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Biomedical subjects

A K Ho

Publications and source records attributed to A K Ho.

At least 37 records · Page 2Linked to original sources
PubMed2001

L-type Ca(2+) channel regulation by pituitary adenylate cyclase-activating polypeptide in vascular myocytes from spontaneously hypertensive rats.

Pituitary adenylate cyclase-activating polypeptide (PACAP), a vasoactive peptide, modulates the L-type Ca(2+) channel current (L channel current) in vascular smooth muscle cells (VSMC) through activation and integration of two intracellular pathways, protein kinase A and protein kinase C (PKC). In the present study we compared the effects of PACAP on the L channel current in VSMC from the spontaneously hypertensive rats (SHR) and normotensive controls, Wistar Kyoto rats (WKY). We found that compared with WKY, VSMC from SHR had a higher L channel current density. Stimulation by PACAP (10 nM) caused an increase in the amplitude of the whole cell current and prolonged open time in VSMC from SHR and WKY, with the increase greater in SHR. These effects of PACAP on the L channel current was mimicked by an activator of PKC. In contrast, PACAP caused a smaller increase in cAMP accumulation in VSMC from SHR than WKY, and there was no difference in the inhibitory effect of 8-bromo-cAMP on the L channel current from both type of cells. The greater increase in amplitude of the L channel current by PACAP in VSMC from SHR persisted in the presence of adenosine cyclic 3',5'-monophosphothioate, Rp-isomer, a cAMP antagonist, but not calphostin C, a PKC inhibitor. Taken together, our results show an increase in L channel current density and an enhanced PACAP effect on the L channel current in VSMC from SHR compared with WKY. This difference in PACAP response appears to be predominately secondary to an increased PKC sensitivity.

Animals↗
PubMed2000

Volume perception in parkinsonian speech.

This study contrasted the volume level of speech production with perceived volume. Fifteen idiopathic patients with Parkinson's disease who have hypophonic dysarthria and 15 healthy age- and sex-matched control subjects participated in this study. Testing took place in a sound-proof room. Ability to regulate volume was tested at three instructional levels of loudness: participants were given no instructions regarding volume (to elicit normal default volume) or were asked to read loudly or quietly. Two types of volume-perception judgments were made. First, an estimate of one's own volume, immediately after speaking (that is, immediate perception), and secondly, an estimation of reading volume after hearing one's own voice played back (that is, playback perception). These perceptual ratings were compared with actual speech volume produced in reading and conversation tasks. It was found that there was less of a difference between patients' production and perception of speech volume compared with that of the control subjects. While patients spoke more quietly than control subjects, they nevertheless perceived (immediate and playback perception) their own speech to be louder than did the control subjects. Patients overestimated the volume of their speech during both reading and conversation. The findings raise the question as to whether impaired speech production is driven by a basic perceptual fault or whether perception is abnormal as a consequence of impaired mechanisms involved in the generation of quiet speech.

Aged↗
PubMed2000

6-Hydroxydopamine-induced developmental cardiac alterations in morphology, calmodulin content, and K(2+)-mediated [Ca(2+)](i)Transient of chicken cardiomyocytes.

Calcium and the calcium-calmodulin-mediated processes have been implicated in cardiac hypertrophy. The purpose of this study was to investigate whether there is a potential role played by the calcium-calmodulin processes in cardiac morphogenesis and malformations, especially in the development of cardiac hypertrophy. Recently, the authors reported that 6-hydroxydopamine, an adrenergic neurotoxin can produce malformations in various organs including ventricular septal lesions and cardiac hypertrophy in the developing chicken embryo. Morphological studies revealed areas of coagulative necrosis, with broken nuclear membranes, swollen mitochondria and dilations of the ventricles, as well as thickening of ventricular walls reminiscent of cardiac hypertrophy. The observation that 6-hydroxydopamine treatment on day 3 of incubation produced a dose-dependent increase in both heart and brain calmodulin levels on day 11 of incubation and an increase in the sensitivity to external potassium induction of intracellular free calcium transient in incubation day 14 chicken cardiomyocytes in culture, leading to an increase in intracellular free calcium is reported here. However, sodium/potassium adenosinetriphosphatase activity showed no significant change on days 12 and 16 of incubation. The effect appears to be relatively specific since 5-hydroxydopamine, a chemical isomer of 6-hydroxydopamine, failed to produce a similar sensitivity change of potassium-induced intracellular calcium transient.

Abnormalities, Drug-Induced↗
PubMed2000

Intracellular pH on translocation of protein kinase C isozymes in rat pinealocytes.

In rat pinealocytes, cytoplasmic alkalization causes protein kinase C (PKC) translocation, but the isozyme involved is not known. In this study, we investigated the effect of cytoplasmic alkalization on membrane-associated PKCalpha, delta, epsilon, and zeta, four isozymes present in the rat pineal gland. Treatment with NH(4)Cl, which had no effect on PKCzeta, caused a sustained increase in membrane-associated PKCalpha, delta, and epsilon that lasted for at least 60 min. The effect of NH(4)Cl on PKCalpha, delta, and epsilon was reduced by sodium propionate, an agent that counteracts the effect of NH(4)Cl on intracellular pH. Both sodium propionate and 5-(N,N-hexamethylene)amiloride (HMA), two treatments that abolished the effect of norepinephrine on cytoplasmic alkalization, also reduced norepinephrine-mediated increases in membrane-associated PKCalpha, delta, and epsilon. In contrast, these two treatments did not have an effect on the increase in membrane-associated PKC isozymes caused by 4beta-phorbol 12-myristate 13-acetate (PMA), an active phorbol ester, even though HMA was effective in abolishing PMA-mediated increases in intracellular pH. These results, apart from demonstrating that cytoplasmic alkalization by itself can cause translocation of PKCalpha, delta, and epsilon in rat pinealocytes, also indicate that the norepinephrine-stimulated cytoplasmic alkalization plays an important role in transducing signals from the adrenergic receptor to selective PKC isozymes. However, PKC translocation stimulated directly by PMA does not appear to be sensitive to changes in intracellular pH.

Amiloride↗
PubMed2000

Assembly and preferential localization of Nup116p on the cytoplasmic face of the nuclear pore complex by interaction with Nup82p.

The yeast Saccharomyces cerevisiae nucleoporin Nup116p serves as a docking site for both nuclear import and export factors. However, the mechanism for assembling Nup116p into the nuclear pore complex (NPC) has not been resolved. By conducting a two-hybrid screen with the carboxy (C)-terminal Nup116p region as bait, we identified Nup82p. The predicted coiled-coil region of Nup82p was not required for Nup116p interaction, making the binding requirements distinct from those for the Nsp1p-Nup82p-Nup159p subcomplex (N. Belgareh, C. Snay-Hodge, F. Pasteau, S. Dagher, C. N. Cole, and V. Doye, Mol. Biol. Cell 9:3475-3492, 1998). Immunoprecipitation experiments using yeast cell lysates resulted in the coisolation of a Nup116p-Nup82p subcomplex. Although the absence of Nup116p had no effect on the NPC localization of Nup82p, overexpression of C-terminal Nup116p in a nup116 null mutant resulted in Nup82p mislocalization. Moreover, NPC localization of Nup116p was specifically diminished in a nup82-Delta108 mutant after growth at 37 degrees C. Immunoelectron microscopy analysis showed Nup116p was localized on both the cytoplasmic and nuclear NPC faces. Its distribution was asymmetric with the majority at the cytoplasmic face. Taken together, these results suggest that Nup82p and Nup116p interact at the cytoplasmic NPC face, with nucleoplasmic Nup116p localization utilizing novel binding partners.

Cell Nucleus↗
PubMed2000

p38MAPK inhibition enhances basal and norepinephrine-stimulated p42/44MAPK phosphorylation in rat pinealocytes.

Interaction between p38MAPK and p42/44MAPK in rat pinealocytes was examined by determining the effects of p38MAPK inhibitors on the phosphorylation of p42/44MAPK using Western blot analysis. Treatment with SB202190, a specific inhibitor of p38MAPK, increased p42/44MAPK phosphorylation in a concentration-dependent manner. SB202190 also enhanced the magnitude and the duration of norepinephrine-activated p42/44MAPK phosphorylation. The effect of SB202190 on p42/44MAPK phosphorylation was abolished by PD98059 or UO126, inhibitors of MEK, suggesting that SB202190 is acting upstream of MEK in activating p42/44MAPK. The SB202190-induced phosphorylation of p42/44MAPK was not blocked by inhibitors of cGMP-dependent kinase (KT5823), protein kinase C (calphostin C) or Ca2+/calmodulin dependent kinase (KN93) suggesting that these pathways may not be involved in the effect of SB202190. SB202190 further increased p42/44MAPK phosphorylation that was stimulated by 8-bromo-cGMP, 4beta phorbol 12-myristate 13-acetate, or ionomycin. In contrast, inhibition of p42/44MAPK phosphorylation by dibutyryl-cAMP persisted when p42/44MAPK phosphorylation was increased by SB202190. Furthermore, inhibition of p42/44MAPK phosphorylation had no effect on p38MAPK activation. These results suggest that inhibition of p38MAPK causes activation of p42/44MAPK and acts synergistically with norepinephrine in the regulation of p42/44MAPK activation in rat pinealocytes.

Animals↗
PubMed2000

Adrenergic regulation of mitogen-activated protein kinase in rat pinealocytes: opposing effects of protein kinase A and protein kinase G.

The role of adrenergic stimulation in the regulation of mitogen-activated protein kinase (MAPK) in rat pinealocytes was investigated by measuring phosphorylated MAPK using Western blot analysis and a MAPK enzymatic assay. Stimulation with the endogenous neurotransmitter, norepinephrine (NE; a mixed alpha- and beta-adrenergic agonist), concentration dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of NE was blocked by PD98059 and U0126 (two inhibitors of MEK). Treatment with prazosin or propranolol significantly reduced the effect of NE on MAPK phosphorylation, suggesting the involvement of both alpha- and beta-adrenergic receptors. Investigation into the intracellular mechanisms of NE action revealed that the increase in MAPK phosphorylation was blocked by KT5823 (a protein kinase G inhibitor), but was enhanced by H89 (a protein kinase A inhibitor). Calphostin C (a protein kinase C inhibitor) and KN93 (a Ca2+/calmodulin-dependent protein kinase inhibitor) also attenuated NE-mediated MAPK activation, but to a lesser degree. Furthermore, inhibition of MAPK phosphorylation by (Bu)2cAMP was effective in reducing MAPK activation by (Bu)2cGMP, an active phorbol ester or ionomycin. These results indicate that the effect of NE on MAPK phosphorylation represents mainly the integration of two signaling mechanisms, protein kinase A and protein kinase G, each having an opposite effect on MAPK phosphorylation.

Animals↗
PubMed1999

Protection of cardiomyocytes by pinacidil during metabolic inhibition and hyperkalemia.

The objective of this study is to understand the mechanism underlying the cardioprotective effects of pinacidil, an ATP-sensitive K+ channel (K(ATP)) opener. We examined the effects of 10 microM pinacidil in cultured chicken cardiomyocytes. Pinacidil caused a concentration-dependent delay in metabolic inhibition-induced increase in intracellular calcium concentration ([Ca2+]i) and creatine phosphokinase release, and this action was antagonized by glyburide, a K(ATP) blocker. Neither verapamil, an L-type Ca2+ channel blocker, nor bepridil, a Na+-Ca2+ exchange inhibitor, affected the time course of increase in [Ca2+]i induced by metabolic inhibition. Pinacidil did not have an effect on the amplitude of K+-induced increase in [Ca2+]i, but accelerated the rate of decline following peak stimulation. In contrast, glyburide reduced the amplitude of K+-induced increase in [Ca2+]i and prolonged the rate of decline. These results provide direct evidence that pinacidil protects cardiomyocytes from metabolic inhibition-induced injury by cyanide (CN) through a delay in the onset of increase in [Ca2+]i, rather than by inhibition of the L-type Ca2+-channels or by alteration of Na+-Ca2+ exchange.

Adenosine Triphosphate↗
PubMed1999

Adrenergic regulation of translocation of protein kinase C isozymes in rat pinealocytes.

Protein kinase C (PKC) is involved in the alpha1-adrenergic-potentiation of beta-adrenergic stimulated cyclic nucleotide responses in rat pinealocytes. In the present study, the PKC isozymes expressed in rat pinealocytes and their regulation by norepinephrine (NE) were investigated. Western blot analysis identified PKC alpha (a classical PKC isozyme), PKC delta and epsilon (novel PKC isozymes), and PKC zeta: (atypical PKC isozymes). NE caused an increase in PKC alpha, delta, and epsilon, but not PKC zeta, in the particulate fraction. BAPTA-AM, which clamps intracellular Ca2+, reduced NE mediated translocation of PKC alpha, delta, and epsilon. Subjecting the animals to stimulus deprivation, which altered adrenergic-stimulated cyclic nucleotide responses, had no effect on the expression of PKC alpha, delta, epsilon, and zeta. Overnight treatment with 4beta-phorbol 12-myristate 13-acetate, an activator of PKC, down-regulated PKC alpha, delta, and epsilon, but not PKC zeta. Our results indicate that all three classes of PKC isozymes (PKC alpha, delta, epsilon, and zeta are expressed in the rat pineal gland. However, selective activation of these PKC isozymes does not appear to account for the differences in the pineal cAMP and cGMP responses to stimulation.

Animals↗
PubMed1999

Speech volume regulation in Parkinson's disease: effects of implicit cues and explicit instructions.

This study examined the regulation of speech volume in hypophonic subjects with Parkinson's disease (PD) and age- and gender-matched controls. The first two experiments investigated the ability of subjects with PD to automatically regulate speech volume in response to two types of implicit cue: (i) background noise (BGN) and (ii) instantaneous auditory feedback (IAF). Control subjects demonstrated the Lombard effect by automatically speaking louder when competing against increasing levels of background noise. They also showed the reverse effect, decreasing speech volume when increasing levels of facilitative instantaneous auditory feedback were provided. Subjects with PD demonstrated decreased overall speech volume; they were less able than controls to appropriately increase volume as background noise increased, and to decrease volume as IAF increased. Thus, subjects with PD demonstrated over-constancy of speech volume and failed to respond to the implicit cues integral to volumetric scaling. A further experiment (3) was carried out to contrast the regulation of volume in response to implicit cue with an explicit attention-driven cue (i.e. instructions regarding volume level). As in Experiments 1 and 2, subjects with PD exhibited reduced speech volume. Under explicit volume instructions, the ability of subjects with PD to regulate volume was normalised. These findings suggest that subjects with PD have the capacity to speak with normal volume provided they consciously attend to speaking loudly. In subjects with PD, overall speech volume was always lower than for control subjects, suggesting a reduction of cortical motor set in the articulatory system similar to that demonstrated by the reduced amplitude of limb movements (hypokinesia) in the motor system.

Aged↗
PubMed1999

Regulation of the L-type Ca2+ channel current in rat pinealocytes: role of basal phosphorylation.

In the present study, the role of phosphoprotein phosphatase in the regulation of L-type Ca2+ channel currents in rat pinealocytes was investigated using the whole-cell version of the patch-clamp technique. The effects of three phosphatase inhibitors, calyculin A, tautomycin, and okadaic acid, were compared. Although all three inhibitors were effective in inhibiting the L-type Ca2+ channel current, calyculin A was more potent than either tautomycin or okadaic acid, suggesting the involvement of phosphoprotein phosphatase-1. To determine the kinase involved in the regulation of these channels, cells were pretreated with H7 (a nonspecific kinase inhibitor), H89 (a specific inhibitor of cyclic AMP-dependent kinase), KT5823 (a specific inhibitor of cyclic GMP-dependent kinase), or calphostin C (a specific inhibitor of protein kinase C). Pretreatment with either H7 or calphostin C decreased the inhibitory effect of calyculin A on the L-type Ca2+ channel current. In contrast, pretreatment with H89 or KT5823 had no effect on the inhibition caused by calyculin A. Based on these observations, we conclude that basal phosphatase activity, probably phosphoprotein phosphatase-1, plays an important role in the regulation of L-type Ca2+ channel currents in rat pinealocytes by counteracting protein kinase C-mediated phosphorylation.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1999

3',5'-cyclic guanosine monophosphate activates mitogen-activated protein kinase in rat pinealocytes.

The role of 3',5'-cyclic guanosine monophosphate (cGMP) in the activation of mitogen-activated protein kinases (MAPKs) was investigated in rat pinealocytes. Treatment with dibutyryl cGMP (DBcGMP) dose-dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of DBcGMP was abolished by PD98059 (a MAPK kinase inhibitor), H7 (a nonspecific protein kinase inhibitor), and KT5823 [a selective cGMP-dependent protein kinase (PKG) inhibitor]. Elevation of cellular cGMP content by treatment with norepinephrine, zaprinast (a cGMP phosphodiesterase inhibitor), or nitroprusside was effective in activating MAPK. Natriuretic peptides that were effective in elevating cGMP levels in this tissue were also effective in activating MAPK. Our results indicate that, in this neuroendocrine tissue, the cGMP/PKG signaling pathway is an important mechanism used by hormones and neurotransmitters in activating MAPK.

Alkaloids↗
PubMed1999

Regulation of parkinsonian speech volume: the effect of interlocuter distance.

This study examined the automatic regulation of speech volume over distance in hypophonic patients with Parkinson's disease and age and sex matched controls. There were two speech settings; conversation, and the recitation of sequential material (for example, counting). The perception of interlocuter speech volume by patients with Parkinson's disease and controls over varying distances was also examined, and found to be slightly discrepant. For speech production, it was found that controls significantly increased overall speech volume for conversation relative to that for sequential material. Patients with Parkinson's disease were unable to achieve this overall increase for conversation, and consistently spoke at a softer volume than controls at all distances (intercept reduction). However, patients were still able to increase volume for greater distances in a similar way to controls for conversation and sequential material, thus showing a normal pattern of volume regulation (slope similarity). It is suggested that speech volume regulation is intact in Parkinson's disease, but rather the gain is reduced. These findings are reminiscent of skeletal motor control studies in Parkinson's disease, in which the amplitude of movement is diminished but the relation with another factor is preserved (stride length increases as cadence-that is, stepping rate, increases).

Aged↗
PubMed1999

Ceramide enhances growth hormone (GH)-releasing hormone-stimulated cyclic adenosine 3',5'-monophosphate accumulation but inhibits GH release in rat anterior pituitary cells.

In this study, the effect of ceramide on GH-releasing hormone (GHRH)-stimulated cAMP accumulation and GH release in rat anterior pituitary cells was investigated. C2-, C6-, and C8-ceramide were found to enhance GHRH-stimulated cAMP accumulation. In contrast, their effects on GHRH-stimulated GH release were inhibitory. Treatment with a glucosylceramide synthase inhibitor produced a similar enhancing effect on cAMP accumulation and an inhibitory effect on GH release. To identify the pathway through which ceramide mediated its effect, it was found that ceramide inhibited GH release stimulated by KCl, BayK 8644, and a GH-releasing peptide, but not that stimulated by ionomycin or an activator of protein kinase C. Direct measurement of intracellular Ca2+ revealed that C2-ceramide inhibited GHRH- and KCl-mediated increases in intracellular Ca2+, suggesting that ceramide probably inhibits GH release through inhibition of the L-type Ca2+ channels. As for its mechanism on cAMP accumulation, the enhancing effect of ceramide on GHRH-stimulated cAMP accumulation was abolished in the presence of a phosphodiesterase inhibitor, isobutylmethylxanthine, suggesting that ceramide enhances the cAMP response through inhibition of its metabolism. Taken together, our results suggest that ceramide plays an important role in the regulation of GHRH-stimulated responses in somatotrophs. By reducing GH secretion while enhancing cAMP accumulation, ceramide may promote the synthesis and storage of GH in rat anterior pituitary cells.

1-Methyl-3-isobutylxanthine↗
PubMed1999

Ceramide inhibits L-type calcium channel currents in rat pinealocytes.

In rat pinealocytes, ceramide can inhibit the KCl- and BayK 8644-mediated potentiation of cAMP and cGMP accumulation, suggesting that the L-type Ca2+ channel is a target of ceramide action. This was examined in the present study using intracellular Ca2+ measurement and patch-clamp studies. In fura-2-loaded pinealocytes, C2- and C6-ceramide inhibited the Ca2+ increase caused by BayK 8644 and KCl, but not that caused by norepinephrine, suggesting an inhibitory effect of ceramide on the L-type Ca2+ channels. Patch-clamp analysis confirmed that C2- and C6-ceramide, but not C2-dihydroceramide (the inactive analog) inhibited the L-type Ca2+ channel current. Furthermore, treatments known to increase cellular ceramide levels, including a glucosylceramide synthase inhibitor and sphingomyelinase, also inhibited this current. The inhibitory effect of ceramide on the current was attenuated by lavendustin A, a tyrosine kinase inhibitor, but not by H7, a serine/threonine kinase inhibitor. The effect of ceramide was mimicked by interleukin-1beta, a cytokine highly expressed in the pineal that is known to activate the sphingomyelin pathway. These results indicate that the sphingomyelin pathway is another important signaling mechanism that regulates the L-type Ca2+ channel, and tyrosine kinase appears to be involved in the effect of ceramide.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1999

6-Hydroxydopamine induced cardiac malformations and alterations of the autonomic nervous system in the developing chicken embryo.

6-Hydroxydopamine (6-OHDA) was injected into the air sac of developing chicken embryos on day E3 in order to study its effects on cardiac development both morphologically and biochemically. A dose-dependent teratogenic effect and fetotoxicity were observed in the 6-OHDA-treated embryos. Cardiac malformations, including ventricular septal lesions, detachment of the apical portions of the ventricles, cardiac hypertrophy, areas of coagulative necrosis with pyknotic nuclei and broken nuclear membranes, and swollen mitochondria were evident from gross histologic and ultrastructural examinations. A LD50 of 0.3 mg/egg on day E11 was obtained. Biochemically, 6-OHDA induced a significant dose-dependent reduction in the total cardiac choline acetyltransferase (ChAT) activities on days E8 and E11, followed by a recovery on days E15 and E20. The effects on muscarinic acetylcholine receptors (mAChRs) were less marked than on ChAT, indicating the effects on the cholinergic nervous system development are primarily presynaptic. There was a significant decrease in the level of norepinephrine (NE) and a delay in the appearance of detectable cardiac NE. It is suggested that 6-OHDA-induced cardiac malformation can be a useful model to study the mechanisms of cardiovascular development.

Adrenergic Agents↗
PubMed1998

Effects of Ca2+ channel blockers on Ca2+ loading induced by metabolic inhibition and hyperkalemia in cardiomyocytes.

The effects of the L-type (nifedipine and verapamil) and the T-type (mibefradil) Ca2+ channel blockers on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by NaCN metabolic inhibition and hyperkalemia were examined in chicken cardiomyocytes using fluorescence imaging with Fura-2. NaCN induced a slow and sustained rise in [Ca2+]i, which was not affected by pretreating the cells for 5 min with nifedipine, verapamil, or mibefradil at 100 nM or 10 microM. Pretreatment of the cells with 10 microM nifedipine, verapamil, or mibefradil for 5 min remarkably inhibited the K+-induced increase in [Ca2+]i. These inhibitory effects diminished after 48-h pretreatment with nifedipine or verapamil but not with mibefradil. Ryanodine also induces an increase in [Ca2+]i, and this effect was enhanced by 48-h pretreatment of the cells with 10 microM verapamil but not with 10 microM mibefradil. We conclude that the NaCN-induced increase in [Ca2+]i is independent of the Ca2+ influx though the L-type or T-type Ca2+ channels. Chronic inhibition of the L-type Ca2+ channels but not T-type channels may enhance the ryanodine receptor-mediated Ca2+ release, which may be responsible for the development of tolerance to their inhibitory effects on K+-induced increase in [Ca2+]i.

Animals↗