Human growth hormone releasing factor (hGRF) modulates calcium currents in human growth hormone secreting adenoma 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.

Involvement of tetrodotoxin-resistant Na+ current and protein kinase C in the action of growth hormone (GH)-releasing hormone on primary cultured somatotropes from GH-green fluorescent protein transgenic mice

GHRH depolarizes the membrane of somatotropes, leading to an increase in intracellular free Ca2+ concentration and GH secretion. Na+ channels mediate the rapid depolarization during the initial phase of the action potential, and this regulates Ca2+ influx and GH secretion. GHRH increases a tetrodotoxin-sensitive somatotrope Na+ current that is mediated by cAMP. TTX-resistant (TTX-R) Na+ channels are abundant in sensory neurons and cardiac myocytes, but their occurrence and/or function in somatotropes has not been investigated. Here we demonstrate expression of TTX-R Na+ channels and a TTX-R Na+ current, using patch-clamp method, in green fluorescent protein-GH transgenic mouse somatotropes. GHRH (100 nm) increased the TTX-R Na+ current in a reversible manner. The GHRH-induced increase in TTX-R Na+ current was not affected by the cAMP antagonist Rp-cAMP or protein kinase A inhibitors KT5720 or H89. The TTX-R current was increased by 8-bromoadenosine-cAMP (cAMP analog), forskolin (adenylyl-cyclase activator), and 3-isobutyl-1-methylxanthine (phosphodiesterase inhibitor), but the additional, GHRH-induced increase in TTX-R Na+ currents was not affected. U-73122 (phospholipase C inhibitor) and protein kinase C (PKC) inhibitors, Gö-6983 and chelerythrine, blocked the effect of GHRH. PKC activators, phorbol dibutyrate and phorbol myristate acetate, increased the TTX-R Na+ current, but GHRH had no further effect on the current. Na+-free extracellular medium significantly reduced GHRH-stimulated GH secretion. We conclude that GHRH-induced increase in the TTX-R Na+ current in mouse somatotropes is mediated by the PKC system. An increase in the TTX-R Na+ current may contribute to the GHRH-induced exocytosis of GH granules from mouse somatotropes.

Involvement of somatostatin receptor subtypes in membrane ion channel modification by somatostatin in pituitary somatotropes

1. Growth hormone (GH) secretion from pituitary somatotropes is mainly regulated by two hypothalamic hormones, GH-releasing hormone (GHRH) and somatotrophin releasing inhibitory factor (SRIF). 2. Somatotrophin releasing inhibitory factor inhibits GH secretion via activation of specific membrane receptors, somatostatin receptors (SSTRs) and signalling transduction systems in somatotropes. 3. Five subtypes of SSTRs, namely SSTR1, 2, 3, 4 and 5, have been identified, with the SSTR2 subtype divided into SSTR2A and SSTR2B. All SSTRs are G-protein-coupled receptors. 4. Voltage-gated Ca(2+) and K(+) channels on the somatotrope membrane play an important role in regulating GH secretion and SRIF modifies both channels to reduce intracellular free Ca(2+) concentration and GH secretion. 5. Using specific SSTR subtype-specific agonists, it has been found that reduction in Ca(2+) currents by SRIF is mediated by SSTR2 and an increase in K(+) currents is mediated by both SSTR2 and SSTR4 in rat somatotropes.

Effect of gsp oncogene on somatostatin receptor subtype 1 and 2 mRNA levels in GHRH-responsive GH3 cells

Growth hormone releasing hormone (GHRH) signals via G protein-coupled receptors (GHRH-R) to enhance intracellular Galphas/adenylyl cyclase/cAMP signaling, which in turn has positive effects on GH synthesis and release, as well as proliferation of the GH-producing cells of the anterior pituitary gland. Some GH-producing pituitary tumors express a constitutively active mutant form of Galphas (gsp oncogene). It has been reported that these tumors are more responsive to octreotide therapy. In this study we used a rat GH-producing cell line (GH3) stably transfected with the human GHRH-R cDNA (GH3-GHRHR cells) as a model to study the effects of gsp oncogene on somatostatin (SRIH) receptor subtype 1 and 2 (sst1 and sst2) mRNA levels. Transient transfection of gsp oncogene in GH3-GHRHR cells for 48 h increased intracellular cAMP levels and GH release. Phosphodiesterase (PDE) 4, sst1 and sst2 mRNA levels were increased by G protein mutation as assessed by real-time RT-PCR. Increased PDE mRNA levels in gsp-transfected cells may be a compensatory mechanism to the constitutive activation of cAMP-dependent pathway by G protein mutation and is consistent with reports of higher PDE expression in human pituitary tumor that express gsp. Our data suggest that higher expression of sst1 and sst2 mRNA induced by the gsp oncogene may be a mechanism by which gsp-positive tumors show a greater response to SRIH. GH3 cells permanently transfected with GHRH-R can be used for in vitro studies of actions of GHRH.

Diverse intracellular signalling systems used by growth hormone-releasing hormone in regulating voltage-gated Ca2+ or K channels in pituitary somatotropes

Influx of Ca2+ via Ca2+ channels is the major step triggering exocytosis of pituitary somatotropes to release growth hormone (GH). Voltage-gated Ca2+ and K+ channels, the primary determinants of the influx of Ca2+, are regulated by GH-releasing hormone (GHRH) through G-protein-coupled intracellular signalling systems. Using whole-cell patch-clamp techniques, the changes of the Ca2+ and K+ currents in primary cultured ovine and human somatotropes were recorded. Growth hormone-releasing hormone (10 nmol/L) increased both L- and T-type voltage-gated Ca2+ currents. Inhibition of the cAMP/protein kinase A (PKA) pathway by either Rp-cAMP or H89 blocked this increase in both L- and T-type Ca2+ currents. Growth hormone-releasing hormone also decreased voltage-gated transient (IA) and delayed rectified (IK) K+ currents. Protein kinase C (PKC) inhibitors, such as calphostin C, chelerythrine or downregulation of PKC, blocked the effect of GHRH on K+ currents, whereas an acute activation of PKC by phorbol 12, 13-dibutyrate (1 micromol/L) mimicked the effect of GHRH. Intracellular dialysis of a specific PKC inhibitor (PKC19-36) also prevented the reduction in K+ currents by GHRH. It is therefore concluded that GHRH increases voltage-gated Ca2+ currents via cAMP/PKA, but decreases voltage-gated K+ currents via the PKC signalling system. The GHRH-induced alteration of Ca2+ and K+ currents augments the influx of Ca2+, leading to an increase in [Ca2+]i and the GH secretion.

Growth hormone secretagogue actions on the pituitary gland: multiple receptors for multiple ligands?

1. Growth hormone (GH) secretion is thought to occur under the reciprocal regulation of two hypothalamic hormones, namely GH-releasing hormone (GHRH) and somatostatin (SRIF), through their engagement with specific cell-surface receptors on the anterior pituitary somatotropes. 2. In addition to GHRH and SRIF, synthetic GH-releasing peptides (GHRP) or GH secretagogue(s) (GHS) regulate GH release through the activation of a novel receptor, the GHS receptor (GHS-R). 3. The cloning of the GHS-R from human, swine and rat identifies a novel G-protein-coupled receptor involved in the control of GH secretion and supports the existence of an undiscovered hormone that may activate this receptor. 4. Varieties of intracellular signalling systems are suggested to mediate the action of GHS, which include changes in intracellular free Ca2+ ([Ca2+]i), cAMP, protein kinases A and C, phospholipase C etc. 5. With regard to the use of signalling systems by GHS, especially a new form of GHRP or GHRP-2, a clear species difference has been demonstrated, supporting the possibility of more than one type of GHS-R.

Growth hormone-releasing hormone decreases voltage-gated potassium currents in GH4C1 cells

The electrophysiological properties of anterior pituitary somatotropes integrally involve the function of voltage-gated K+ currents. In this study, we have used GH4C1 cell lines to investigate the effect of human GHRH on voltage-gated K+ currents. Because of a clear 'rundown' of the K+ current with classic whole cell recording (WCR) without ATP in pipette solution, nystatin-perforated WCR was the major recording configuration used. Using a low Ca2+ (0.5 mM) bath solution containing Co2+ (1 mM) and TTX (1 microM), GH4 cells predominantly exhibited an outward delayed rectifier K+ current (IK). Local application of growth hormone releasing hormone (GHRH) (100 nM) reversibly reduced the amplitude of the K+ currents (to 83% of control). There was no effect of GHRH on the activation curve of the K+ current and no difference observed using 2.5 mM Ca2+ or low Ca2+ (0.5 mM Ca2++1 mM Co2+) bath solutions. Under the condition of low Ca2+ bath solution, the application of apamin (1 microM) or charybdotoxin (1 microM), two specific blockers of the Ca2+-activated K+ current, did not alter the K+ current or the response to GHRH. This reduction in the K+ current by GHRH was also observed with classic WCR with a pipette solution containing ATP (2 mM). The GHRH-induced reduction in the K+ current was completely abolished by the presence of GDP-beta-s (500 microM) in the pipette solution or by addition of PKC inhibitors, calphostin C (100 nM) and chelerythrine (1 microM), in bath solution. Inhibitor for cAMP-PKA system (Rp-cAMP and H89) did not affect the K+ current response to GHRH. These results suggest that GHRH reduces the voltage-gated K+ current in GH4C1 cells, a response that is mediated by G-proteins and PKC system but not by cAMP-PKA system. The reduction in the K+ current may partially contribute to the GHRH-stimulated growth hormone (GH) secretion.

Human GHRH reduces voltage-gated K+ currents via a non-cAMP-dependent but PKC-mediated pathway in human GH adenoma cells

1. Whole-cell voltage-gated K+ currents and the K+ current response to growth hormone-releasing hormone (GHRH) were characterised in primary cultures of human acromegalic somatotropes. 2. Both delayed rectifier (IK) and transient (IA) K+ currents were recorded from human somatotropes held at -80 mV and bathed in a solution containing Cd2+ (1 mM), TTX (1 microM) and a low concentration of Ca2+ (0.5 mM). Only IK was recorded, however, when a holding potential of -40 mV was used. 3. GHRH (10 nM) immediately and significantly reduced the amplitude of both IA and IK. While the reduction in the amplitude of IA was fully reversed following the removal of GHRH, the amplitude of IK had only partially recovered 10 min after GHRH removal. In addition, GHRH shifted the voltage-dependent inactivation curve of IA by 13.5 mV in the negative direction. 4. In a low Ca2+ and Cd2+-containing solution, the Ca2+-activated K+ channel blockers apamin (100 nM and 1 microM) and charybdotoxin (1 microM) did not alter K+ currents or the effect of GHRH on the recorded K+ currents. 5. The whole-cell K+ currents and their responses to GHRH were unaffected by the application of 8-bromo-cAMP (100 microM), Rp-cAMP (100 microM) or the protein kinase A (PKA) inhibitor H89 (1 microM). In addition, intracellular dialysis of the PKA inhibitory peptide PKI (10 microM) had no effect on the K+ current response to GHRH. 6. While the application of protein kinase C (PKC) inhibitors calphostin C (100 nM) or chelerythrine (1 microM) did not affect the amplitude of the K+ currents, the K+ current response to GHRH was significantly attenuated. Downregulation of PKC with phorbol 12,13-dibutyrate (PDBu, 0.5 microM for 16 h) also abolished the K+ current response to GHRH. In addition, intracellular dialysis of somatotropes with the PKC inhibitory peptide PKC19-36 (1 microM) prevented the GHRH-induced decrease in the amplitude of the voltage-gated K+ currents. Local application of PDBu (1 microM) significantly reduced the amplitude of the voltage-gated K+ currents in a similar manner to that induced by GHRH, but without clear recovery upon removal. 7. This study demonstrates that GHRH decreases voltage-gated K+ currents via a PKC-mediated pathway in human adenoma somatotropes, rather than by the cAMP-PKA pathway that is usually implicated in the actions of GHRH.

Estrogen transiently increases delayed rectifier, voltage-dependent potassium currents in ovine gonadotropes

Treatment of gonadotropes with estrogen (E) changes the electrophysiological response to gonadotropin-releasing hormone (GnRH) such that the cells are hyperpolarised immediately after stimulation with GnRH and then generate action potentials more frequently than non-E-treated cells. We investigated the role of K+ current in this altered response to GnRH using cultures of ewe pituitary cells enriched for gonadotropes. K+ current density was measured using nystatin-perforated whole-cell recordings in the voltage clamp mode. Treatment of cells with E for 16-20 h significantly (p < 0.01) increased the unit K+ current to 180% of that in vehicle-treated cells. Outward current in these cells flows predominantly through voltage-dependent, delayed rectifier K+ channels (IK), and E alters the magnitude of this current. The effect of E to increase the K+ current was dose- and time-dependent and was maximal after 16-20 h. The unit K+ current values returned to pre-treatment levels after 36 h of E treatment. Several cells were studied both before and after E treatment and the average effect of E on these cells was to increase the unit K+ current by 90%. The time-course of the effect of E on K+ current density is the same as the effect of E to increase LH release in vitro and in vivo. We conclude that the increase in K+ current may be an important part of the mechanism whereby E acts on gonadotropes to facilitate the LH surge which triggers ovulation.

Differential management of Ca2+ oscillations by anterior pituitary cells: a comparative overview

Most electrical and ionic properties of anterior pituitary cells are common to all pituitary cell types; only gonadotropes exhibit a few cell specific features. Under basal conditions, the majority of pituitary cells in vitro, irrespective of their cell type, display spontaneous action potentials and [Ca2+]i transients that result from rhythmic Ca2+ entry through L-type Ca2+ channels. The main function of these action potentials is to maintain cells in a readily activable responsive state. We propose to call this state a 'pacemaker mode', since it persists in the absence of extrinsic stimulation. When challenged by hypothalamic releasing hormones, cells exhibit two distinct response patterns: amplification of pacemaker activity or shift to internal Ca2+ release mode. In the internal Ca2+ release mode, [Ca2+]i oscillations are not initiated by entry of external Ca2+, but by release of Ca2+ from intracellular stores. In somatotropes and corticotropes, GHRH or CRH triggers the pacemaker mode in silent cells and amplifies it in spontaneously active cells. In contrast, in gonadotropes GnRH activates the internal Ca2+ release mode in silent cells and switches already active cells from the pacemaker to the internal Ca2+ release mode. Interestingly, homologous normal and tumoral cells display the same type of activity in vitro, in the absence or presence of hypothalamic hormones. Pacemaker and internal Ca2+ release modes are likely to serve different purposes. Pacemaker activity allows long-lasting sequences of [Ca2+]i oscillations (and thus sustained periods of secretion) that stop under the influence of hypothalamic inhibitory peptides. In contrast, the time during which cells can maintain internal Ca2+ release mode depends upon the importance of intracellular Ca2+ stores. This mode is thus more adapted to trigger secretory peaks of large amplitude and short duration. On the basis of these observations, theoretical models of pituitary cell activity can be proposed.

Effect of growth hormone-releasing peptide-2 (GHRP-2) and GH-releasing hormone (GHRH) on the the cAMP levels and GH release from cultured acromegalic tumours

There is a difference between the sheep and rat somatotrophs in the response to growth hormone-releasing peptide-2 (GHRP-2), which raises the question of what the response may be in human somatotrophs. In the present study, cells were obtained from seven human acromegalic tumours and the effects of GHRP-2 were studied. Cells were dissociated and kept in primary culture for 1-3 weeks before experimentation. Application of GHRP-2 for 30 min induced a significant increase in GH secretion from the cultured cells from all seven tumours whereas human GH-releasing hormone (hGHRH) at a dose of 10 nM induced a significant GH release in only four of seven tumours. The intracellular levels of cAMP in all seven tumours were significantly increased by both 10 nM GHRP-2 and GHRH, but the response to GHRH was significantly higher than the response to GHRP-2. The adenylyl cyclase inhibitor, MDL 12330A, blocked the effect of GHRH and GHRP-2 on intracellular cAMP levels, whereas the Ca2+ channel blocker Co2+ (0.5 mM) did not attenuate the cAMP response. For the tumours in which GH secretion was increased by GHRH and GHRP-2, the cAMP antagonist Rp-cAMP blocked the GH response to GHRH but not to GHRP-2. When a protein kinase A (PKA) inhibitor (H89) was applied, GHRH stimulated GH release was blocked, but cAMP accumulation was not affected. The response to GHRP-2 was not altered by H89. Calphostin C [a protein kinase C (PKC) inhibitor] reduced the effect of GHRP-2 on the secretion of GH but did not affect the response to GHRH. Both GHRH and GHRP-2 increased the intracellular Ca2+ concentration in a concentration-dependent manner. We conclude that (1) GHRH increases GH secretion from human GH tumours via the cAMP pathway whereas GHRP-2 increases GH secretion mainly via the PKC pathway; (2) GHRH increases cAMP (without GH release) in a subset of tumours whereas GHRP-2 increases cAMP levels (slightly) and GH secretion in all tumours; and (3) GHRP-2 and GHRH do not act on the same receptor on human somatotrophs derived from acromegalic tumours.

Regulation by growth hormone-releasing hormone and somatostatin of a Na+ current in the primary cultured rat somatotroph

The purpose of the present study is to characterize Na+ current activated by GH-releasing hormone (GHRH) and to investigate the effect of somatostatin (SRIF) on that current, because the Na+ current has been suggested to play a pivotal role in the process of GHRH-induced GH secretion. Primary-cultured pituitary somatotrophs were prepared from male Wistar rats. Whole-cell membrane currents were recorded and analyzed by a perforated patch clamp system. To isolate Na+ current, K+ and Ca2+ were replaced with Cs+ and Mg2+, respectively, in the extracellular solution, and cesium aspartate was used for the pipette solution. Furthermore, tetrodotoxin and nifedipine were added to the extracellular solution to eliminate the voltage-gated currents. Under these conditions, GHRH activated a mean inward Na+ current (-1.86 +/- 0.33 pA, mean +/- SE) at potentials between -50 and -20 mV and a smaller current (-0.59 +/- 0.13 pA) at potentials between -100 and -80 mV, which were completely blocked by protein kinase A blocker (H-89). In addition, SRIF (1-10 nM) partially suppressed these Na+ currents, which were not affected by phosphatase inhibitors (okadaic acid and calyculin A). These results suggest that GHRH activates the Na+ current through phosphorylation by protein kinase A and that SRIF partially suppressed this current and that the current was larger at more positive potentials than at more negative potentials.

Withdrawal of somatostatin augments L-type Ca2+ current in primary cultured rat somatotrophs

It is known that withdrawal of somatostatin (SRIF) augments the growth hormone (GH) releasing hormone (GRF)-induced GH secretion. To investigate the mechanism of this augmentation in GH secretion, effects of GRF and SRIF on L-type Ca2+ current (Ba2+ was used as a charge carrier) or primary cultured rat somatotroph were studied by perforated patch clamp technique. The reason is that GRF-induced GH secretion is thought to be causally related to the influx of Ca2+ through L-type Ca2+ channels. 10 mM GRF augmented maximum amplitude of L-type Ba2+ current by 12.2% (n = 12). Subsequent application of SRIF slightly suppressed the currents but the suppression never exceeded the control level of the current. Removal of SRIF, however, promptly augmented the L-type Ba2+ current by 26.8%. Such off-response of SRIF was not observed in cells treated overnight with 100 ng/ml pertussis toxin. Further, specific inhibitor of protein kinase A, H-89 at 1 microM reversibly suppressed the augmentation of L-type Ba2+ current to control level. At 10 microM, H-89 suppressed L-type Ba2+ current by more than 40% from control level. These results suggest that (1) L-type Ca2+ channel of somatotroph is probably phosphorylated in a basal condition and may be slightly modulated by GRF through increased level of cAMP; (2) SRIF only slightly suppress the channel activity; (3) Withdrawal of SRIF facilitates the activity of L-type Ca2+ channel via PTX-sensitive G-protein, although the precise mechanism of this facilitation is unknown. The augmentation by SRIF-pretreatment of GRF-induced GH secretion may be at least partly due to the facilitation of the activity of L-type Ca2+ channel.

Modulation of Ca2+ influx in the ovine somatotroph by growth hormone-releasing factor

Voltage-gated Ca2+ currents were recorded using the nystatin-perforated whole cell recording configuration on the ovine somatotrophs. With the use of Ca(2+)-tetraethylammonium chloride bath solution and Cs+ electrode solution, two types of Ca2+ currents were obtained with a predominant long-lasting (L) current blocked by nifedipine. A transient (T) current was isolated in the presence of nifedipine (3 microM) and was not blocked by omega-conotoxin (5 microM), but diminished to 47 +/- 5% of control by Ni2+ (0.3 mM) or to 52 +/- 10% of control by amiloride (0.5 mM). The nifedipine-blockable L-type current was not affected by omega-conotoxin (5 microM); it was, however, attenuated to 80 +/- 4% of control by Ni2+ (0.3 mM) and to 48 +/- 6% of control by amiloride (0.5 nM). Cd2+ (1 mM) totally prevented both T and L currents. Application of growth hormone-releasing factor (GRF, 10 nM) reversibly increased the amplitude of both Ca2+ currents without modifying their kinetic properties. The effect of GRF was observed approximately 30 s after application, peaked (142 +/- 11% of control, n = 5) rapidly, and lasted > 10 min if GRF treatment was continuous. Intracellular Ca2+ concentration ([Ca2+]i) was increased by GRF (10 nM) within seconds, reaching a peak within 30 s and lasting > 250 s. Blockade of Ca2+ channels (Cd2+, 1 mM) or the use of Ca(2+)-free solution reduced basal [Ca2+]i and significantly (P < 0.05) diminished the effect of GRF on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion channels and the signal transduction pathways in the regulation of growth hormone secretion

The secretion of GH from pituitary somatotrophs is mainly regulated by alterations in the levels of intracellular free Ca(2+) concentrations ([Ca(2+)](i)) that depend on the influx of Ca(2+) through voltage-gated Ca(2+) channels in the cell membrane. Hypothalamic stimulatory and inhibitory factors bind to specific receptors on the cell membrane to regulate membrane potential and activate second-messenger systems. The receptors are G-protein coupled, and activated G proteins directly influence membrane ion channels to regulate Ca(2+) influx. The function of cAMP-dependent protein kinase A is also modulated by receptor-coupled G proteins leading to the phosphorylation of Ca(2+) channel proteins and further alteration of Ca(2+) influx.

Actions of growth-hormone-releasing hormone on rat pituitary cells: intracellular calcium and ionic currents

Actions of growth-hormone-releasing hormone (GHRH) on single rat anterior pituitary cells were studied using indo-1 fluorescence to monitor changes in intracellular calcium, [Ca2+]i, and perforated-patch recording to measure changes in membrane potential and ionic currents. GHRH elevated [Ca2+]i in non-voltage-clamped cells by a mechanism that was dependent upon extracellular Na+ and Ca2+ and was blocked by the dihydropyridine Ca(2+)-channel blocker, nitrendipine. Resting cells had a fluctuating membrane potential whose a mean value depolarized by 9 mV in response to GHRH. The membrane-permeant cAMP analogue, 8-(4-chlorophenylthio)cAMP, mimicked the action of GHRH on membrane potential. Under voltage clamping, GHRH activated a small inward current (1-5 pA). Two types of response could be distinguished. The type I response had an inward current that was largest at more negative potentials (-90 mV), and the type II response had inward current that was larger at more positive potentials (-40 to -70 mV). Both types of response were reversible and blocked by removal of extracellular Na+. These results suggest that the rise in [Ca2+]i produced by GHRH in non-voltage-clamped cells results from the activation via cAMP of a Na(+)-dependent conductance, which depolarizes the cell and increases the Ca2+ influx through voltage-gated Ca2+ channels.

Effects in vitro of new growth hormone releasing peptide (GHRP-1) on growth hormone secretion from ovine pituitary cells in primary culture

Continuous perifusion of pituitary cells was used to study the effects of a newly synthesized GHRP (GHRP-1 or KP 101) on growth hormone (GH) secretion from ovine pituitary cells and these have been compared to effects of growth hormone-releasing factor (GRF) and the original growth hormone-releasing peptide (GHRP-6). GH was continuously released at a constant rate during perifusion and secretion was increased by KP 101, GHRP-6 and GRF in a dose-dependent manner. The half-maximal effective dose of KP 101 and GHRP-6 was 10(-7) M, an order of magnitude higher than that for GRF. The maximal effects of KP 101 and GHRP-6 were similar but significantly less than the maximal effect of GRF. Blockade of calcium channels with Cd2+ (2 mM) totally and reversibly abolished the releasing effects of all three peptides. Like GHRP-6, the GH release induced by KP 101 was not affected by a GRF antagonist ([Ac-Tyr1, D-Arg2]-GRF 1-29, 1 microM) which significantly reduced the effect of GRF on GH release. For each peptide, the response to a second application (1 h after the first application) was lower than the first response. When GRF (or KP 101, GHRP-6) was applied first and then KP 101 or GHRP-6 (or GRF) given 1 h later, the second response was not attenuated. Only a small additive effect on the release of GH by GRF was obtained by the co-administration of either KP 101 or GHRP-6. This result was achieved with maximal doses of the peptides, but not with half-maximal doses.(ABSTRACT TRUNCATED AT 250 WORDS)