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Endocrinology Vol. 138, No. 12 5096-5100
Copyright © 1997 by The Endocrine Society

ARTICLES

Regulation by Growth Hormone-Releasing Hormone and Somatostatin of a Na+ Current in the Primary Cultured Rat Somatotroph1

Masakatsu Kato and Yasuo Sakuma

Department of Physiology I, Nippon Medical School, Sendagi 1, Bunkyo Tokyo 113, Japan

Address all correspondence and requests for reprints to: M. Kato, Department of Physiology I, Nippon Medical School, Sendagi 1, Bunkyo Tokyo 113, Japan. E-mail: mkato{at}nms.ac.jp


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
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.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
GH SECRETION from the anterior pituitary is mainly regulated by two hypothalamic peptides, GH-releasing hormone (GHRH) and somatostatin (SRIF). In the process of GHRH-induced GH secretion, it is generally accepted that cAMP and Ca2+ are involved (1). An elevation of intracellular free Ca2+ concentration is caused by Ca2+ influx through L-type Ca2+ channels, probably via cAMP (2, 3). In addition to these, GHRH has been suggested to increase the membrane Na+ permeability via cAMP, thereby depolarizing the cells (4, 5, 6). This depolarization activates the voltage-gated Ca2+ channels, which facilitates Ca2+ influx. An increase in Na+ permeability by GHRH has been demonstrated electrophysiologically in primary cultured rat somatotrophs and in human (h) GH-secreting adenoma cells. In the rat preparation, two types of response were reported (7). The type I response had an inward Na+ current that was largest at more negative potentials (-90 mV), and the type II had inward Na+ current that was larger at more positive potentials (-40 to -70 mV). On these currents, however, a mechanism of the activation was not investigated. In hGH-secreting adenoma cells, GHRH activated the other type of current, a nonselective cation current without clear voltage sensitivity (8). This current was shown to be blocked by inhibitors of protein kinase A (PKA), suggesting that a phosphorylation is necessary to activate the current.

An action of SRIF has been investigated electrophysiologically in several laboratories. An early report showed that SRIF hyperpolarized somatotrophs (9). More recently, SRIF has been reported to activate a delayed rectifier K+ current, a transient outward K+ current (10), and an inward rectifier K+ current (11) in somatotrophs. In hGH-secreting adenoma cells, SRIF activated a pertussis toxin-sensitive K+ current (12, 13). In addition, SRIF suppressed the voltage-gated Ca2+ currents in somatotrophs (14, 15).

Thus, both peptides modulate the membrane potential of somatotroph by activating or inhibiting various types of ion channels, which should relate to the regulation of GH secretion. Among those, the Na+ current must play a pivotal role in the process of GHRH-induced GH secretion. Therefore, we decided to further examine this current in primary cultured rat somatotrophs and found that GHRH activated Na+ current, which was larger at potentials more positive than -50 mV. This current was blocked completely by PKA inhibitor and partially by SRIF. We did not observe an activation of the current that was larger at more negative potentials (-90 mV), as observed by others (7, 8).


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
The methods employed here were the same as those previously described (16).

Pituitary cells in primary culture
Anterior pituitaries from male Wistar rats (~250–300 g BW) were dispersed by 0.2% trypsin (Type III, Sigma, St. Louis, MO) and 0.25% collagenase (Type I, Sigma). The cell suspension was applied on two-stage discontinuous Percoll density gradient centrifugation. Somatotroph-enriched fraction was obtained from the middle layer of the second-stage density gradient centrifugation, composed of 1.0, 1.076, and 1.086 g/ml. More than 90% of the cells thus obtained were GH immunoreactive. Cells were plated on poly-L-lysine-coated glass coverslips and incubated in MEM (Nissui Pharmaceutical Co., Tokyo, Japan), supplemented with 2 mM L-glutamine and 0.2% BSA (fraction V, Sigma) for approximately 1–2 days.

Electrophysiology
For electrophysiological recording and data analysis, the List EPC-9 patch clamp system (Physio-Tech, Tokyo, Japan) was used. Whole-cell currents were measured by the perforated patch clamp technique with standard procesure (17). The final concentration of amphotericin B (Seikagaku Corporation, Tokyo, Japan) in the pipette solution was approximately 20–100 µg/ml. The pipette solution (Cs-asp medium) consisted of (mM) 95 cesium aspartate, 47.5 CsCl, 1.0 MgCl2, 0.1 EGTA, and 10 HEPES (pH 7.2). In the experiments presented in Fig. 1Go, Cs+ was replaced with K+ in the pipette solution (K-asp medium). The standard extracellular solution consisted of (mM) 137.5 NaCl, 5 KCl, 2.5 CaCl2, 0.8 MgCl2, 0.6 NaHCO3, 10 glucose, and 20 HEPES (pH 7.35). To isolate an Na+ current, K+ and Ca2+ were replaced with Cs+ and Mg2+, respectively, in the standard extracellular solution (Na+-medium). In some experiments, Na+ was replaced equiosmotically with N-methyl-D-glucamine (NMDG). Pipettes had resistance of approximately 5–7 M{Omega}. Currents were filtered at 2.3 kHz, digitized at 10 kHz and recorded. Cells with seal resistance more than 10 G{Omega} and series resistance less than 35 M{Omega} were chosen for study. About 50% of series resistance was electronically compensated. Leak currents were not subtracted, and liquid junction potential was not compensated. Cell capacitance was 3.91 ± 0.65 pF (mean ±SD, n = 84). Cells were perfused at a flow rate of 1 ml/min throughout the experiments in room temperature (24–27 C).



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  		Figure 1. hGHRH-induced Na+ current. The membrane potential was held at -80 mV and the ramp pulse (-100 to 0 mV/sec) was applied every 5 sec. The current traces shown are the averages of 10 traces. The voltage clamp was turned off for a short period during the experiment to observe the membrane potential under the current clamp condition (insets). Standard extracellular solution (A and B) and Na+-free (NMDG) solution (C) were used. For the pipette solution, K-asp was used. All records shown here were obtained from the same cell. The control currents were recorded both in the standard extracellular solution and in the Na+-free solution. Then the currents were recorded with hGHRH in both extracellular solutions. Finally the recovered current was recorded. A, Upper trace, control current; lower trace, the current with 10 nM hGHRH. Left inset, the control record of the membrane potential; right inset, the membrane potential record with hGHRH. B, The control current is the same as that in A. The current in recovery is that after 30-min washout of hGHRH. C, Replacement of extracellular Na+ with impermeant molecule (NMDG) reduced the control inward current and suppressed the hGHRH-induced inward current. Inset, the membrane potential record in Na+-free solution without hGHRH. Calibration bars at the lower left of each figure indicate 10 pA. In insets, the voltage records under voltage clamp indicate -80 mV (the holding potential), 0 mV (top of the each voltage pulse), and 5 sec (interval between the pulses).

 
Chemicals
hGHRH (1–44 amide) and SRIF (1–14 amide) were obtained from Peptide Institute Inc. (Osaka, Japan). Tetrodotoxin (TTX), nifedipine, H-89, okadaic acid, and calyculin A were obtained from Seikagaku Corporation.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
hGHRH-induced Na+ current
The membrane potential was held at -80 mV, and the ramp pulse (-100 to 0 mV/sec) was applied every 15 sec in all experiments except those for Fig. 1Go, where the pulse was applied every 5 sec. Fig. 1Go shows the averaged current records in the standard extracellular solution with K-asp medium of the pipette solution. In the current records, there was the region of negative slope conductance at -50 mV to -30 mV. Application of 10 nM hGHRH elicited a small inward current (lower trace in Fig. 1AGo). In current clamp condition, trains of action potentials were seen before application of hGHRH in this cell (left inset in Fig. 1AGo), and the depolarization was induced by 10 nM hGHRH (right inset in Fig. 1AGo). This cell recovered almost to the control level after a 30-min wash with standard extracellular solution (Fig. 1BGo). The control and recovered traces are almost identical. Replacement of extracellular Na+ with the impermeant molecule NMDG reduced the basal inward current (Fig. 1CGo). Without extracellular Na+, 10 nM hGHRH elicited no current in the voltage clamp (Fig. 1CGo) and no voltage change in current clamp condition (data not shown). Similar results, except the recovery, were obtained in 7 cells of 17 examined. Among these 7 cells, we completed the experiment with 2 cells but lost the other 5 cells after recording the current with hGHRH in both standard extracellular and Na+-free solution.

Effect of H-89 on hGHRH-induced current
To isolate the Na+ current, Na+-medium was employed with Cs-asp medium in the pipette (Fig. 2AGo, trace 1). Nifedipine (10 µM) and TTX (2 µM) were added to the Na+-medium to block the voltage-gated currents (Fig. 2AGo, trace 2). Under these conditions, 10 nM hGHRH elicited an inward current activated at potentials more positive than -50 mV (Fig. 2Go, traces 3 and 3–2). This hGHRH-activated current was reverted to the control level by a subsequent application of A-kinase inhibitor H-89 (10 µM) in the presence of hGHRH (traces 4 and 4–2). At the end of the recording, 10 nM SRIF was applied together with hGHRH and H-89, which further elicited outward current carried probably by some residual K+ (traces 5 and 5–2). A time course of the effects of drugs and peptides was shown in Fig. 3Go. Mean currents at potentials between -50 and -20 mV were calculated and plotted against time. In 13 cells of 23 examined, 10 nM hGHRH elicited an inward current of more than 0.5 pA. Those cells were considered as responsive to hGHRH. The mean inward currents induced by hGHRH were -1.86 ± 0.33 pA (n = 13) at potentials between -50 and -20 mV and -0.59 ± 0.13 pA (n = 13) at potentials between -100 and -80 mV. These inward currents were completely blocked by subsequent application of 10 µM H-89 (Fig. 3AGo, for the mean current at potentials between -50 and -20 mV). The hGHRH-induced currents were reduced to 0.62 ± 0.19 pA (n = 13) at potentials between -50 and -20 mV and to 0.015 ± 0.14 pA (n = 14) at potentials between -100 and -80 mV by 10 µM H-89. In addition, hGHRH did not elicit the current when H-89 was applied before and during the application of hGHRH (Fig. 3BGo). Wash-out of H-89 revealed the hGHRH-induced inward current and reapplication of H-89 blocked the current (Fig. 3BGo). We examined 8 cells in this time sequence of application and found that 3 cells elicited an inward current (-1 ± 0.4 pA) by hGHRH after wash-out of H-89. Furthermore, as shown in Fig. 3BGo, H-89 alone inhibited the basal inward current with amplitudes of 0.98 ± 0.09 pA (n = 3) in the cells responsive to hGHRH and of 1.04 ± 0.47 pA (n = 5) in those nonresponsive to hGHRH.



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  		Figure 2. Effect of H-89. The cells were superfused with Na+-medium and patch-clamped with a pipette containing Cs-asp medium. The voltage clamp protocol is the same as that of Fig. 1Go, except that the pulse was applied every 15 sec. The currents shown are the averages of approximately 5–10 traces. A, Trace 1, control; trace 2, the current with 2 µM TTX and 10 µM nifedipine. All subsequent experiments were carried out with TTX and nifedipine. B, Trace 2 is the same as shown in A. hGHRH elicited an inward current at potentials more positive than -50 mV (trace 3). Subsequent application of 10 µM H-89 suppressed the hGHRH-induced current (trace 4). SRIF with hGHRH and H-89 evoked an outward current (trace 5). C, The difference currents were calculated by subtracting the current with TTX and nifedipine from that with hGHRH; that with hGHRH and H-89; and that with hGHRH, H-89, and SRIF (and then the currents were plotted). These traces were indicated by 3–2, 4–2, and 5–2, respectively. For visual purposes, the currents were filtered at 100 Hz and were shown in expanded current scale. Ordinate, membrane current (pA); abscissa, membrane potential (mV).

 


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  		Figure 3. Time course of effects of drugs and peptides. The mean current at membrane potentials between -50 and -20 mV were calculated and plotted against time. Timing and duration of applications are indicated by horizontal bars in each graph. Data for A were obtained from the same cell as that of Fig. 2Go and those for B were from the different cell. Ordinate, mean current (pA); abscissa, time (sec).

 
Effect of SRIF on hGHRH-induced current
In the presence of SRIF (1–10 nM), 10 nM hGHRH did not elicit the current, but a wash-out of SRIF alone revealed the inward current (Figs. 4Go and 5Go). In 8 cells of 14 examined, hGHRH elicited -2.13 ± 0.62 pA at potentials between -50 and -20 mV and -0.73 ± 0.23 pA at potentials between -100 and -80 mV. SRIF suppressed these currents to -0.79 ± 0.19 pA and to -0.44 ± 0.17 pA, respectively. These blockades by SRIF were not affected by phosphatase inhibitor, okadaic acid (100 nM). In 4 cells of 14 examined, 10 nM hGHRH elicited -1.7 ± 0.47 pA at potentials between -50 and -20 mV and -0.73 ± 0.27 pA at potentials between -100 and -80 mV. SRIF reduced these currents to -0.82 ± 0.25 pA and -0.38 ± 0.19 pA, respectively, in the presence of 100 nM okadaic acid. Okadaic acid alone did not produce a consistent effect on the basal current. Another phosphatase inhibitor, calyculin A, also was ineffective on the inhibitory action of SRIF.



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  		Figure 4. Effect of SRIF on hGHRH-induced Na+ current. A, Trace 1, the current with 2 µM TTX and 10 µM nifedipine. All subsequent experiments were carried out with TTX and nifedipine. SRIF (10 nM) elicited an outward current (trace 2). Subsequent application of 10 nM hGHRH, in the presence of SRIF, elicited a tiny current (trace 3). Traces 2 and 3 were almost identical and covered each other in the graph. Washout of SRIF alone elicited an inward current (trace 4) bigger than the control current (trace 1). B, The difference currents were calculated in the same manner as those in Fig. 2CGo and were plotted. 2–1, the current by SRIF; 3–1, that by SRIF and hGHRH; 4–1, that by hGHRH after washout of SRIF. This revealed that hGHRH elicited a small inward current (<1 pA) in the presence of SRIF. The hGHRH-induced current was larger at more positive potentials (-50 to -20 mV) than at negative potentials (-100 to -80 mV). Ordinate, membrane current (pA); abscissa, membrane potential (mV).

 


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  		Figure 5. Time course of effects of drugs and peptides. The mean current at membrane potentials between -50 and -20 mV were calculated and plotted against time. Timing and duration of the applications are indicated by horizontal bars in each graph. Data for A were obtained from the same cell as that in Fig. 4Go and those for B were from the different cell. Ordinate, mean current (pA); abscissa, time (sec).

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Present experiments confirmed the existence of hGHRH-induced Na+ current in the rat somatotroph and further characterized the current. First, a possible involvement of the voltage-gated currents was excluded by using TTX and nifedipine, because the voltage-gated Na+ and L-type Ca2+ currents have been shown to be augmented by hGHRH (3, 15, 16). The result showed that hGHRH-induced Na+ current was not caused by an augmentation of these voltage-gated currents. In addition, we observed the hGHRH-induced Na+ current, which was larger at the membrane potentials between -50 and -20 mV, whereas Naumov et al. (7) reported two different types of currents, namely type I and II, in rat somatotrophs. The type I response had an inward current that was largest at more negative potentials (-90 mV), and the type II response had an inward current that was larger at more positive potentials (-70 to -40 mV). We observed the current that was larger at potentials of approximately -50 to -20 mV. This current may reflect the type II response. However, we did not observe the hGHRH-induced current similar to the type I response. This discrepancy might be caused by the concentration of hGHRH used and to the type of hGHRH. We used hGHRH at a concentration of 10 nM, whereas Naumov et al. used 50 nM hGHRH. The peptide used in the present experiments was hGHRH (1–44 amide), whereas that in their experiments was hGHRH (1–29 amide).

Second, the hGHRH-induced Na+ current was completely suppressed by a PKA inhibitor, H-89, which indicates that the current may be activated through phosphorylation by PKA of the channel itself or some other protein(s) related to the channel activation. In this respect, the Na+ current in normal somatotrophs has a similar property to that in GH-secreting adenoma cells, because nonselective cation current activated by hGHRH in GH-secreting adenoma cells also was inhibited by H-89 (8). Interestingly, H-89 alone suppressed the basal inward current both in the cells responsive to hGHRH and in those nonresponsive to hGHRH. This indicates that the basal level of cellular cAMP may activate an inward current to a certain extent. This Na+ current may contribute to the resting membrane potential (-60 to -40 mV) of the primary cultured rat somatotrophs.

Third, SRIF partially suppressed the hGHRH-induced Na+ current. This action of SRIF may have a physiological significance, either in the cells that lack the SRIF-activated K+ channels or in the cells in which those channels are in malfunction. To investigate the mechanism of action, the effect of phosphatase inhibitors was examined on the action of SRIF, because SRIF is reported to modulate the channel activity by dephosphorylation through activation of phosphatase (18). However, two kinds of phosphatase inhibitors, okadaic acid and calyculin A (data not shown), had no effect on the action of SRIF. Further work is needed to elucidate a mechanism of action of SRIF, especially a possible involvement of GTP-binding proteins.

In conclusion, hGHRH activated Na+ current in primary cultured rat somatotrophs. The current was larger at more positive potentials (-50 to -20 mV) than at more negative potentials (-100 to -80 mV). This hGHRH-activated current was suppressed by PKA inhibitor and SRIF. The inhibitory effect of SRIF was not affected by phosphatase inhibitor.


   Footnotes
 
1 This work was supported, in part, by Grant-in-Aid 08680872 from the Ministry of Education, Science, and Culture of Japan. Back

Received May 29, 1997.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

  1. Bilezikjian LM, Vale WW 1983 Stimulation of adenosine 3',5'-monophostropinphate production by growth hormone releasing factor and its inhibition by somatostatin in anterior pituitary cells in vitro. Endocrinology 113:1726–1731[Abstract]
  2. Kato M, Suzuki M 1991 Inhibition by nimodipin of growth hormone (GH) releasing factor-induced GH secretion from rat anterior pituitary cells. Jpn J Physiol 41:63–74[CrossRef][Medline]
  3. Chen C, Zhang, McNeill P, Pullar M, Cummins T, Clarke IJ 1993 Human growth hormone releasing factor (hGRF) modulates calcium currents in human growth hormone secreting adenoma cells. Brain Res 604:345–348[CrossRef][Medline]
  4. Kato M, Hattori M, Suzuki M 1988 Inhibition by extracellular Na+ replacement of GRF-induced GH secretion from rat pituitary cells. Am J Physiol 254:E476–E481
  5. Kato M, Suzuki M 1989 Growth hormone releasing factor depolarizes rat pituitary cells in Na+-dependent mechanism. Brain Res 476:145–148[CrossRef][Medline]
  6. Kato M, Hoyland J, Sikdar SK, Mason WT 1992 Imaging of intracellular calcium in rat anterior pituitary cells in response to growth hormone releasing factor. J Physiol (London) 447:171–189[Abstract/Free Full Text]
  7. Naumov AP, Herrington J, Hille B 1994 Actions of growth hormone releasing hormone on rat pituitary cells: intracellular calcium and ionic currents. Pflugers Arch 427:414–421[CrossRef][Medline]
  8. Takano K, Takei T, Teramoto A, Yamashita N 1996 GHRH activates a nonselective cation current in human GH-secreting adenoma cells. Am J Physiol 270:E1050–E1057
  9. Israel JM, Denef C, Vincent JD 1983 Electrophysiological properties of normal somatotrophs in culture. Neuroendocrinology 37:193–199[Medline]
  10. Chen C, Zhang J, Vincent JD, Israel JM 1990 Somatostatin increases voltage-dependent potassium currents in rat somatotrophs. Am J Physiol 259:C854–C861
  11. Sim SM, Lussier BT, Kraicer J 1991 Somatostatin activates an inwardly rectifying K+ conductance in freshly dispersed rat somatotrophs. J Physiol (London) 441:615–637[Abstract/Free Full Text]
  12. Yamashita N, Shibuya N, Ogata E 1986 Hyperpolarization of the membrane potential caused by somatostatin in dissociated human pituitary adenoma cells that secrete growth hormone. Proc Natl Acad Sci USA 83:6198–6202[Abstract/Free Full Text]
  13. Yamashita N, Kojima I, Shibuya N, Ogata E 1987 Pertussis toxin inhibits somatostatin-induced K+ conductance in human pituitary tumor cells. Am J Physiol 253:E28–E32
  14. Chen C, Zhang J, Vincent JD, Israel JM 1990 Two types of voltage-dependent calcium current in rat somatotrophs are reduced by somatostatin. J Physiol (London) 425:29–42[Abstract/Free Full Text]
  15. Kato M 1995 Withdrawal of somatostatin augments L-type Ca2+ current in primary cultured rat somatotrophs. J Neuroendocrinol 7:855–859[CrossRef][Medline]
  16. Kato M 1996 Growth hormone releasing hormone augments voltage-gated Na+ current in cultured rat pituitary cells. Am J Physiol 270:C125–C130
  17. Rae J, Cooper K, Gates P, Watsky M 1991 Low access resistance perforated patch recordings using amphotericin B. J Neurosci Methods 37:15–26[CrossRef][Medline]
  18. Armstrong DL, White RE 1992 An enzymatic mechanism for potassium stimulation through pertussis-toxin-sensitive G proteins. Trends Neurosci 15:403–408[CrossRef][Medline]



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