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Biomedical subjects

A K Ho

Publications and source records attributed to A K Ho.

At least 91 records · Page 5Linked to original sources
PubMed1993

Evidence for a calmodulin inhibitory substance(s) isolated from human meningiomas.

Calmodulin (CaM), a calcium-binding protein, is present in human tumor tissues and in meningioma. Following a purification procedure using DEAE-cellulose and the polymeric resin 3520, the CaM content of tumor extracts was assayed using CaM-deficient phosphodiesterase (PDE). In the presence of low amounts of the extracts, a concentration dependent stimulation of PDE was observed. However, further addition of higher concentrations of the extract produced a marked inhibition of the CaM stimulation of PDE in 13 of 15 specimens. A wide range (2.44-51.31 units/1 mg tumor [wet weight]) of inhibitor concentration was noted. However, no detectable inhibitory activity of this magnitude was observed in normal human meningeal extracts. The final extracts showed no calcineurin-phosphatase activity in the presence of Ni++, a known activator of this phosphatase. SDS-polyacrylamide gel (10%) electrophoresis of the extracts revealed the typical calmodulin band at 17 kDa plus two additional bands with apparent molecular masses of 21 and 36 kDa respectively. These bands were not seen using normal meningeal extracts.

Aged↗
PubMed1993

Cyclic adenosine monophosphate is not part of the transduction chain by which cell-swelling induces secretion in either normal or tumor-derived GH4C1 pituitary cells.

We have evaluated whether cyclic adenosine monophosphate (cAMP) generation plays a role in the burst of hormone secretion caused by osmotic cell-swelling in tumor-derived and normal pituitary cells. Up to 45% hyposmolarity induced a dose-dependent increase in prolactin (PRL) secretion, but had no effect on cAMP generation. However, hyposmolarity inhibited forskolin-induced cAMP accumulation in a dose-dependent manner, but had no effect on 3-isobutyl-1-methyl-xanthine (IBMX)-induced cAMP accumulation. Ca2+ depletion of the medium partially blocked the inhibitory effect of hyposmolarity on forskolin-induced cAMP generation and completely blocked stimulation of PRL secretion by hyposmolarity. High levels of K+ in medium inhibited forskolin-induced cAMP generation, while thyrotropin-releasing hormone (TRH) enhanced it. Our results indicate that in both GH4C1 and normal pituitary cells, cAMP is not a regulatory factor for hormone secretion induced by cell-swelling.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Salmonella bacteraemia occurring concurrently with the first presentation of systemic lupus erythematosus.

There is increasing recognition of the association between Salmonella infections and systemic lupus erythematosus (SLE). However, these infections have usually occurred in patients with established SLE who were receiving immunosuppressive therapy. We describe two young women with previously undiagnosed SLE who developed Salmonella bacteraemia. Neither of these patients had received immunosuppressive drugs. From these and two previously reported patients, it may be concluded that SLE per se is likely to be associated with a defect in host defences to Salmonella bacteria.

Adult↗
PubMed1993

Altered pineal adrenergic-stimulated cyclic nucleotide responses in spontaneously hypertensive rats.

In this study, we examined the pineal adrenergic-stimulated cyclic nucleotide responses in spontaneously hypertensive rats (SHR) and their genetic control, Wistar-Kyoto rats (WKY). Treatment with norepinephrine stimulated cAMP and cGMP contents up to 50- and 12-fold in WKY pinealocytes, compared with a 35- and 4-fold increase in SHR. By contrast, there was no difference in the isoproterenol-stimulated cAMP and cGMP contents, suggesting a reduced alpha 1-adrenergic potentiation of beta-adrenergic-stimulated cGMP response in SHR pinealocytes. The altered potentiation mechanism was examined using agents that activate protein kinase C or elevate intracellular Ca2+. In the presence of a protein kinase C activator, the isoproterenol-stimulated cAMP response was potentiated to a similar degree in WKY and SHR pinealocytes. In contrast, the potentiating effects of ionomycin and KCl on the isoproterenol-stimulated cAMP and cGMP responses in SHR pinealocytes were markedly reduced. These results indicate that in the SHR pineal gland, an altered intracellular Ca(2+)-mediated event may account for the reduction in alpha 1-adrenergic potentiation of beta-adrenergic-stimulated cyclic nucleotide responses.

Animals↗
PubMed1993

Mechanisms of action of a second generation growth hormone-releasing peptide (Ala-His-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2) in rat anterior pituitary cells.

The mechanism by which GH-releasing peptides elicit GH secretion has remained largely unknown. In this study, the effects of a second generation GH-releasing peptide, Ala-His-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2(GHRP-1), on cAMP, intracellular Ca2+ ([Ca2+]i), and GH release were examined using rat pituitary gland static monolayer cell cultures. It was found that GHRP-1 increased GH release in a dose-dependent manner up to 3-fold, while having no effect on cAMP levels. In contrast, simultaneous elevations of cAMP and GH were observed after treatment with GHRH. To further define the underlying mechanism of GHRP-1-mediated GH release, its effect on [Ca2+]i was determined using a fluorescent Ca2+ indicator, fura-2. GHRP-1 dose dependently increased [Ca2+]i up to 45.5 nM +/- 5.6 nM. A similar elevation of [Ca2+]i was observed after GHRH treatment. Similar to GHRH, GHRP-1-induced increases in [Ca2+]i and GH release were inhibited by somatostatin. Furthermore, the GHRP-1-induced increases in [Ca2+]i and GH were also suppressed by nifedipine. The interaction between the voltage-dependent Ca2+ channels and GHRP-1 was investigated in cells maximally stimulated by KCl. The addition of GHRP-1 had no effect on the KCl-stimulated GH release. To investigate the possible interaction between the adenylyl cyclase pathway and GHRP-1, cells were maximally stimulated with forskolin or (Bu)2cAMP. Addition of GHRP-1 stimulated GH release beyond that observed using cAMP elevating agents. Similar results were obtained in the presence of a protein kinase C, 4 beta-phorbol 12-myristate 13-acetate (PMA). The GHRP-1-stimulated GH release was additive to that observed with PMA stimulation. Based on these findings, it was concluded that 1) GHRP-1 treatment leads to an increase in [Ca2+]i; 2) unlike GHRH, GHRP-1 releases GH via a Ca(2+)-dependent, cAMP-independent mechanism; 3) GHRP-1-induced increases in [Ca2+]i and GH release are sensitive to somatostatin inhibition; and 4) cAMP-elevating agents and PMA have an additive effect on the GHRP-1-stimulated GH release, indicating these agents stimulate GH release via a mechanism separate from that of GHRP-1.

Amino Acid Sequence↗
PubMed1993

Evidence of muscarinic acetylcholine receptors and GTP-binding proteins in peritoneal mesothelial cells in vitro.

Preliminary evidence of muscarinic acetylcholine receptors (AchRs) in rabbit and human peritoneal mesothelial cells grown in tissue culture is reported. Atropine displaceable binding of L-quinuclidinyl[phenyl-4-3H]-benzilate (QNB), an antagonist that binds to all AchR subtypes, and [N-methyl-3H]-4-diphenylacetoxy-N-methyl-piperidine methiodide (4-DAMP), an antagonist specific for AchR subtypes 1 and 3, to homogenates of mesothelial cells was approximately 400 and 120 fmol ligand bound/milligram cell protein, respectively. A similar value for specific [3H]-QNB binding was observed in living cells exposed to ligand and atropine for 1 hour at 37C. Guanosine 5'-triphosphate (GTP)-binding proteins measured as beta,gamma-imido[8-3H] guanosine 5'-triphosphate binding to mesothelial cell membranes was approximately 9 pmol/mg cell protein. Our finding of both muscarinic receptors and GTP-binding proteins in mesothelial cells suggests first, the presence of a functional muscarinic receptor system(s) in peritoneal mesothelial cells, and second, the potential susceptibility of these cells to direct pharmacological manipulation by muscarinic drugs.

Animals↗
PubMed1992

Spotted fever in Hong Kong.

A previously healthy 7-year-old Hong Kong-born Caucasian child developed sudden onset fever, followed by a generalized rash and systemic symptoms of rigor and prostration, mucous membrane involvement (conjunctivitis) and arthralgia. He lives in a rural area of Hong Kong and has been in contact with various domestic animals--rodents, dogs and cows. Chloramphenicol 50 mg/kg/day was given on day 4 with rapid response. Subsequent Weil-Felix test and specific serology suggested the diagnosis of rickettsial infection of the spotted fever group. To our knowledge, this is the first confirmed case of spotted fever reported in Hong Kong.

Child↗
PubMed1992

Ethanol reduces norepinephrine-stimulated melatonin synthesis in rat pinealocytes.

In this study, the in vitro effects of ethanol on norepinephrine-stimulated cyclic AMP (cAMP), N-acetyltransferase (NAT), and melatonin (MT) production were examined in dispersed rat pinealocytes. Cellular cAMP content was determined 15 min after treatment; whereas NAT activity and MT release in the medium were determined 4.5 h after treatment. It was found that ethanol less than or equal to 200 mM had no effect on norepinephrine-stimulated cAMP response, whereas 25 mM ethanol resulted in a significant inhibition of norepinephrine-stimulated NAT and MT levels. Furthermore, ethanol was equally effective in inhibiting the dibutyryl cAMP-stimulated NAT and MT levels. The inhibitory action of ethanol was not due to a direct effect or a delay in the onset of NAT activity. When alcohols with different chain lengths were used, it was found that their inhibitory potencies were related to their chain lengths with butanol greater than propanol greater than ethanol greater than methanol. Taken together, these findings indicate that (1) ethanol has an inhibitory action on norepinephrine-stimulated MT synthesis, (2) one site of ethanol action is distal to cAMP elevation, and (3) the inhibitory effect of ethanol on pineal MT synthesis appears to be secondary to its hydrophobic action.

Alcohols↗
PubMed1992

Intracellular pH on protein kinase C and ionomycin potentiation of isoproterenol-stimulated cyclic AMP and cyclic GMP production in rat pinealocytes.

In rat pinealocytes, alpha 1-adrenergic activation, which leads to cytoplasmic alkalinization, also potentiates the beta-adrenergic stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) responses. Both elevation of intracellular calcium ([Ca2+]i) and activation of protein kinase C are involved in the potentiation mechanism. Recently, intracellular pH has also been found to modulate the adrenergic-stimulated cyclic nucleotide responses, suggesting intracellular pH may also affect the potentiation mechanism. This possibility was examined in the present study. Cytoplasmic alkalinization by ammonium chloride had an enhancing effect on the isoproterenol and ionomycin-stimulated cAMP and cGMP accumulation. In comparison, cytoplasmic acidification by sodium propionate reduced the isoproterenol and ionomycin-stimulated cAMP and cGMP responses. Direct measurement of [Ca2+]i indicated that neither ammonium chloride nor sodium propionate had an effect on the ionomycin-stimulated elevation of [Ca2+]i, suggesting their effects on cyclic nucleotide responses may be independent of [Ca2+]i. In cells stimulated by isoproterenol and an activator of protein kinase C, ammonium chloride had an enhancing effect on both cAMP and cGMP responses, whereas sodium propionate had no effect. Taken together, these results suggest that a site distal to elevation of [Ca2+]i and activation of protein kinase C, of importance to the potentiation mechanism, is modulated by intracellular pH.

Ammonium Chloride↗
PubMed1992

Mycobacteria as a cause of infective exacerbation in bronchiectasis.

In 91 patients with bronchiectasis seen over 6 years, a positive mycobacterial culture was obtained in 12 cases (13%). The organisms isolated were Mycobacterium tuberculosis in nine cases, Mycobacterium avium in two cases and Mycobacterium tuberculosis and chelonei were obtained on separate occasions in one case. Computed tomography and/or bronchography showed that the bronchiectatic changes commonly involved the lower lobes and to a lesser extent, the middle and lingula lobes. In none of these 12 cases was tuberculosis strongly suspected on clinical or radiological grounds. We conclude that mycobacterial infections are common in patients with bronchiectasis and sputum should be cultured for mycobacteria periodically in these patients. In doubtful cases, bronchoscopy may be helpful to obtain a positive mycobacterial culture.

Adult↗
PubMed1992

Differential effects of intracellular calcium elevating agents on adrenergic-stimulated cyclic nucleotide and melatonin synthesis in rat pinealocytes.

In this study, the role of elevation of intracellular Ca2+ and activation of protein kinase C on adrenergic-stimulated cyclic nucleotide accumulation and melatonin synthesis in rat pinealocytes was investigated. It was found that whereas KCl, ionomycin, and ouabain, three Ca(2+)-elevating agents, had a potentiating effect on adrenergic-stimulated cyclic AMP response, their effects on melatonin synthesis were inhibitory. Similar inhibition was also observed when dibutyryl cyclic AMP was used to stimulate melatonin synthesis. By determining intracellular Ca2+ directly, it was found that the enhancing effects of these agents on the cyclic AMP response but not their inhibitory effects on melatonin synthesis paralleled their abilities to elevate intracellular Ca2+. In comparison, activation of protein kinase C significantly enhanced the adrenergic-stimulated cyclic AMP response and, to a lesser degree, the adrenergic-stimulated N-acetyltransferase and melatonin levels. These results indicate that (i) Ca(2+)-elevating agents have opposite effects on adrenergic-stimulated cyclic AMP and melatonin production; (ii) a post cyclic AMP event of importance to melatonin synthesis is inhibited by these agents; and (iii) the mechanism of inhibition may not be directly related to their effect on intracellular Ca2+.

Animals↗
PubMed1992

Inhibitors of Na(+)-H+ exchange block stimulus-provoked pineal melatonin synthesis.

In rat pinealocytes, amiloride can modulate adrenergic-stimulated cyclic nucleotide accumulation. In this study, the effect of amiloride on melatonin production was characterized. Addition of 5-(N,N-hexamethylene)amiloride, a potent inhibitor of the Na(+)-H+ antiport, dose dependently inhibited norepinephrine- and isoproterenol-stimulated N-acetyltransferase (NAT) activity and melatonin production. Similar inhibition was also observed when pineal melatonin synthesis was stimulated directly with forskolin or dibutyryl adenosine 3',5'-cyclic monophosphate (cAMP), suggesting that the site of inhibition is distal to cAMP accumulation. Similarities between the inhibitory potencies of amiloride derivatives on the Na(+)-H+ antiport and pineal melatonin synthesis indicate that the observed inhibition on pineal melatonin synthesis by amilorides may be secondary to their actions on the Na(+)-H+ antiport. Further studies also indicate that the inhibitory effect of amilorides was not secondary to its cytotoxic actions and that amilorides had no direct antagonistic action on NAT activity. Our findings, therefore, suggest that, in addition to their effects on cyclic nucleotide accumulation, the Na(+)-H+ antiport also plays an important role in the cAMP-mediated melatonin synthesis in the rat pineal gland.

Amiloride↗
PubMed1992

Inhibitory action of ethanol on L-type Ca2+ channels and Ca(2+)-dependent guanosine 3',5'-monophosphate accumulation in rat pinealocytes.

It has previously been shown that the K+ potentiation of vasoactive intestinal peptide-stimulated cAMP and cGMP responses was inhibited by ethanol in rat pinealocytes, suggesting an inhibitory action of ethanol on the voltage-dependent Ca2+ channels (VDCC). In this study, using the whole cell version of the patch clamp technique, we found that ethanol reduced the amplitude, but did not change the voltage dependence or the time course of activation or inactivation of the L-type VDCC. The inhibitory effect of ethanol on this current was concentration dependent, and ethanol (100 mM) resulted in a 40% inhibition of this current. However, in fura-2-loaded cells, total increases in intracellular Ca2+ ([Ca2+]i) caused by ethanol and BayK 8644 did not differ from the [Ca2+]i signal caused by BayK 8644 alone, suggesting that the inhibitory action of ethanol on VDCC may not be related to a reduction in [Ca2+]i. Although there was no change in the total [Ca2+]i signal, ethanol (25-200 mM) dose-dependently inhibited the potentiation effects of depolarizing concentrations of K+ and BayK 8644 on the isoproterenol-stimulated cGMP, but not the cAMP, response. Therefore, the cGMP response appears to be more sensitive to the inhibitory action of ethanol, and a site distal to elevation of [Ca2+]i of importance to the potentiation mechanism may be inhibited by ethanol. This was confirmed by the finding that ethanol was effective in inhibiting the A23187 potentiation of isoproterenol-stimulated cGMP response. These results suggest that 1) the L-type VDCC was inhibited by ethanol; 2) the Ca(2+)-mediated potentiation of the isoproterenol-stimulated cGMP response was sensitive to the inhibitory action of ethanol; and 3) although ethanol inhibits the VDCC, it alone cannot explain the inhibitory effect of ethanol on BayK 8644- and K(+)-mediated potentiation of the isoproterenol-stimulated cGMP response.

Animals↗
PubMed1992

Intracellular pH and growth hormone-releasing factor-stimulated adenosine 3'5'-monophosphate, intracellular calcium and growth hormone release from rat anterior pituitary cells.

In this study, we examined the effect of changes in intracellular pH (pHi) on basal and GH-releasing factor (GRF)-stimulated cyclic AMP (cAMP), intracellular Ca2+ and GH release using a static monolayer culture prepared from dispersed rat anterior pituitary cells. To modulate pHi, two approaches were used: variation of extracellular pH (pHo) and addition of sodium propionate and ammonium chloride which alter pHi directly. Direct pHi measurement with 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein showed that for pHo values between 6.9 and 7.6, a change in pHo of 0.1 units resulted in a change in pHi of 0.045 units. Sodium propionate (30 mmol/l) reduced pHi by 0.06 units whereas ammonium chloride (30 mmol/l) increased pHi by 0.1 units. Increasing pHo from 6.6 to 7.8 enhanced the maximal GRF-stimulated cAMP and GH responses by 80% and 300% respectively, indicating that the GRF-stimulated cAMP and GH release were both pH-dependent. Acute elevation of pHo from 6.6 to 7.8 also increased basal GH release by sixfold. Reduction of pHi by sodium propionate, however, had no significant effect on GRF-stimulated cAMP levels while the corresponding GRF-stimulated GH release was reduced by up to 40%. In comparison, elevation of pHi by ammonium chloride enhanced the GRF-stimulated cAMP release by up to 75% and the corresponding increase in GH was less than 20%. When the relationship between pHi and intracellular Ca2+ was determined with the fluorescent Ca2+ indicator, Fura-2, it was found that increasing pHo and treatment with ammonium chloride increased intracellular Ca2+, while sodium propionate and reducing pHi had no effect on intracellular Ca2+. These results indicate that activation of adenylate cyclase and mobilization of intracellular Ca2+, two intracellular signalling pathways of importance to GH secretion, are both sensitive to changes in pHi.

Ammonium Chloride↗
PubMed1992

The laboratory diagnosis of rickettsial diseases in Hong Kong.

Indirect immunoperoxidase (IP) tests against antigens of Rickettsia typhi, R. sibirica and R. tsutsugamushi were carried out on serum samples from 10 patients known to have high titres against one or more Proteus OX antigens in the Weil-Felix (WF) test. The IP test confirmed rickettsial infection in nine of the 10 patients, giving unequivocal indications of the grouping of the infecting Rickettsia species and, in one instance, showing a diagnostic IgM titre in a specimen taken on day 3 of the patient's illness. IP tests confirmed four cases of spotted fever group rickettsial infection, three cases of urban or murine typhus and two of scrub typhus.

Adolescent↗
PubMed1991

Inhibitory effects of ethanol on the calcium-dependent potentiation of vasoactive intestinal peptide-stimulated cAMP and cGMP accumulation in rat pinealocytes.

In rat pinealocytes, ethanol has been shown recently to inhibit the alpha 1-adrenergic potentiation of vasoactive intestinal peptide (VIP)-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) responses, with the cGMP response being more sensitive to the inhibition. Two intracellular events known to be of importance to the potentiation mechanism are activation of protein kinase C and elevation of intracellular calcium [( Ca2+]i). In this study, we examined the effects of ethanol on these two intracellular mechanisms with an activator of protein kinase C, and two agents that elevate [Ca2+]i, depolarizing concentrations of K+ and A23187. Using dispersed pinealocytes, ethanol (up to 175 mM) was found to have no effect on the 4 beta-phorbol 12-myristate 13-acetate (PMA, an activator of protein kinase C) potentiated VIP-stimulated cAMP response. As for the cGMP response, full potentiation requires both activation of protein kinase C and simultaneous elevation of [Ca2+]i. This could be achieved by stimulating the VIP-treated cells with a combination of PMA and 15 mM K+. In the presence of ethanol, the amplification effect of PMA and K+ on the VIP-stimulated cGMP response was inhibited with an IC50 value of 125 mM. In contrast, similar concentrations of ethanol had no effect on the corresponding cAMP response. These findings suggest that the potentiation of cAMP response by protein kinase C is not affected by ethanol. When depolarizing concentrations of K+ were used to potentiate the VIP-stimulated cAMP and cGMP accumulation, ethanol inhibited the K+ potentiation of VIP-stimulated cAMP and cGMP responses with IC50 values of 50 and 30 mM, respectively. The A23187 potentiation of VIP-stimulated cGMP response was also sensitive to the inhibitory effect of ethanol with an IC50 value of 120 mM. In comparison, the corresponding IC50 value for the cAMP response was greater than 175 mM. Based on our findings, we conclude that ethanol likely inhibits a Ca(2+)-dependent event(s) that is critical to the alpha 1-adrenergic-mediated potentiation of VIP-stimulated cAMP and cGMP responses.

Animals↗
PubMed1991

Evidence for the simultaneous translocation of muscarinic acetylcholine receptor and G protein by carbachol.

Interactions between guanine nucleotide regulatory proteins (G proteins) and muscarinic acetylcholine receptors (mAChRs) were studied in vivo following carbachol treatment. Rat brain homogenates were separated by high speed ultracentrifugation into heavy and light membrane and 300,000 g supernate fractions. The G proteins were partially purified by Sephadex-G200 and heptylamine-Sepharose and the mAChRs by (3,2'-aminobenzhydryloxy)-tropane-(ABT)-affinity chromatographies. Radioligand binding assays showed that acute carbachol induced a biphasic translocation of the mAChRs and G proteins into the light membrane fraction with an initial release at 5-10 min and a second phase at 60 min. Portions of the released mAChRs and the G proteins, were found in the 300,000 g supernates and light membranes and were eluted in the same peak fractions from a Sephadex G-200 column. This dually labelled peak dissociated in the presence of digitonin, suggesting close association between the mAChR and G protein. ABT-affinity chromatography yielded dually labelled mAChR-G protein fractions which eluted as a single radioactive peak on a second ABT column. The partially purified G proteins from these fractions were photoaffinity labelled with 8-azidoguanosine-5'-triphosphate, [gamma-32P]. SDS-PAGE autoradiography revealed the presence of Go alpha and Gi alpha which may be released simultaneously with the mAChRs from the plasma membrane. In addition, 110,000 molecular weight polypeptide was dually labelled by [3H]-PrBCM and [gamma-32P]-8-azido-GTP suggesting the presence of a "mAChR-G protein complex." These findings provide direct evidence for the release of mAChRs and G proteins and a mAChR-G protein complex by agonist occupation of the mAChRs.

Animals↗