Growth hormone-releasing factor mobilizes cytosolic free calcium through different mechanisms in two somatotrope subpopulations from porcine pituitary

Homologous and heterologous in vitro regulation of pituitary receptors for somatostatin, growth hormone (GH)-releasing hormone, and ghrelin in a nonhuman primate (Papio anubis)

Secretion of GH by pituitary somatotrophs is primarily stimulated by GHRH and ghrelin and inhibited by somatostatin through the activation of specific receptors [GHRH receptor (GHRH-R), GH secretagogue receptor (GHS-R) and somatostatin receptors (sst1-5), respectively]. However, we have shown that somatostatin, at low doses, can also stimulate GH release, directly and specifically, in primary pituitary cultures from a nonhuman primate (baboons, Papio anubis) and pigs. To determine whether somatostatin, GHRH, and ghrelin can also regulate the expression of their receptors in primates, pituitary cultures from baboons were treated for 4 h with GHRH or ghrelin (10(-8) m) or with high (10(-7) m) and low (10(-15) m) doses of somatostatin, and GH release and expression levels of all receptors were measured. GHRH/ghrelin decreased the expression of their respective receptors (GHRH-R and GHS-R). Both peptides increased sst1, only GHRH decreased sst5 expression, whereas sst2 expression remained unchanged. The effects of GHRH/ghrelin were completely mimicked by forskolin (adenylate cyclase activator) and phorbol 12-myristate 13-acetate (protein kinase C activator), respectively, indicating the regulation of receptor subtype levels by GHRH and ghrelin involved distinct signaling pathways. In contrast, high-dose somatostatin did not alter GH release but increased sst1, sst2, and sst5 expression, whereas GHRH-R and GHS-R expression were unaffected. Interestingly, low-dose somatostatin increased GH release and sst1 mRNA but decreased sst5 and GHRH-R expression, similar to that observed for GHRH. Altogether, our data show for the first time in a primate model that the primary regulators of somatotroph function (GHRH/ghrelin/somatostatin) exert both homologous and heterologous regulation of receptor synthesis which is dose and subtype dependent and involves distinct signaling pathways.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Ghrelin induces growth hormone secretion via a nitric oxide/cGMP signalling pathway

The presence of ghrelin and its receptor, growth hormone (GH) secretagogue receptor, in the hypothalamus and pituitary, and its ability to stimulate GH release in vivo and in vitro, strongly support a significant role for this peptide in the control of somatotroph function. We previously demonstrated that ghrelin elicits GH secretion directly in somatotrophs by activating two major signalling cascades, which involve inositol phosphate and cAMP. In as much as nitric oxide (NO) and its mediator cGMP have been recently shown to contribute substantially to the response of somatotrophs to key regulatory hormones, including GH-releasing hormone, somatostatin and leptin, we investigated the possible role of this signalling pathway in ghrelin-induced GH release in vitro. Accordingly, cultures of pituitary cells from prepuberal female pigs were challenged with ghrelin (10(-8) m, 30 min) in the absence or presence of activators or blockers of key steps of the NO synthase (NOS)/NO/guanylate cyclase (GC)/cGMP route and GH secretion was measured. Two distinct activators of the NO route, S-nitroso-N-acetylpenicillamine (SNAP) (5 x 10(-4) m) and L-arginine methyl ester hydrochloride (L-AME) (10(-3) m), comparably stimulated GH secretion when applied alone. The presence of L-AME enhanced ghrelin-stimulated GH secretion, whereas SNAP did not alter its effect. Conversely, two different NOS/NO pathway inhibitors, N(w)-nitro-L-arginine methyl ester hydrochloride (10(-5) m) or haemoglobin (20 microg/ml), similarly blocked ghrelin-induced (but not basal) GH release, thus indicating that NO contributes critically to ghrelin action in somatotrophs. Moreover, incubation with a permeable cGMP analogue, 8-Br-cGMP (10(-8) m) stimulated GH secretion, but did not modify the stimulatory action of ghrelin, suggesting that cGMP could mediate the action of NO. Indeed, inhibition of GC by 10 microm LY-53,583 did not alter basal GH secretion but abolished the GH-releasing action of ghrelin. Taken together, our results provide novel evidence indicating that ghrelin requires activation of the NOS/NO route, and its subsequent GC/cGMP signal transduction pathway, as necessary steps to induce GH secretion from somatotrophs.

Direct pituitary effects of kisspeptin: activation of gonadotrophs and somatotrophs and stimulation of luteinising hormone and growth hormone secretion

Recent, compelling evidence indicates that kisspeptins, the products of KiSS-1 gene, and their receptor GPR54, represent key elements in the neuroendocrine control of reproduction, and that they act primarily by regulating gonadotrophin-releasing hormone (GnRH) secretion at the hypothalamus. Conversely, and despite earlier reports showing GPR54 expression in the pituitary, the potential physiological roles of kisspeptins at this gland have remained elusive. To clarify this issue, cultures of rat pituitary cells were used to evaluate expression of KiSS-1 and GPR54, and to monitor the ability of kisspeptin-10 to stimulate Ca(2+) responses in gonadotrophs and to elicit luteinising hormone (LH) secretion in vitro. The results obtained show that both GPR54 and KiSS-1 are expressed in the pituitary of peripubertal male and female rats. Moreover, kisspeptin-10 induced a rise in free cytosolic Ca(2+) concentration ([Ca(2+)](i)) in approximately 10% of male rat pituitary cells. Intriguingly, kisspeptin-responsive cells included not only gonadotrophs, in which a 62.8 +/- 16.0%[Ca(2+)](i) rise was observed, but also somatotrophs, wherein kisspeptin induced a 60.3 +/- 5.5%[Ca(2+)](i) increase. Accordingly, challenge of dispersed pituitary cells with increasing kisspeptin-10 concentrations induced dose-related LH and growth hormone (GH) secretory responses, which were nevertheless of lower magnitude than those evoked by the primary regulators GnRH and GH-releasing hormone, respectively. In particular, 10(-8) M kisspeptin caused maximal increases in LH release (218.7 +/- 23.6% and 180.4 +/- 7.2% in male and female rat pituitary cells, respectively), and also stimulated maximally GH secretion (181.9 +/- 14.9% and 260.2 +/- 15.9% in male and female rat pituitary cells, respectively). Additionally, moderate summation of kisspeptin- and GnRH-induced LH responses was observed after short-term incubation of male rat pituitary cells. In conclusion, our results provide unequivocal evidence that kisspeptins exert direct pituitary effects in peripubertal male and female rats and suggest a possible autocrine/paracrine mode of action. The precise relevance and underlying mechanisms of this potential new actions of kisspeptins (i.e. the direct modulation of gonadotrophic and somatotrophic axis at the pituitary) deserve further analysis.

Subpopulations of somatotropes with differing intracellular calcium concentration responses to secretagogues

Multiple secretagogues stimulate the release of growth hormone (GH). The present studies examined the ability of chicken somatotropes to respond to GH secretagogues with increased intracellular calcium concentrations ([Ca2+]i). It was hypothesized that there are subsets of the somatotrope population with different responsiveness to the various secretagogues. Somatotropes were identified and distinguished from other adenohypophyseal cells, by their unique ability to respond to GH-releasing hormone with increased [Ca2+]i with immunocytochemistry used as a post-hoc confirmatory test. Large increases in [Ca2+]i (222 +/- 16 nM) were evoked by thyrotropin-releasing hormone in only 73% of the somatotropes. Similarly, [Ca2+]i was increased by perifusion with pituitary adenylate cyclase-activating peptide in 85% and by leptin but only in 51% of somatotropes. Ghrelin acutely increased [Ca2+]i in only 21% of somatotropes. Perfusion with gonadotropin-releasing hormone elevated [Ca2+]i, but in only 40% of somatotropes. The kinetics of calcium transients and the magnitude of the response differed from those observed in the presumptive gonadotropes. It is concluded that there are subsets of the somatotrope population in the anterior pituitary gland with differences in their ability to respond to various secretagogues.

Neuropeptide Y modulates growth hormone but not luteinizing hormone secretion from prepuberal gilt anterior pituitary cells in culture

Pituitary cells, from seven 160- to 170-day-old pigs, were studied in primary culture to determine the affects NPY on LH and GH secretion at the level of the pituitary. On day 4 of culture, medium was discarded, plates were rinsed twice with serum-free medium and cells were cultured in 1 ml fresh medium without serum and challenged individually with 10(-10), 10(-8) or 10(-6) M [Ala(15)]-h growth hormone-releasing factor-(1-29)NH(2) (GRF); 10(-9), 10(-8) or 10(-7) M GnRH or 10(-9), 10(-8), 10(-7) or 10(-6) M NPY individually or in combinations with 10(-9) or 10(-8) M GnRH or 10(-8) or 10(-6)M GRF. Cells were exposed to treatment for 4 h at which time medium was harvested and quantified for LH and GH. Basal LH secretion (control; n = 7 pituitaries) was 12 +/- 6 ng/well. Relative to control at 4 h, 10(-9), 10(-8) and 10(-7) M GnRH increased (P < 0.01) LH secretion by 169, 176 and 197%, respectively. Neuropeptide-Y did not alter (P > 0.4) basal LH secretion nor 10(-8) M GnRH-induced increase in LH secretion but 10(-9) M GnRH-stimulated LH secretion was reduced by NPY and was not different from control or GnRH alone. Basal GH secretion (control; n = 7 pituitaries) was 56 +/- 12 ng/well. Relative to control at 4 h, 10(-10), 10(-8) and 10(-6) M GRF increased GH secretion by 111%, 125% (P < 0.01) and 150% (P < 0.01), respectively. Only 10(-6) M (134%) and 10(-7) M (125%) NPY increased (P < 0.04) basal GH secretion. Addition of 10(-9), 10(-8) and 10(-7) M NPY in combination with 10(-8) M GRF suppressed (P < 0.04) GRF-stimulated GH secretion. However, 10(-9) M NPY enhanced (P < 0.06) the GH response to 10(-6) M GRF. These results demonstrate that NPY may directly modulate GH secretion at the level of the pituitary gland.

Differential contribution of nitric oxide and cGMP to the stimulatory effects of growth hormone-releasing hormone and low-concentration somatostatin on growth hormone release from somatotrophs

There is increasing evidence that nitric oxide (NO) produced by NO synthase (NOS), and their signalling partners, guanylyl cyclase and cGMP, play a relevant role in growth hormone (GH) secretion from somatotrophs. We previously demonstrated that both GH-releasing hormone (GHRH; 10(-8) M) and low concentrations of somatostatin (10(-15) M) stimulate pig GH release in vitro, whereas a high somatostatin concentration (10(-7) M) inhibits GHRH-induced GH secretion. To ascertain the possible contribution of the NOS-NO and guanylyl cyclase-cGMP routes to these responses, cultures of pituitary cells from prepubertal female pigs were treated (30 min) with GHRH (10(-8) M) or somatostatin (10(-7) or 10(-15) M) in the absence or presence of activators or blockers of key steps of these signalling cascades, and GH release was measured. Two distinct activators of NO route, SNAP (5x10(-4) M) or L-AME (10(-3) M), similarly stimulated GH release when applied alone (with this effect being blocked by 10(-7) M somatostatin), but did not alter the stimulatory effect of GHRH or 10(-15) M somatostatin. Conversely, two NO pathway inhibitors, NAME (10(-5) M) or haemoglobin (20 microg/ml) similarly blocked GHRH- or 10(-15) M somatostatin-stimulated GH release. 8-Br-cGMP (10(-8) to 10(-4) M) strongly stimulated GH release, suggesting that cGMP may function as a subsequent step in the NO pathway in this system. Interestingly, 10(-7) M somatostatin did not inhibit the stimulatory effect of 8-Br-cGMP. Moreover, although 8-Br-cGMP did not modify the effect of GHRH, it enhanced GH release stimulated by 10(-15) M somatostatin. Accordingly, a specific guanylyl cyclase inhibitor, LY-83, 583 (10(-5) M) did not alter 10(-15) M somatostatin-induced GH release, whereas it blocked GHRH-induced GH secretion. These results demonstrate for the first time that the NOS/NO signalling pathway contributes critically to the stimulatory effects of both GHRH and low-concentration somatostatin on GH release, and that, conversely, the subsequent guanylyl cyclase/cGMP step only mediates GHRH- and not low-concentration somatostatin-induced GH secretion from somatotrophs.

Intracellular signaling mechanisms mediating ghrelin-stimulated growth hormone release in somatotropes

Ghrelin is a newly discovered peptide that binds the receptor for GH secretagogues (GHS-R). The presence of both ghrelin and GHS-Rs in the hypothalamic-pituitary system, together with the ability of ghrelin to increase GH release, suggests a hypophysiotropic role for this peptide. To ascertain the intracellular mechanisms mediating the action of ghrelin in somatotropes, we evaluated ghrelin-induced GH release from pig pituitary cells both under basal conditions and after specific blockade of key steps of cAMP-, inositol phosphate-, and Ca2+-dependent signaling routes. Ghrelin stimulated GH release at concentrations ranging from 10-10 to 10-6 m. Its effects were comparable with those exerted by GHRH or the GHS L-163,255. Combined treatment with ghrelin and GHRH or L-163,255 did not cause further increases in GH release, whereas somatostatin abolished the effect of ghrelin. Blockade of phospholipase C or protein kinase C inhibited ghrelin-induced GH secretion, suggesting a requisite role for this route in ghrelin action. Unexpectedly, inhibition of either adenylate cyclase or protein kinase A also suppressed ghrelin-induced GH release. In addition, ghrelin stimulated cAMP production and also had an additive effect with GHRH on cAMP accumulation. Ghrelin also increased free intracellular Ca2+ levels in somatotropes. Moreover, ghrelin-induced GH release was entirely dependent on extracellular Ca2+ influx through L-type voltage-sensitive channels. These results indicate that ghrelin exerts a direct stimulatory action on porcine GH release that is not additive with that of GHRH and requires the contribution of a multiple, complex set of interdependent intracellular signaling pathways.

The in vitro regulation of growth hormone secretion by orexins

Orexins, orexigenic neuropeptides, have recently been discovered in lateral hypothalamus and play an important role in the regulation of pituitary hormone secretion. Two subtypes of orexin receptors (orexin-1 and orexin-2) have been demonstrated in pituitaries. In this experiment, the effects of orexins on voltage-gated Ca2+ currents and the GH release in primary cultured ovine somatotropes were examined. Voltage-gated Ca2+ currents were isolated in ovine somatotropes as L, T, and N currents using whole-cell patch-clamp techniques and specific Ca2+ channel blocker and toxin. Application of orexin-A or orexin-B (100 nM) significantly, dose-dependently, and reversibly increased only nifedipine-sensitive L-type Ca2+ current. Inhibitors of PKC (calphostin C, PKC inhibitory peptide) but not inhibitors of PKA (H89, PKA inhibitory peptide) cancelled the increase in the L current by orexins. Co-administration of orexin-A and GHRH (10 nM) showed an additive effect on the L current. Specific intracellular Ca2+-store-depleting reagent, thapsigargin (1 microM), did not affect the orexin-induced increase in the L current. Orexin-B alone slightly increased GH release and co-administration of orexin-A and GHRH synergistically stimulated GH secretion in vitro. It is therefore suggested that orexins may play an important role in regulating GHRH-stimulated GH secretion through an increase in the L-type Ca2+ current and the PKC-mediated signaling pathways in ovine somatotropes.

Secretory plasticity of pituitary cells: a mechanism of hormonal regulation

Pituitary somatotropes and melanotropes have enabled us to investigate the molecular basis and functional dynamics underlying secretory plasticity, an ability of endocrine cells to adapt their activity to the changing physiologic requirements, which generates discrete cell subpopulations within each cell hormonal type. Porcine somatotropes comprise two morphologically distinct subpopulations of low- (LD) and high-density (HD) cells, separable by Percoll gradient, that respond differently to hypothalamic regulators. In LD somatotropes, somatostatin (SRIF) inhibits growth hormone (GH)-releasing hormone (GHRH)-induced GH secretion. Conversely, SRIF alone stimulates GH release from HD somatotropes. These disparate SRIF actions entail a molecular signaling heterogeneity, in that SRIF increases cAMP levels in HD but not in LD cells as a requisite to stimulate GH release. GHRH-stimulated GH release also involves differential signaling in LD and HD cells: although it acts primarily through the cAMP/extracellular Ca2+ route in both somatotrope subsets, full response of LD somatotropes also requires the inositol phosphate/intracellular Ca2+ pathway. Amphibian melanotropes, which regulate skin adaptation to background color by secreting POMC-derived alpha-melanocyte-stimulating hormone (alphaMSH), also comprise two subpopulations with divergent secretory phenotypes. LD melanotropes show high biosynthetic and secretory activities and high responsiveness to multiple hypothalamic factors. Conversely, HD melanotropes constitute a hormone-storage subset poorly responsive to regulatory inputs. Interestingly, in black-adapted animals most melanotropes acquire the highly-secretory LD phenotype, whereas white-background adaptation, which requires less alphaMSH, converts melanotropes to the storage HD phenotype. These same interconversions can be reproduced in vitro using appropriate hypothalamic factors, thus revealing the pivotal role of the hypothalamus in regulating the functional dynamics of the secretory plasticity. Furthermore, this regulation likely involves a precise control of the secretory pathway, as suggested by the differential distribution in LD and HD melanotropes of key components of the intracellular transport, processing, and storage of secretory proteins. Hence, molecular signaling heterogeneity and unique secretory pathway components seem to relevantly contribute to the control of secretory plasticity, thereby enabling endocrine cells to finely adjust their dynamic response to the specific hormonal requirements.

Somatostatin stimulates GH secretion in two porcine somatotrope subpopulations through a cAMP-dependent pathway

Somatostatin (SRIF) inhibits GH release from rat somatotropes by reducing adenylate cyclase (AC) activity and the free cytosolic calcium concentration ([Ca(2+)](i)). In contrast, we have reported that SRIF can stimulate GH release in vitro from pig somatotropes. Specifically, 10(-7) and 10(-15) M SRIF stimulate GH release from a subpopulation of high density (HD) somatotropes isolated by Percoll gradient centrifugation, whereas in low density (LD) somatotropes only 10(-15) M SRIF induces such an effect. To ascertain the signaling pathways underlying this phenomenon, we assessed SRIF effects on second messengers in cultured LD and HD cells by measuring cAMP, IP turnover, and [Ca(2+)](i). Likewise, contribution of the corresponding signaling pathways to SRIF-induced GH release was evaluated by blocking AC, PLC, extracellular Ca(2+) influx, or intracellular Ca(2+) mobilization. Both 10(-7) and 10(-15) M SRIF increased cAMP, IP turnover, and [Ca(2+)](i) in HD cells. Conversely, in LD cells 10(-7) M SRIF reduced [Ca(2+)](i), but did not alter cAMP or IP, and 10(-15) M SRIF was without effect. Interestingly, SRIF-stimulated GH release was abolished in both subpopulations by AC blockade, but not by PLC inhibition. Furthermore, SRIF-induced GH release was not reduced by blockade of extracellular Ca(2+) influx through voltage-sensitive channels or by depletion of thapsigargin-sensitive intracellular Ca(2+) stores. Therefore, SRIF stimulates GH secretion from cultured porcine somatotrope subpopulations through an AC/cAMP pathway-dependent mechanism that is seemingly independent of net increases in IP turnover or [Ca(2+)](i). These novel actions challenge classic views of SRIF as a mere inhibitor for somatotropes and suggest that it may exert a more complex, dual function in the control of porcine GH release, wherein molecular heterogeneity of somatotropes would play a critical role.

Expression of Ca(2+)-mobilizing endothelin(A) receptors and their role in the control of Ca(2+) influx and growth hormone secretion in pituitary somatotrophs

The expression and coupling of endothelin (ET) receptors were studied in rat pituitary somatotrophs. These cells exhibited periods of spontaneous action potential firing that generated high-amplitude fluctuations in cytosolic calcium concentration ([Ca(2+)](i)). The message and the specific binding sites for ET(A), but not ET(B), receptors were found in mixed pituitary cells and in highly purified somatotrophs. The activation of these receptors by ET-1 led to an increase in inositol 1,4,5-trisphosphate production and the associated rise in [Ca(2+)](i) and growth hormone (GH) secretion. The Ca(2+)-mobilizing action of ET-1 lasted for 2-3 min and was followed by an inhibition of action potential-driven Ca(2+) influx and GH secretion to below the basal levels. As in somatostatin-treated cells, the ET-1-induced inhibition of spontaneous electrical activity and Ca(2+) influx was accompanied by the inhibition of adenylyl cyclase and by the stimulation of inward rectifier potassium current. In contrast to somatostatin, ET-1 did not inhibit voltage-gated Ca(2+) channels. During prolonged agonist stimulation a gradual recovery of Ca(2+) influx and GH secretion occurred. In somatotrophs treated with pertussis toxin overnight, the ET-1-induced Ca(2+)-mobilizing phase was preserved, but it was followed immediately by facilitated Ca(2+) influx and GH secretion. Both somatostatin- and ET-1-induced inhibitions of adenylyl cyclase activity were abolished in pertussis toxin-treated cells. These results indicate that the transient cross-coupling of Ca(2+)-mobilizing ET(A) receptors to the G(i)/G(o) pathway in somatotrophs provides an effective mechanism to change the rhythm of [Ca(2+)](i) signaling and GH secretion during continuous agonist stimulation.

Growth hormone (GH)-releasing factor differentially activates cyclic adenosine 3',5'-monophosphate- and inositol phosphate-dependent pathways to stimulate GH release in two porcine somatotrope subpopulations

Somatotropes comprise two morphologically and functionally distinct subpopulations of low (LD) and high (HD) density cells. We recently reported that GRF induces different patterns of increase in the cytosolic free Ca2+ concentration in single porcine LD and HD somatotropes, which for LD cells required not only Ca2+ influx but also intracellular Ca2+ mobilization. This suggested that GRF may activate multiple signaling pathways in pig LD and HD somatotropes to stimulate GH secretion. To address this question, we first assessed the direct GRF effect on second messenger activation in cultures of LD and HD cells by measuring cAMP levels and [3H]myo-inositol incorporation. Secondly, to determine the relative importance of cAMP- and inositol phosphate (IP)-dependent pathways, and of intra- and extracellular Ca2+, GRF-induced GH release from cultured LD and HD somatotropes was measured in the presence of specific blockers. GRF increased cAMP levels in both subpopulations, whereas it only augmented IP turnover in LD cells. Accordingly, adenylate cyclase inhibition by MDL-12,330A abolished GRF-stimulated GH release in both subpopulations, whereas phospholipase C inhibition by U-73122 only reduced this effect partially in LD cells. Likewise, blockade of Ca2+ influx with Cl2Co reduced GRF-stimulated GH secretion in both LD and HD somatotropes, whereas depletion of thapsigargin-sensitive intracellular Ca2+ stores only decreased the secretory response to GRF in LD cells. These results demonstrate that GRF specifically and differentially activates multiple signaling pathways in two somatotrope subpopulations to stimulate GH release. Thus, although the prevailing signaling cascade employed by GRF in both subpopulations is adenylate cyclase/cAMP/extracellular Ca2+, the peptide also requires activation of the phospholipase C/IP/intracellular Ca2+ pathway to exert its full effect in porcine LD somatotropes.

Pituitary adenylate cyclase-activating polypeptides 38 and 27 increase cytosolic free Ca2+ concentration in porcine somatotropes through common and distinct mechanisms

Ca2+ plays an essential role in pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated growth hormone (GH) secretion from porcine somatotropes. Here, Indo-1 microfluorimetry was used to investigate the dynamics of free cytosolic Ca2+ concentration ([Ca2+]i) in single porcine somatotropes in response to PACAP38 and PACAP27. We also evaluated the relative contributions of extra- and intracellular Ca2+ sources and of cAMP-dependent protein kinase (PKA) and phospholipase C (PLC). A high proportion of somatotropes responded to PACAP38 (79.4%) and PACAP27 (68.4%) with [Ca2+]i rises that could be followed by a refractory plateau (type 1 response), or by a decrease in [Ca2+]i during which somatotropes were responsive to a subsequent PACAP pulse (type II response). Although Ca2+ profiles in response to both peptides were similar, PACAP38-induced [Ca2+]i rises were higher. Somatotrope response to PACAP38 or PACAP27 was markedly reduced by removing extracellular Ca2+, blocking Ca2+ entry through L-type voltage sensitive Ca2+ channels (VSCC), or inhibiting PKA. Conversely, Ca2+ depletion from intracellular stores or PLC inactivation did not affect the response to PACAP27 but considerably reduced maximal [Ca2+]i induced by PACAP38. We conclude that both peptides stimulate extracellular Ca2+ influx through L-type VSCC by a PKA-dependent mechanism. However, PACAP38 also triggers a PLC-mediated Ca2+ mobilization from intracellular stores, thereby indicating that the two molecular forms of PACAP activate common and distinct second messenger pathways within porcine somatotropes.