Biomedical subjects

W K Ho

Publications and source records attributed to W K Ho.

At least 55 records · Page 3Linked to original sources

PubMed2000

Permeability characteristics of monovalent cations in atrial myocytes of the rat heart.

We investigated the permeability of Cs+ and Na+ through various ion channels in rat atrial myocytes using the whole-cell voltage-clamp technique. With isotonic CsCl (140 mM) on both sides of the membrane and nominally [Ca2+]o-free conditions, depolarising clamp pulses induced an increase of outward currents which showed a biphasic time course. Repolarisation to the holding potential induced inward tail currents. With isotonic NaCl, depolarisation also induced outward currents which showed a monotonic decay, but inward tail currents were not observed. Both in NaCl and CsCl, currents were hardly affected by TEA (10 mM), 4-AP (5 mM) and DIDS (100 microM). Nicardipine (1 M) almost completely blocked time-dependent outward currents in isotonic NaCl solution, leaving only time-independent currents which showed linear I-V relationship. In isotonic CsCl conditions, nicardipine blocked outward current considerably, but there still remained time-dependent outward currents and inward tail currents. Addition of E-4031 (2-20 M) which is known as a specific blocker of the rapidly activating delayed rectifier K+ current (IKr) completely blocked these time-dependent outward and inward currents, leaving only a time-independent current. Time-independent currents recorded in the presence of nicardipine and E-4031 were inhibited by GdCl3, which is known to block non-selective cation (NSC) currents. From these results, it was suggested that NSC current in atrial myocytes can be investigated in isotonic Cs+ or Na+ solution in the presence of Ca2+ channel and IKr blockers.

Animals↗

PubMed2000

Inhibitory effects of purified green tea epicatechins on contraction and proliferation of arterial smooth muscle cells.

AIM: The present study was aimed to examine the vasorelaxant and antiproliferative responses to purified green tea epicatechin mixture, (-)epicatechin and (-)epigallocatechin gallate on rat arterial smooth muscle cells. METHODS: Changes in isometric force were measured by Grass force transducer and effects on cell proliferation were evaluated using [3H]thymidine incorporation assay. RESULTS: Epicatechin mixture, (-)epicatechin and (-)epigallocatechin gallate, which we isolated and purified from jasmine green tea, concentration-dependently, reduced the contractile response to phenylephrine in rat isolated aortic rings with (-)epigallocatechin gallate being more effective. These three agents also inhibited [3H]thymidine incorporation into DNA in cultured rat aortic smooth muscle cells in a concentration-dependent manner. CONCLUSION: The purified epicatechin derivatives from jasmine green tea relaxed the isolated rat arteries preconstricted by phenylephrine and inhibited aortic smooth muscle cell proliferation.

Animals↗

PubMed2000

Effect of the external nasal dilator on nasal minimal cross-sectional area in orientals as assessed by acoustic rhinometry.

OBJECTIVE: This study aimed to evaluate the effect of the external nasal dilator on the dimension of the nasal valve in Orientals. DESIGN: A cohort study of normal subjects. SETTING: Academic institution. METHODS: The nasal fossae of normal subjects were assessed by acoustic rhinometry before and after application of the external nasal dilator. MAIN OUTCOME MEASURES: The minimal cross-sectional area of the nasal fossae and the total cross-sectional area of the nose. RESULTS: Nasal fossae of 25 normal subjects were evaluated. There was a significant increase of 0.10 cm2 (SD = 0.16) or a 17% increase in the minimal cross-sectional area of the 50 nasal cavities after application of the external nasal dilator (Wilcoxon's matched-pairs signed rank test, p = .0001). A significant increase in the total minimal cross-sectional area for the whole nose after application was also present (0.19 cm2, SD = .27, or 16%; Wilcoxon's matched-pairs signed rank test, p = .0032). CONCLUSIONS: The external nasal dilator results in an increase in the minimal cross-sectional area of the nasal airway in Orientals.

Cohort Studies↗

PubMed1999

Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit.

1. Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca2+-activated K+ current (IK(Ca)) and non-selective cation currents (INSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (Vm) was -39.2 +/- 0.9 mV (n = 72). It did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5.6 +/- 1. 1 Hz and -7.7 +/- 0.7 mV (n = 12), respectively. In the voltage-clamp mode, spontaneous transient outward currents (STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca2+ stores by caffeine or ryanodine, and the removal of extracellular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG+ caused hyperpolarization Vm of without affecting STHPs. Removal of extracellular Ca2+ induced a marked depolarization of Vm along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The permeability ratio of K+ : Cs+ : Na+ : Li+ was 1.7 : 1.3 : 1 : 0. 9, indicating that it is a non-selective cation current (INSC). The reversal potential of this current in control conditions was calculated to be -13.9 mV. The current was blocked by millimolar concentrations of extracellular Ca2+ and Mg2+. 6. From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca2+-activated K+ (KCa) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.

Animals↗

PubMed1999

Long-term sensorineural hearing deficit following radiotherapy in patients suffering from nasopharyngeal carcinoma: A prospective study.

BACKGROUND: This was a prospective study to evaluate the effect of radical external irradiation on inner ear function after treatment of nasopharyngeal carcinoma. METHODS: Pure tone audiograms were performed at regular intervals before, after, and up to 4.5 years following completion of radiotherapy. RESULTS: Two hundred ninety-four patients (526 ears) were included. Within 3 months after radiotherapy, deterioration of bone conduction threshold at 4 kHz and pure tone average (average of 0.5 kHz, 1 kHz, and 2 kHz) were noted in 164 ears (31%) and 75 ears (14%), respectively. Patients older than 50 years and ears with threshold below 60 dB at 4 kHz before radiotherapy were significant factors (p < 0.01 and p < 0. 001) associated with a 4 kHz loss. In 40% of these ears, recovery was evident at 2 years. With follow-up for 4.5 years, the probability of significant threshold deterioration increased with time. CONCLUSION: Sensorineural hearing loss started soon after radiotherapy. Early changes could be reversible while the probability of persistent hearing loss continued to increase.

Adolescent↗

PubMed1999

Lack of allelic association of dopamine D4 receptor gene polymorphisms with Parkinson's disease in a Chinese population.

Parkinson's disease (PD) is a neurodegenerative disease caused by a multitude of environmental, neurochemical, and genetic factors. The gene for human dopamine D4 receptor (DRD4) has been considered as a plausible candidate for the pathogenesis of PD. Different dopamine D4 receptor allelic forms have variable affinity toward certain neuroleptics such as clozapine, suggesting a role for dopamine D4 receptors in neurologic disorders. To test the hypothesis that the DRD4 polymorphism is associated with the susceptibility to Parkinson's disease, we have examined differences in allele frequencies of different DRD4 polymorphisms in 101 Chinese patients with PD and in 105 age-matched control subjects in Hong Kong. The DRD4 gene was analyzed by a non-radioactive polymerase chain reaction-based Southern hybridization with chemiluminescence detection. The number of polymorphic 48 base pair tandem repeats in exon 3 was identified in each study subject. The DRD4 alleles with high frequencies in the control subjects are 4-repeat allele (72.4%), 2-repeat allele (21.4%), and 7-repeat allele (3.8%) which accounted for over 97% of the total alleles in the elderly Chinese population. The most prevalent genotype in the control subjects is the 4/4 (47.6%), followed by 4/2 (38.6), 4/7 (7.6%), and 2/2 (3.0%). None of the variable number tandem repeat polymorphism showed evidence for genetic association with Parkinson's disease.

Aged↗

PubMed1999

Blockade of HERG channels expressed in Xenopus oocytes by external H+.

We have investigated the effect of external H+ concentration ([H+]o) on the human-ether-a-go-go-related gene (HERG) current (IHERG), the molecular equivalent of the cardiac delayed rectifier potassium current (IKr), expressed in Xenopus oocytes, using the two-microelectrode voltage-clamp technique. When [H+]o was increased, the amplitude of the IHERG elicited by depolarization decreased, and the rate of current decay on repolarization was accelerated. The activation curve shifted to a more positive potential at lower external pH (pHo) values (the potential required for half-maximum activation, V1/2, was: -41.8 mV, -38.0 mV, -33.7 mV, -26.7 mV in pHo 8.0, 7.0, 6.6, 6.2, respectively). The maximum conductance (gmax) was also affected by [H+]o: a reduction of 7.9%, 14.6%, and 22.8% was effected by decreasing pHo from 8.0 to 7.0, 6.6, and 6.2, respectively. We then tested whether this pH effect was affected by the external Ca2+ concentration, which is also known to block HERG channels. When the extracellular Ca2+ concentration was increased from 0.5 mM to 5 mM, the shift in V1/2 caused by increasing [H+]o was attenuated, suggesting that these two ions compete for the same binding site. On the other hand, the decrease in gmax caused by increasing [H+]o was not significantly affected by changing external Ca2+ levels. The results indicate that HERG channels are inhibited by [H+]o by two different mechanisms: voltage-dependent blockade (shift of V1/2) and the decrease in gmax. With respect to the voltage-dependent blockade, the interaction between H+ and Ca2+ is competitive, whereas for the decreasing gmax, their interaction is non-competitive.

Animals↗

PubMed1999

Blockade of the delayed rectifier K+ currents, IKr, in rabbit sinoatrial node cells by external divalent cations.

We investigated the actions of various divalent cations on the delayed rectifier K+ currents (IKr) in rabbit sinoatrial node cells using the whole-cell voltage-clamp technique in isotonic K+ solutions. External divalent cations decreased the amplitude of currents, accelerated the time course of deactivation and shifted the activation to positive potentials in a dose-dependent manner. The concentrations for half-maximum inhibition of the steady-state currents (KM) obtained at 0 mV were 0.63, 1.36, 1.65 and 2.16 mM for Ni2+, Co2+, Mn2+ and Ba2+, respectively. The effect was voltage dependent (KM decreased e-fold for 12.2-16.8 mV hyperpolarization), but the dependence did not vary significantly among different cations. Acceleration of the time course of current deactivation by the increase of divalent cation concentration was well fitted by the voltage-dependent block model, and the binding rate constant (k1) was obtained. The binding rates for the ions took the following order: Ni2+ >Co2+ >Mn2+ >Ba2+. The degree of the shift of activation occurred in the same order: Ni2+ >Co2+ >Mn2+ >Ba2+. From these results, we conclude that IKr channels are non-selectively blocked by most divalent cations from the external side and that the binding site is located deep inside the channel, resulting in a steep voltage dependence of the blockade.

Animals↗

PubMed1999

Clinicopathological analysis of elective neck dissection for N0 neck of early oral tongue carcinoma.

BACKGROUND: The study aims at evaluation of the efficacy of elective neck dissection as a staging and therapeutic procedure for N0 neck of early carcinoma of the oral tongue by whole organ serial sectioning. METHODS: There were 50 stage I and II patients. The neck dissection specimens were whole organ sectioned in 3-mm intervals for the evaluation of nodal metastasis. RESULTS: There were 18 (36%) patients with subclinical nodal metastasis. The total number of metastatic nodes were 31 (1%) among all 2,826 nodes being examined. The metastatic foci had a median size of 3 mm and occupied a median of 6% of the cross sectional area of the involved nodes. The ipsilateral level II nodes were the commonest (26%) site of metastasis. Metastatic nodes were present in 34% patients who had negative preoperative radiological assessment and in 20% patients who had negative intraoperative frozen section sampling of neck nodes. Patients with subclinical nodal metastasis had a high incidence of regional recurrence (62%) and low survival (27%) when postoperative radiotherapy was not given after elective neck dissection. CONCLUSIONS: Ipsilateral level I,II,III neck dissection is an adequate diagnostic procedure for staging of the N0 neck of early oral tongue carcinoma. Its diagnostic role cannot be replaced by the available pre-operative radiological screening and intra-operative frozen section sampling. However, elective selective neck dissection is an effective but not adequate therapeutic procedure, and post-operative adjuvant radiotherapy and chemotherapy have to be considered for all pathologically positive necks.

Adolescent↗

PubMed1999

Blockade of HERG channels expressed in Xenopus laevis oocytes by external divalent cations.

We have investigated actions of various divalent cations (Ba2+, Sr2+, Mn2+, Co2+, Ni2+, Zn2+) on human ether-a-go-go related gene (HERG) channels expressed in Xenopus laevis oocytes using the voltage clamp technique. All divalent cations inhibited HERG current dose-dependently in a voltage-dependent manner. The concentration for half-maximum inhibition (Ki) decreased at more negative potentials, indicating block is facilitated by hyperpolarization. Ki at 0 mV for Zn2+, Ni2+, Co2+, Ba2+, Mn2+, and Sr2+ was 0.19, 0.36, 0. 50, 0.58, 2.36, and 6.47 mM, respectively. The effects were manifested in four ways: 1) right shift of voltage dependence of activation, 2) decrease of maximum conductance, 3) acceleration of current decay, and 4) slowing of activation. However, each parameter was not affected by each cation to the same extent. The potency for the shift of voltage dependence of activation was in the order Zn2+ > Ni2+ >/= Co2+ > Ba2+ > Mn2+ > Sr2+, whereas the potency for the decrease of maximum conductance was Zn2+ > Ba2+ > Sr2+ > Co2+ > Mn2+. The kinetics of activation and deactivation were also affected, but the two parameters are not affected to the same extent. Slowing of activation by Ba2+ was most distinct, causing a marked initial delay of current onset. From these results we concluded that HERG channels are nonselectively blocked by most divalent cations from the external side, and several different mechanism are involved in their actions. There exist at least two distinct binding sites for their action: one for the voltage-dependent effect and the other for reducing maximum conductance.

Animals↗

PubMed1999

Characterization of a functional AT1A angiotensin receptor in pancreatoma AR4-2J cells.

Functional angiotensin receptors were characterized in the rat pancreatic acinar cell line AR4-2J. Angiotensin II stimulated a dose-dependent release of amylase and production of inositol phosphates. Results of high-performance liquid chromatography separation of inositol phosphates indicated that angiotensin stimulated the rapid accumulation of inositol 1,3,4-trisphosphate. Angiotensin II and angiotensin III were at least an order of magnitude more potent than angiotensin I in the stimulation of amylase release. The angiotensin II-stimulated amylase release was blocked by losartan, a selective AT1 angiotensin antagonist. The selective AT2 angiotensin receptor ligands CGP42112 did not alter angiotensin II-stimulated amylase released. However, CGP42112 stimulated amylase release at micromolar concentrations with a potency similar to angiotensin I. Analysis of mRNA expression by reverse transcription polymerase chain reaction suggested that AT1A was the predominant type-I angiotensin receptor expressed in the AR4-2J cells.

Amylases↗

PubMed1999

Jasmine green tea epicatechins are hypolipidemic in hamsters (Mesocricetus auratus) fed a high fat diet.

These studies were designed to test the hypolipidemic activity of green tea epicatechins (GTE) isolated from jasmine green tea. In Experiment 1, three groups of hamsters were given a semisynthetic diet containing 200 g lard/kg and 1 g cholesterol/kg for 4 wk. The control group received distilled water, and the other two groups received either 15 g/L green tea water extract (GTWE) or 5.0 g/L GTE solution. Both the GTWE and GTE groups had lower concentrations of serum total cholesterol (TC) and triacylglycerols (TG) than the controls (P < 0.05). In Experiment 2, four groups of hamsters received tap water as the drinking fluid, but they were given the same high fat and cholesterol diet supplemented with 0 (control), 1.1, 3.4 or 5.7 g GTE/kg diet. The hypolipidemic effect of jasmine GTE was dose dependent. In Experiment 3, the time-course of changes in serum TC and TG was monitored in hamsters given the high fat diet supplemented with 5.7 g GTE/kg in comparison with that of controls. The hypolipidemic effects of dietary GTE were evident after feeding for 2 wk. Dietary supplementation of GTE did not affect liver fatty acid synthase. However, GTE-supplemented hamsters had higher fecal excretions of total fatty acids, neutral sterols and acidic sterols compared with the control group. In Experiment 4, hamsters were fed nonpurified diet; the control group drank distilled water, and the GTE group drank distilled water containing 5.0 g GTE/L. No differences in activities of 3-hydroxy-3-methyl glutaryl coenzyme A reductase and intestinal acyl CoA:cholesterol acyltransferase were observed. This study suggests that the hypolipidemic activity of GTE is not due to inhibition of synthesis of cholesterol or fatty acid but is most likely mediated by its influence on absorption of dietary fat and cholesterol.

Animals↗

PubMed1999

Different contractile properties between intralobar and extralobar pulmonary arteries of the rabbit.

In pulmonary circulation, small muscular resistance arteries are known to have different receptor properties and sensitivity to neurotransmitters from those of large elastic conduit arteries. It is, however, not yet certain whether the different properties are primarily due to the diameter or the location of arteries. In the present study, we compared the contractile responses to various agonists among large extralobar (ELPA, diameter: 2-3 mm), large intralobar (ILPA, diameter: 2-3 mm), and small intralobar pulmonary arteries (SPA, diameter: 300-500 microm) of the rabbit. There were no differences in normalized dose-response curves to KCl among three groups. Half maximum doses (EC50 in mM) were 38.0+/-2.0 (n=8, mean+/-SEM) in ELPA, 36.9+/-2.4 (n=10) in ILPA, and 39.0+/-0.9 (n=12) in SPA. Responses to phenylephrine, epinephrine, histamine, serotonin, and PGF2alpha were normalized and expressed as a relative contraction against maximum tension to KCl. In ELPA, the contractile responses to various agents showed the following sequence: KCl>epinephrine>phenylephrine>serotonin>PGF2alpha>histamine. In ILPA and SPA, the sequence was: KCl>histamine>PGF2alpha>serotonin. There was little response to phenylephrine and epinephrine in ILPA and SPA. These results demonstrate that the difference of contractile responses between ELPA and ILPA was more prominent than that between ILPA and SPA, suggesting that the location is more important than the diameter itself in determining the characteristic contractile responses of pulmonary arteries.

Animals↗

PubMed1999

Measurement of nasal geometry by acoustic rhinometry in normal-breathing Asian children.

OBJECTIVE: The purpose of this study was to obtain normal values of nasal geometry in Asian children with no nasal problems using the acoustic rhinometry method. METHOD: Acoustic rhinometric measurement of the nasal fossae was performed in pediatric patients coming for general surgical operations with no nasal problem. RESULTS: A cohort study of 183 children in an acute care hospital with full elective and emergency surgical services was undertaken. The mean minimal cross-sectional area (MCA), which was functionally important for nasal breathing, was found to be 0.32 cm2 (SD = 0.13) and situated at 1.40 cm (SD = 0.26) from the anterior nare. Normal values for area, length, and volume of the nose in these children as a group and at different ages from 1 to 11 years old were obtained. CONCLUSIONS: Acoustic rhinometry was found to be well tolerated by the children. The MCA was confirmed to be situated at the anterior part of the nose and had a positive correlation with an increase in age.

Acoustics↗

PubMed1998

Expression of grass carp growth hormone by baculovirus in silkworm larvae.

A total of five recombinant Bombyx mori nuclear polyhedrosis viruses (BMNPV) carrying the grass carp (Ctenopharyngodon idellus) growth hormone (GH) cDNA were constructed in this study. Two of them were able to express the hormone up to a level of 12 microgram/ml medium when cultured B. mori cells were infected for 4 days. Inoculation of the viruses into silkworm (B. mori) host significantly increased the level of GH achievable. The amount of hormone produced per larva was estimated to be around 1 mg. The recombinant grass carp GH had immunological and biological activities similar to the native hormone. The N-terminal sequence of the recombinant hormone was the same as the native one, indicating that the fish signal peptide was correctly processed by the insect cells. Silkworm powder prepared from larvae infected with the recombinant virus was used as food supplement for fish. Compared with the control, this dietary supplement was effective in increasing the growth rate of juvenile carp.

Animals↗

PubMed1998

Brain-derived neurotrophic factor rapidly potentiates synaptic transmission through NMDA, but suppresses it through non-NMDA receptors in rat hippocampal neuron.

Brain-derived neurotrophic factor (BDNF) rapidly enhances synaptic transmission among the hippocampal neurons. In order to examine which component of glutamate receptors participates in synaptic potentiation by BDNF, we have studied the effect of glutamate antagonists on excitatory postsynaptic currents (EPSCs) enhanced by BDNF, using cultured embryonic hippocampal neurons. In the presence of AP5, a N-methyl-D-aspartate (NMDA) antagonist, BDNF depressed the EPSCs. In contrast, BDNF enhanced the EPSCs in the presence of a non-NMDA antagonist CNQX. Our results suggest that BDNF acutely activates synaptic transmission via NMDA, but suppresses it via non-NMDA receptors in the hippocampus.

2-Amino-5-phosphonovalerate↗

PubMed1998

Voltage-dependent blockade of HERG channels expressed in Xenopus oocytes by external Ca2+ and Mg2+.

1. We expressed the human eag-related gene (HERG), which is known to encode the delayed rectifier K+ current (IKr) in cardiac muscle, in Xenopus oocytes. Using a two-microelectrode voltage clamp technique, the effect of external Ca2+ and Mg2+ on the HERG current (IHERG) was investigated. 2. When [Ca2+]o was increased, the amplitude of outward IHERG elicited by depolarization decreased, and the rate of current onset slowed. The rate of current decay observed on repolarization was greatly accelerated. The threshold and fully activated potential of IHERG shifted to a more positive potential. On the other hand, the inactivation property represented by the negative slope of the I-V curve and the instantaneous conductance of IHERG were little affected by [Ca2+]o. 3. The effect of [Ca2+]o on IHERG can be interpreted using the channel blockade model. The blockade is voltage dependent; smaller dissociation constants (KM) at more negative potentials indicate that block is facilitated by hyperpolarization. KM changes e-fold for 14.5 mV and the fractional electrical distance of the binding site calculated from this value is 0.86. 4. Blockade by a low concentration of Ca2+ (0.5 mM) was inhibited by increasing [K+]o (from 2 to 20 mM), whereas blockade by a high concentration of Ca2+ (5 mM) was not affected by varying [K+]o, indicating that there is competition between permeating ions and blocking ions. 5. The effect of [Mg2+]o on IHERG was qualitatively similar to that of [Ca2+]o, but the potency was lower. 6. These results suggest that external Ca2+ and Mg2+ block the HERG channel in a voltage- and time-dependent manner, resulting in a voltage dependence which has been regarded as a property of the activation gate.

Animals↗

PubMed1998

Somatostatin inhibits (d-Arg6, Pro9-NEt) salmon gonadotropin-releasing hormone- and dopamine D1-stimulated growth hormone release from perifused pituitary cells of chinese grass carp, ctenopharyngodon idellus.

In this study, a heterologous radioimmunoassay (RIA) for grass carp GH has been validated and used to monitor the kinetics of GH release from perifused grass carp pituitary cells. To establish the anatomical specificity of GH antiserum used in this RIA, immunohistochemical staining was performed in grass carp pituitary sections. Somatotrophs recognized by this GH antiserum were located mainly in the proximal pars distalis without overlapping with gonadotrophs located in the same area or with lactotrophs located in the rostral pars distalis. The immunoreactivity of somatotrophs was abolished by preabsorbing GH antiserum with purified grass carp GH, suggesting that the possibility of a cross-reactivity of antiserum with other grass carp pituitary hormones is unlikely. Using 125I-labeled carp GH as the RIA tracer, parallelism was observed among the displacement curves of grass carp GH standard, grass carp serum, and culture medium conditioned by grass carp pituitary cells, suggesting that this RIA can be used to quantitate grass carp GH levels in biological samples. Using an in vitro column perifusion system, a superactive gonadotropin-releasing hormone (GnRH) analog (d-Arg6, Pro9-NEt)-sGnRH(sGnRHa, 0.3-30 nM), dopamine (DA, 0.1-10 muM), and the nonselective DA agonist apomorphine (0.1-10 muM) stimulated GH release from grass carp pituitary cells in a dose-dependent manner. The GH-releasing effect of DA was mimicked by the D1 agonists SKF38393 (0.1-10 muM) and SKF77434 (0.1-10 muM), but not by the D2 agonist LY171555 (3 muM). In addition, the GH response to DA (1 muM) was blocked by the D1 antagonist SCH23390 (5 muM) but not by the D2 antagonist (+/-) sulpiride (5 muM), suggesting that the GH-releasing action of DA is mediated through receptors resembling mammalian D1 receptors. Somatostatin-14 (SRIF14, 0.01-100 nM), unlike sGnRHa and DA, induced a dose-dependent suppression on basal GH release. At a high dose (100 nM), SRIF14 also abolished the GH responses to sGnRHa (100 nM), DA (10 muM), and the D1 agonist SKF38393 (3 muM). These results, as a whole, provide evidence that GH release in the grass carp is under the direct regulation of GnRH, DA, and SRIF at the pituitary cell level. The present study also suggests that DA D1 receptors are present in grass carp pituitary cells mediating the GH-releasing action of DA.

Animals↗