromoadenosine 3',5'-cyclic monophosphate, or a low concentration (100 pM) of the hypothalamic factor pituitary adenylate cyclase-activating polypeptide (PACAP) stimulated Ca2+ step responses or repetitive Ca2+ transients, which were blocked by the removal of extracellular Ca2+ by the dihydropyridine (DHP) (+)PN 200-110 or by preincubation with the protein kinase A (PKA) antagonist H-89 (10 microM). Thus activation of the cAMP/PKA system in alpha T3-1 gonadotrophs stimulates Ca2+ influx through DHP-sensitive (L-type) Ca2+ channels. In contrast, high PACAP concentrations (100 nM) stimulated biphasic Ca2+ spike-plateau responses. The Ca2+ spike was independent of extracellular Ca2+, and similar responses were observed by microperfusion of individual cells with D-myo-inositol 1,4,5-trisphosphate, suggesting the involvement of the phospholipase C (PLC) signaling pathway. The Ca2+ plateau depended on Ca2+ influx, was blocked by (+)PN 200-110, but was only partially blocked by H-89 pretreatment. In conclusion, PACAP stimulates [Ca2+]i increases in alpha T3-1 gonadotrophs through both the PLC and adenylate cyclase signaling pathways. Furthermore, this is the first clear demonstration that the cAMP/PKA system can mediate changes in [Ca2+]i in gonadotroph-like cells.

PACAP/VIP receptor subtypes, signal transducers, and effectors in pituitary cells

Rat anterior pituitary tissue expresses mRNA for PVR1 and PVR3, as well as a low level of PVR2. The PVR1 appears to be highly expressed in gonadotroph-like cells, while somatotroph-like cells apparently express the PVR3. We have recently demonstrated the expression of mRNA for both PVR2 and PVR3 in corticotroph-like AtT20 cells (FIG.3). If normal corticotrophs express the same mRNA as AtT20 cells, this may partly explain the low levels of PVR2 seen in normal pituitary tissue. Significant levels of at least two PVR1 splice variants mRNAs (PVR1s and PVR1hop) were expressed in clonal gonadotroph-like alpha T3-1 cells and normal rat anterior pituitary tissue. However, these splice variants are reported to have almost identical pharmacological characteristics in terms of binding, and the activation of AC and PLC. Further experiments are necessary to determine the functional consequences of differential splice variant expression in such cells. Interestingly, all three pituitary-cell lines studied expressed mRNA for the PVR3 (FIG.3), whereas earlier binding studies demonstrate a predominance of PACAP-preferring binding sites on normal anterior pituitary-cell membranes. In addition, it is clear that the different PVR subtypes can couple to different intracellular messenger systems. Thus it will be important to determine the expression of the different PVR subtypes in normal anterior pituitary-cell types if we are to begin to understand the regulation of pituitary-cell regulation by PACAP. Such questions form the basis of some of the ongoing studies in our laboratory.

Pituitary adenylate cyclase-activating polypeptide regulates [Ca(2+)](i) and electrical activity in pituitary cells through cell type-specific mechanisms

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a recently identified hypothalamic factor that acts on a variety of anterior pituitary cell types. It is clear, however, that its actions are not mediated by the same intracellular signaling mechanisms in each cell type. The signaling pathways by which PACAP regulates changes in [Ca(2+)], and electrical activity in rat somatotrophs and gonadotrophs is described in the present article. Finally, the possibility that the differences in PACAP-regulated signaling in anterior pituitary cells is due to the differential expression and coupling of PACAP receptor subtypes is discussed.

PACAP and VIP stimulate Ca2+ oscillations in rat gonadotrophs through the PACAP/VIP type 1 receptor (PVR1) linked to a pertussis toxin-insensitive G-protein and the activation of phospholipase C-beta

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are hypothalamic factors that play roles in the regulation of anterior pituitary cell activity. PACAP exists in 2 forms physiologically, a 38 amino acid form (PACAP38) and a form possessing the N-terminal 27 amino acids of PACAP38 (PACAP27). We have previously shown that PACAP38 stimulates an increase in [Ca2+]i in rat gonadotrophs. In an attempt to identify the PACAP receptor type underlying this effect, we compared the potency of PACAP38, PACAP27 and VIP to stimulate Ca2+ changes in identified single rat gonadotrophs. All 3 peptides at 100 nM were capable of stimulating high amplitude Ca2+ oscillations, which were also observed in the absence of extracellular Ca2+. The order of potency of these peptides was PACAP38 > PACAP27 > VIP, and a potent antagonist of the PACAP/VIP type II binding site ([4-CI-D-Phe6, Leu17]-VIP) failed to block these responses, suggesting that these effects are mediated through a PACAP/VIP type 1 receptor (PVR1). The Ca2+ responses to PACAP38 and VIP were unaffected by overnight treatment of the cells with pertussis toxin (PTX; 250 ng/ml) indicating that these responses are mediated by a PTX-insensitive G-protein. Finally, the Ca2+ responses stimulated by PACAP38 and VIP were blocked by the phospholipase C-beta blocker U73122 (5 microM). In summary, PACAP stimulates Ca2+ oscillations in rat gonadotrophs through the activation of the PVR1 linked to a PTX-insensitive G-protein and the activation of phospholipase C-beta. VIP can stimulate the same pathway in rat gonadotrophs, although it is at least 100 fold less potent than PACAP38.

Differential expression of pituitary adenylate cyclase-activating polypeptide/vasoactive intestinal polypeptide receptor subtypes in clonal pituitary somatotrophs and gonadotrophs

Pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are hypothalamic factors believed to play a role in the regulation of anterior pituitary cell function. However, little is known about the expression of PACAP/VIP receptor (PVR) subtypes in such cells. Three PVR subtypes have recently been cloned: the PACAP-selective PVR1, and PVR2 and PVR3, which exhibit similar affinities for PACAP and VIP. In the present study we used the reverse transcription-polymerase chain reaction with PVR-specific primers to identify the PVR messenger RNAs (mRNAs) expressed in the somatotroph-like GH4C1 and the gonadotroph-like alpha T3-1 cell lines. In parallel, the effects of PACAP and VIP on intracellular signaling were studied. GH4C1 cells were found to express mRNA only for the PVR3, and neither PVR1 nor PVR2 mRNA was found. PACAP and VIP stimulated Ca2+ influx responses in individual GH4C1 cells and were equipotent in stimulating cAMP production (EC50, 15 nM) in GH4C1 cell populations, but failed to stimulate inositol phospholipid (PI) turnover, results consistent with the expression of a PVR3. In contrast, alpha T3-1 cells expressed mRNA for PVR1 and PVR3, but not PVR2. The predominant splice variant forms of PVR1 observed were PVR1s and PVR1hop, although the other forms (PVR1hiphop and PVR1hip) were also seen at much lower levels. PACAP stimulated a Ca2+ store-dependent Ca2+ spike and a sustained Ca2+ influx in individual alpha T3-1 cells, whereas VIP only stimulated Ca2+ influx. PACAP (EC50, 3 nM) was approximately 1000-fold more potent than VIP (EC50, approximately 3 microM) in stimulating cAMP production. PACAP also stimulated PI turnover (EC50, approximately 20 nM), whereas VIP stimulated PI turnover only at very high (10 microM) concentrations. These results are indicative of the expression of a PVR1. Rat anterior pituitary tissue expressed mRNAs for PVR1, PVR3, and low levels of PVR2. The coexpression of different PVRs in the same cell type and the differential expression of PVRs in different cell types would allow for a complex regulation of anterior pituitary gland function by PACAP and VIP.

Pituitary adenylate cyclase-activating polypeptide increases [Ca2]i in rat gonadotrophs through an inositol trisphosphate-dependent mechanism

Pituitary adenylate cyclase-activating polypeptide (PACAP) increases cAMP production and stimulates hormone release from a variety of anterior pituitary cells. However, in anterior pituitary gonadotrophs PACAP stimulates oscillations in cytosolic free Ca2+ concentration ([Ca2+]i) that appear to be independent of cAMP. To study the mechanisms involved in this response, we used the patch-clamp technique to microperfuse various agents into single rat gonadotrophs while monitoring [Ca2+]i with microfluorometry. Extracellular application of PACAP to single gonadotrophs stimulated high amplitude (> 1 microM) oscillations in [Ca2+]i, which were blocked by intracellular application of GDP beta S (guanosine 5'-O-2-thiodiphosphate), indicating the involvement of a G-protein. To identify the intracellular messenger(s) involved, we microperfused gonadotrophs with cAMP, inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), or heparin, an antagonist of the Ins(1,4,5)P3 receptor. A high concentration of cAMP (100 microM) had no significant effect on basal [Ca2+]i and did not alter the PACAP-stimulated Ca2+ response. Heparin, but not its inactive isoform, completely blocked the PACAP-stimulated increase in [Ca2+]i, while Ins(1,4,5)P3 stimulated oscillations in [Ca2+]i very similar to those observed in response to PACAP. These results strongly suggest that PACAP mobilizes Ca2+ through an Ins(1,4,5)P3-dependent mechanism. The fact that PACAP stimulates two signaling pathways in pituitary cells could substantially enhance the signaling potential of this hypothalamic peptide.

Pituitary adenylate cyclase-activating polypeptide regulates cytosolic Ca2+ in rat gonadotropes and somatotropes through different intracellular mechanisms

The hypothalamic factor pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates an increase in the cytoplasmic free Ca2+ ion concentration ([Ca2+]i) in GH-secreting somatotropes and LH-secreting gonadotropes of the rat anterior pituitary gland. The dynamics of the PACAP-induced Ca2+ responses and their dependence on extracellular Ca2+ are markedly different in the two cell types, suggesting separate mechanisms of action of PACAP in somatotropes and gonadotropes. The present study reports a full characterization of the Ca2+ responses seen in the two cell types over a wide range of PACAP concentrations. In addition, the involvement of the cAMP-dependent protein kinase (PKA) system in the mediation of PACAP-stimulated Ca2+ was tested using the R-isomer of cAMP (RpcAMPs) as a specific inhibitor of PKA. In identified somatotropes, PACAP (10(-11)-10(-6) M) stimulated Ca2+ responses in 49% of the cells tested, with two types of Ca2+ response profile observed. The first was a slow rise in [Ca2+]i to a new level (Ca2+ step; 28% of somatotropes); the second was characterized by repetitive transient rises in [Ca2+]i (Ca2+ transients; 21% of somatotropes). The range of PACAP concentrations tested (10(-11)-10(-6) M) did not markedly alter the number of cells responding or the type of response observed. In some gonadotropes, PACAP stimulated Ca2+ step responses similar to those seen in somatotropes; however, the most common response observed was a rapid, high amplitude, but transient spike of [Ca2+]i, which was often accompanied by rapid oscillations in [Ca2+]i. This response profile was termed a Ca2+ spike-oscillations response, and the proportion of cells exhibiting this response increased from 25% at a PACAP concentration of 10(-11) M to 73% at 10(-6) M PACAP. PACAP-induced Ca2+ responses in somatotropes were blocked by pretreatment with the cAMP antagonist RpcAMPs (10(-3) M). In addition, other factors known to increase cAMP in somatotropes (GH-releasing factor and 8-bromo-cAMP) stimulated Ca2+ responses in these cells that were qualitatively similar to those induced by PACAP. In contrast, the Ca2+ responses in gonadotropes were insensitive to the cAMP antagonist RpcAMPs, and the membrane-permeable cAMP analog 8-bromo-cAMP failed to stimulate responses in this cell type. These results suggest that the intracellular mechanisms of PACAP action in the two cell types are markedly different. In somatotropes, the rise in [Ca2+]i stimulated by PACAP is probably through the production of cAMP and the activation of protein kinase-A.(ABSTRACT TRUNCATED AT 400 WORDS)

Pituitary adenylate cyclase-activating polypeptide specifically increases cytosolic calcium ion concentration in rat gonadotropes and somatotropes

The hypothalamic peptide, pituitary adenylate cyclase-activating polypeptide (PACAP), is a potent stimulator of cAMP accumulation in the anterior pituitary gland, though its physiological function has yet to be defined. To establish the target cells of PACAP action we have measured PACAP-induced changes in cytosolic free calcium ion concentration ([Ca2+]i) in single identified anterior pituitary cells. This was achieved by combining fura-2 videomicroscopy, to measure [Ca2+]i, and reverse hemolytic plaque assays, to identify the secreted hormone. PACAP (100 nM) increased [Ca2+]i in 32% of all pituitary cells. These responses were predominantly seen in identified gonadotropes and somatotropes, but rarely in corticotropes or lactotropes. PACAP induced two forms of Ca2+ response in gonadotropes; a "Ca2+ spike" (independent of extracellular Ca2+) in 72% of responding gonadotropes, and an extracellular Ca(2+)-dependent "Ca2+ plateau" (28% of cells). In somatotropes, PACAP stimulated either Ca2+ plateau responses (58% of responding somatotropes) or repetitive "Ca2+ transients" (42% of cells), both of which were dependent upon extracellular Ca2+. PACAP, therefore, produces distinct changes in [Ca2+]i in gonadotropes and somatotropes, which may be related to distinct intracellular messenger pathways. The identification of these cell types as targets of PACAP action suggests a role in the regulation of reproduction and growth.

Evidence for localized calcium mobilization and influx in single rat gonadotropes

Dynamic video-imaging microscopy was used to investigate the spatial and temporal nature of Ca2+ mobilization and Ca2+ influx in acutely dissociated, fura-2-loaded, rat gonadotropes. Addition of luteinizing hormone-releasing hormone (LHRH) to an isolated gonadotrope stimulated a wave of Ca2+ originating from a specific locus of the cell. This probably reflects Ca2+ mobilization from an intracellular store, since this response was unaffected by the removal of extracellular Ca2+. Application of the dihydropyridine-sensitive Ca2+ channel agonist Bay K 8644 (Bay K) stimulated a rise in cytosolic free Ca2+ concentration in the rat gonadotrope. This response was blocked by the removal of extracellular Ca2+ and probably reflects the influx of Ca2+ across the cell membrane. High speed (30 frames.s-1) imaging of the Bay K-induced Ca2+ influx revealed a wave of Ca2+ originating from a localized part of the cell membrane, which, in general, was spatially distinct from the LHRH-induced Ca2+ wave produced in the same cell. This suggests that Ca2+ channels in the cell membrane may be clustered in a specific area of the cell membrane. The velocity of the LHRH-induced Ca2+ mobilization wave was faster (mean = 79 +/- 5 microns.s-1, n = 9) than the Bay K-induced Ca2+ influx wave (39 +/- 7 microns.s-1, n = 9) (p less than or equal to 0.01, Wilcoxon signed rank test) measured in the same cells. Thus, both Ca2+ mobilization from intracellular stores and Ca2+ influx through the cell membrane appear to be spatially localized in the rat gonadotrope. These findings may have important implications in the intracellular regulation of Ca(2+)-dependent cell functions such as hormone biosynthesis and secretion.

Calcium homeostasis in bovine somatotrophs: calcium oscillations and calcium regulation by growth hormone-releasing hormone and somatostatin

The free intracellular calcium ion concentration ([Ca2+]i) was measured in single cells of a population containing 65-80% somatotrophs, using the fluorescent Ca(2+)-indicator Fura-2 and digital imaging microscopy. Spontaneous oscillations in [Ca2+]i ranging in frequency up to 1.5 oscillations per minute were observed in 30% of somatotrophs. These Ca2+ oscillations were blocked by the Ca2+ channel blocker CoCl2 and were thus proposed to be the result of influx of Ca2+ into the cell, possibly as the result of spontaneous electrical activity. GHRH (10-100 nM) increased [Ca2+]i in 61% of the cells studied, although the amplitude and dynamics of the response varied from cell to cell. Typically [Ca2+]i rose from 170 +/- 26 nM to 321 +/- 44 nM (n = 13) in response to a challenge with 66 nM GHRH. GHRH also increased the frequency of Ca2+ oscillations in a number of cells, and some previously quiescent cells showed Ca2+ oscillations following addition of GHRH. Forskolin, which raises cAMP levels in bovine anterior pituitary cells, also stimulated a sustained rise in [Ca2+]i in 10 out of 14 cells tested. Somatostatin (SS) (10-80 nM) rapidly reduced basal [Ca2+]i, blocked Ca2+ oscillations, and blocked the [Ca2+]i response to GHRH. The Ca2+ channel blocker CoCl2 (4 mM) had similar actions on [Ca2+]i to those of SS. These results suggest that GHRH and SS may regulate GH release by modulating Ca2+ entry into the cell through the cell membrane. The [Ca2+]i oscillations seen in a proportion of the somatotrophs were modulated in frequency by GHRH and SS, and are probably generated by influx of Ca2+ through channels in the cell membrane. Thus GH secretion may be regulated by changes in the mean level of [Ca2+]i, which in turn, may be influenced by the frequency of [Ca2+]i oscillations in bovine somatotrophs.

Whole-cell recordings of ionic currents in bovine somatotrophs and their involvement in growth hormone secretion

1. The whole-cell mode of the patch-clamp technique was used to record voltage-activated cationic currents in immunocytochemically identified bovine somatotrophs. 2. In current-clamp mode, cells had a resting membrane potential of -83.0 +/- 4.5 mV, and an input resistance of 8.4 +/- 2.1 G omega. Cells rarely fired action potentials spontaneously, but fired one to three action potentials in response to a suprathreshold current pulse. 3. Under voltage clamp, in Ca2+-free media, the action potential was shown to be composed of a TTX-sensitive inward Na+ current and an outward K+ current. 4. The isolated Na+ current had a threshold of approximately -50 mV, and rapidly activated and then inactivated to a small steady-state current. Peak Na+ current amplitude with 140 mM-external Na+ was 341.1 +/- 33.5 pA (n = 14) at a membrane potential of -32.1 +/- 2.4 mV (n = 14). 5. With Ca2+ or Ba2+ (5-30 mM) as the only membrane-permeable cation, voltage pulses to potentials more positive than -55 mV from a holding potential of -80 mV revealed a rapidly activating current component that was followed by a second, very slowly inactivating, current component, most clearly seen with Ba2+. Both components were maximally activated between 0 and +10 mV, were TTX insensitive, but were blocked by 4 mM-Co2+. 6. Three components of the isolated K+ current were identified (IA, IK and IK(Ca] by their voltage sensitivity, Ca2+ dependence and their response to 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 7. Growth hormone (GH)-releasing hormone (GHRH) applied to cells under whole-cell voltage clamp had no effect on either steady-state or voltage-activated ionic currents. This is probably due to dialysis of cytoplasmic compounds vital for GHRH activation of the cell. 8. Both basal and GHRH-stimulated GH secretion were unaffected by TTX, implying that the Na+ action potential is not critical for such release. In contrast the Ca2+ channel blocker Co2+ attenuated GH release in both cases. The K+ channel blocker TEA stimulated GH release above basal and GHRH-stimulated levels.

Control of secretion in anterior pituitary cells--linking ion channels, messengers and exocytosis

Normal anterior pituitary cells, in their diversity and heterogeneity, provide a rich source of models for secretory function. However, until recently they have largely been neglected in favour of neoplastic, clonal tumour cell lines of pituitary origin, which have enabled a number of studies on supposedly homogeneous cell types. Because many of these lines appear to lack key peptide and neurotransmitter receptors, as well as being degranulated with accompanying abnormal levels of secretion, we have developed a range of normal primary anterior pituitary cell cultures using dispersion and enrichment techniques. By studying lactotrophs, somatotrophs and gonadotrophs we have revealed a number of possible transduction mechanisms by which receptors for hypothalamic peptides and neurotransmitters may control secretion. In particular, the transduction events controlling secretion from pituitary cells may differ fundamentally from those found in other cell types. Patch-clamp recordings in these various pituitary cell preparations have revealed substantial populations of voltage-dependent Na+, Ca2+ and K+ channels which may support action potentials in these cells. Although activation of these channels may gate Ca2+ entry to the cells under some conditions, our evidence taken with that of other laboratories suggests that peptide-receptor interactions leading to hormone secretion occur independently of significant membrane depolarization. Rather, secretion of hormone and rises in intracellular calcium measured with new probes for intracellular calcium activity, can occur in response to hypothalamic peptide activation in the absence of substantial changes in membrane potential. These changes in intracellular calcium activity almost certainly depend on both intracellular and extracellular calcium sources. In addition, strong evidence of a role for multiple intracellular receptors and modulators in the secretory event suggests we should consider the plasma membrane channels important for regulation of hormone secretion to be predominantly agonist-activated, rather than of the more conventional voltage-dependent type. Likewise, evidence from new methods for recording single ion channels suggests the existence of intracellular sites for channel modulation, implying they too may play an important role in secretory regulation. We shall consider new data and new technology which we hope will provide key answers to the many intriguing questions surrounding the control of pituitary hormone secretion. We shall highlight our work with recordings of single ion channels activated by peptides, and recent experiments using imaging of intracellular ionized free calcium.(ABSTRACT TRUNCATED AT 250 WORDS)