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Biomedical subjects

A K Ho

Publications and source records attributed to A K Ho.

At least 109 records · Page 6Linked to original sources
PubMed1991

Differential involvement of the arachidonic acid cascade on the alpha 1-adrenergic potentiation of vasoactive intestinal peptide- versus beta-adrenergic-stimulated cyclic AMP and cyclic GMP accumulation in rat pinealocytes.

In the rat pineal gland, alpha 1-adrenergic agonists, which stimulate arachidonic acid release, also potentiate vasoactive intestinal peptide (VIP)- or beta-adrenergic-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) accumulation. In this study, the possible involvement of the arachidonic acid pathway in the potentiation mechanism was examined in dispersed rat pinealocytes using two inhibitors of the arachidonic acid cascade, indomethacin and nordihydroguaiaretic acid. These two inhibitors appeared to have differential effects on the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses. Whereas nordihydroguaiaretic acid was effective in suppressing both the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP responses, indomethacin inhibited selectively the VIP-mediated cAMP and cGMP responses. The role of arachidonic acid metabolites was further determined using several prostaglandins--A2, I2, E2, and F2 alpha--and leukotrienes--B4, C4, and D4. Of the seven compounds tested, prostaglandins E2 and F2 alpha stimulated basal cAMP but not cGMP accumulation. The prostaglandin E2- and F2 alpha-stimulated cAMP responses were additive to those stimulated by VIP or beta-adrenergic receptors. The other five compounds had no effects on basal or VIP- or beta-adrenergic-stimulated cAMP or cGMP accumulation. Taken together, these findings indicate that the arachidonic acid cascade is likely involved in the alpha 1-adrenergic potentiation of VIP- or beta-adrenergic-stimulated cAMP and cGMP accumulation. However, the specific arachidonic acid metabolite involved in the potentiation mechanisms of VIP- versus beta-adrenergic-stimulated cyclic nucleotide responses may be different.

Animals↗
PubMed1991

Intracellular pH on adrenergic-stimulated cAMP and cGMP production in rat pinealocytes.

In rat pinealocytes, activation of alpha 1-adrenergic receptors leads to increases in intracellular pH (pHi). In this study, the role of pHi on adrenergic regulation of cyclic nucleotide accumulation was investigated using ammonium chloride, which increased pHi, and sodium propionate, which reduced pHi. Ammonium chloride significantly enhanced the norepinephrine-stimulated adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) responses, while it selectively potentiated the isoproterenol (ISO)-stimulated cGMP response. Reduction of pHi by sodium propionate reduced the norepinephrine-stimulated cGMP accumulation by 70%, and its effect on the ISO-stimulated cGMP response was stimulatory. Treatment with sodium propionate effectively neutralized the enhancing effects of ammonium chloride on the adrenergic-stimulated cAMP and cGMP responses. These effects of sodium propionate and ammonium chloride on cyclic nucleotides appeared to reflect altered rate of synthesis, and they were also in part secondary to changes in intracellular Ca2+. Our findings indicate that the receptor-mediated changes in pHi may play an integral part in the adrenergic regulation of cAMP and cGMP production in rat pinealocytes.

Ammonium Chloride↗
PubMed1991

See-saw signal processing: reciprocal effects of stimulus deprivation on vasoactive intestinal peptide-stimulated adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate accumulation in rat pinealocytes.

In the present study the effects of stimulus deprivation on vasoactive intestinal peptide (VIP)- and alpha 1-adrenergically mediated amplification of VIP-stimulated cAMP and cGMP accumulation were examined. Dispersed pinealocytes were prepared from either Sprague-Dawley rats maintained for 2 weeks in a normal lighting schedule providing 14 h of light/day (LD cells) or from animals maintained in constant lighting (LL cells). LL treatment enhanced the VIP-stimulated cAMP response up to 2-fold, while reducing the peak VIP-stimulated cGMP by 70%. In LL cells, phenylephrine potentiated the VIP-stimulated cAMP response, but did not potentiate the VIP-stimulated cGMP response. Potentiation of the cAMP response to VIP can be produced in LD cells by treatment with agents that elevate intracellular Ca2+ (depolarizing concentrations of K+ or A23187) or an activator of protein kinase-C [14 beta-phorbol 12-myristate 13-acetate (PMA)]. LL treatment abolished the potentiating effects of K+ or A23187 on cAMP and cGMP responses in VIP-treated cells. In contrast, LL treatment augmented the PMA potentiation of VIP-stimulated cAMP response. The potentiation effects of PMA and K+ on the cGMP response in VIP-treated cells, however, were suppressed by LL treatment. To further investigate the involvement of postreceptor mechanisms, forskolin was used to stimulate pineal cAMP and cGMP accumulation. LL treatment had similar effects on the forskolin-stimulated cyclic nucleotide responses, with one exception. Depolarizing concentrations of K+ potentiated the forskolin-stimulated cAMP response while having no effect on the VIP-stimulated cAMP responses. These findings suggest that LL treatment results in a larger VIP-stimulated cAMP response, while its effect on the cGMP response is inhibitory. LL treatment appears to inhibit a step distal to elevation of intracellular Ca2+ which is of importance to the alpha 1-adrenergic potentiation of VIP-stimulated cAMP and cGMP responses.

Animals↗
PubMed1990

Infective endocarditis caused by Streptococcus suis serotype 2.

Infective endocarditis caused by Streptococcus suis serotype 2 is not uncommon in pigs but is rare in human beings. We describe the case of a pig-farmer with endocarditis due to S. suis serotype 2 and in whom prolapse of the mitral valve was the predisposing cardiac lesion. Streptococcus suis, a possible cause of infective endocarditis in endemic areas, may be confused with other group D streptococci. In suspected cases a history of contact with pigs or raw pork should be sought.

Agricultural Workers' Diseases↗
PubMed1990

Phosphorylation-dephosphorylation of muscarinic acetylcholine receptors: evidence for the in vivo and in vitro release of receptors from rat brain plasma membrane.

The effects of phosphorylation on muscarinic acetylcholine receptors (mAChRs) were studied in vitro and in vivo using rat brain plasma membrane and receptors partially purified at least 2500-fold. Purified mAChRs were phosphorylated in vitro by cAMP-dependent protein kinase and dephosphorylated by calcineurin. Phosphorylation of purified mAChRs was enhanced by carbachol and blocked by atropine. The filtrate which passed through glass fiber filters and high speed supernates were assayed for mAChRs by an ammonium sulfate precipitation method. Following incubation of the plasma membrane under phosphorylating conditions and ultracentrifugation at 300,000 g, the mAChRs appeared in the high speed supernate. This release was stimulated by adding carbachol to the incubation medium. In rats treated with carbachol, brain mAChRs redistributed from the heavy into the light membrane fractions. Ultrastructural examination of the light membrane fractions and the 300,000 g supernatant fractions after in vivo and in vitro carbachol treatment calcineurin increased the reincorporation of added partially purified receptors into the plasma membrane. The release and reincorporation of mAChRs strongly imply that there is a translocation and recycling of mAChRs between plasma membrane and cytosol in vivo. The significance and the function of the translocation of mAChRs remain to be investigated.

Animals↗
PubMed1990

Inhibitory effects of amilorides on pinealocyte adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate accumulation: possible involvement of postreceptor mechanisms.

In rat pinealocytes, alpha 1-adrenergic receptor activation increases intracellular pH (pHi) through Ca2+/protein kinase-C-dependent activation of the Na+/H+ antiporter. Using a series of amiloride analogs, norepinephrine stimulation of cGMP accumulation is also found to be pHi dependent. In this study, we examined the postreceptor mechanisms involved in the amiloride effects on cyclic nucleotide accumulation using agents that simulate alpha 1-adrenoceptor activation. Four amiloride analogs, with a 500-fold difference in their inhibitory potency of the Na+/H+ antiporter, were used. 5-(N,N-Hexamethylene)amiloride (HA), the most active inhibitor of the Na+/H+ antiporter, had a stimulatory effect on isoproterenol (ISO)-stimulated cAMP, while its effect on cGMP was inhibitory. The other three amiloride derivatives had no effect on the ISO-stimulated cAMP or cGMP responses. All four amilorides (at 10 microM) had no effect on the phenylephrine potentiation of cAMP responses in beta-adrenergically stimulated cells, while they inhibited the potentiation of cGMP accumulation according to their inhibitory potency on the Na+/H+ antiporter. Using depolarizing concentrations of K+, it was found that HA was additive to the submaximal potentiation by K+ on ISO-stimulated cAMP, while its effect on cGMP was inhibitory. Amiloride hydrochloride dihydrate, the amiloride that is least potent in its inhibitory action on the Na+/H+ antiporter, had no effect on the K+ potentiation of either cAMP or cGMP. Using 4 beta-phorbol 12-myristate 13-acetate in cells treated with 10 mM K+ and ISO, it was found that HA was additive to phorbol 12-myristate 13-acetate and K+ potentiation of the cAMP response, while its effect on the cGMP response was inhibitory. Amiloride hydrochloride had no effect on either the cAMP or cGMP response. It can be concluded from these studies that 1) HA has a stimulatory effect on the beta-adrenoceptor-Gs-adenylate cyclase pathway that is independent of inhibition of the Na+/H+ antiporter; 2) postreceptor mechanisms are involved in HA's effects on cAMP and cGMP accumulation; and 3) the action of HA on cGMP is likely to be related to its effect on the Na+/H+ antiporter.

Amiloride↗
PubMed1989

Evidence of alpha 1-adrenergic----protein kinase C----Na+/H+ antiporter-dependent increase in pinealocyte intracellular pH. Role in the adrenergic stimulation of cGMP accumulation.

The regulation of intracellular pH (pHi) in isolated rat pinealocytes was studied using the fluorescent pH indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. Resting pHi was 7.09 when the extracellular pH (pHe) was 7.2. Treatment of pinealocytes with the physiological regulator of pineal function, norepinephrine, resulted in a concentration-dependent increase in pHi. Further analysis indicated that norepinephrine is probably acting via an alpha 1-adrenergic----[Ca2+]i----Ca2+/phospholipid- dependent protein kinase (protein kinase C) mechanism to activate the Na+/H+ antiporter, thereby causing cytoplasmic alkalization. A potential influence of cytosolic alkalization on the responsiveness of cyclic nucleotides to adrenergic agonists was also studied. Five analogs of the antiporter inhibitor amiloride reduced norepinephrine stimulation of cGMP accumulation with the same relative potency as they act on the antiporter. In contrast, although inhibitory effects of these compounds on cAMP accumulation were detectable, they occurred at 10-100-fold higher concentrations, and the relative potency of these inhibitors did not indicate they were acting via the antiporter. These findings provide evidence that 1) alpha 1-adrenergic receptor activation increases pinealocyte pHi through Ca2+----protein kinase C-dependent activation of the Na+/H+ antiporter; and 2) norepinephrine stimulation of cGMP accumulation is pHi-dependent. It would appear that alpha 1-adrenergic regulation of pHi via the Na+/H+ antiporter may be of general importance in the control of cGMP accumulation.

Adrenergic alpha-Agonists↗
PubMed1989

Forskolin stimulates pinealocyte cGMP accumulation. Dramatic potentiation by an alpha 1-adrenergic----[Ca2+]i mechanism involving protein kinase C.

The effect of forskolin on cGMP regulation was investigated using dispersed rat pinealocytes. Forskolin stimulated cGMP accumulation in a concentration-dependent manner; this response was strongly potentiated by an alpha 1-adrenergic----[Ca2+]i mechanism involving protein kinase C. These findings provide further evidence that activation of two receptor-regulated signal transduction mechanisms may be commonly required for maximal stimulation of cGMP accumulation, and establish a new experimental approach to the study of cGMP regulation.

Adenylyl Cyclases↗
PubMed1989

Calmodulin content in human central nervous system tumors.

Benign and malignant brain tumors and normal cerebral cortex were assayed for calmodulin content by enzymatic and radioimmunoassay techniques. Normal cerebral cortex contained more (8.31 +/- 1.27 vs 3.30 +/- 0.42 micrograms/mg protein) calmodulin than the brain tumors. The contents of calmodulin in the malignant glioblastomas were significantly higher than the meningiomas (5.41 +/- 0.31 vs 2.97 +/- 0.16 micrograms/mg protein). These differences were independent of tumor location and persisted when calmodulin content was normalized for DNA rather than protein content. This data supports differences in the tissue calmodulin contents with normal cortex greater than primary malignant tumors greater than benign tumors greater than metastatic tumor tissue.

Brain Neoplasms↗
PubMed1989

Soybean metal-binding proteins: calmodulin purification by hydrophobic interaction with polymer 3520.

Polymer 3520, a non-polar styrene divinylbenzene polymer, provides a simple way to purify calmodulin (CAM) from soybeans. This polymer, which selectively adsorbs CAM by hydrophobic interaction within the polymer matrix, contains no exchangeable groups; thus, interaction with CAM requires no Ca++ ions, and elution is achieved with 50% ethanol. Purification by this form of reversed-phase liquid chromatography is a substantial improvement over the conventional method, which requires high salt in elution buffers. CAM in soybean meal is first extracted with 80% ethanol in the presence of EGTA at room temperature and then chromatographed directly on a polymer 3520 column to yield pure CAM. Addition of non-ionic detergent (Nonidet P-40) to the ethanolic extract helps to separate extraneous proteins, lipids, sugars, and isoflavones. Such isolated CAM migrates as a single band during polyacrylamide gel electrophoresis and reversed-phase HPLC, and it retains activity stimulatory to phosphodiesterase.

Adsorption↗
PubMed1989

Effect of pinealectomy on the undernutrition-induced suppression of the reproductive axis in rats.

Food restriction is known to sensitize the reproductive axis of rats to the inhibitory action of the pineal gland and its hormone, melatonin. The present study investigated the effect of pinealectomy on the undernutrition-related reproductive suppression. Three groups of animals (control, sham pinealectomy or pinealectomy) were subjected to either 3 weeks of 50% food restriction or ad libitum feeding under a lighting regimen of 14 h light and 10 h dark. At the end of the treatment period, body weight, testes weight, accessory organ weights, serum LH and serum testosterone were determined. In the pinealectomized animals, accessory organ weights and serum testosterone levels increased significantly. As compared to the controls, underfed pinealectomized animals had a partial recovery of the reduced accessory organ weights and normalization of the serum testosterone levels. These findings suggests that pinealectomy has a protective effect on the gonads of both control and underfed animals and that underfed animals are more sensitive to this protective mechanism.

Animals↗
PubMed1988

Calmodulin content in human prolactin-secreting pituitary adenoma: an inverse relationship to serum prolactin levels.

Calmodulin content was evaluated in 3 prolactin-secreting pituitary adenomas (prolactinoma) and 3 normal anterior pituitary glands. The calmodulin content in the normal anterior pituitary tissue was quite consistent, 3.32 +/- 0.016 micrograms/mg protein. In contrast, calmodulin content varied almost 4-fold in the prolactinoma tissue (10.97, 8.50 and 3.00 micrograms/mg protein). Preoperative serum prolactin levels varied inversely with the prolactinoma calmodulin content (125, 257 and 3526 ng/ml, respectively). This study reveals that prolactinoma calmodulin content differs from normal, although it is not uniformly elevated as in other transformed tissues and that elevation of prolactinoma calmodulin content does not positively correlate with serum prolactin levels.

Adenoma↗
PubMed1988

Protein kinase C: subcellular redistribution by increased Ca2+ influx. Evidence that Ca2+-dependent subcellular redistribution of protein kinase C is involved in potentiation of beta-adrenergic stimulation of pineal cAMP and cGMP by K+ and A23187.

Phenylephrine is known to stimulate translocation of protein kinase C in rat pinealocytes (Sugden, D., Vanecek, J., Klein, D.C., Thomas, T.P., and Anderson, W. B. (1985) Nature 314, 359-361). In the present study, the receptor mediating this effect was found to belong to the alpha 1-adrenoceptor subclass. Activation of this receptor is also known to produce a sustained increase in [Ca2+]i by increasing net influx (Sugden, A. L., Sugden, D., and Klein, D. C. (1985) J. Biol. Chem. 261, 11608-11612), which points to the possible importance of Ca2+ influx in the subcellular redistribution (activation) of protein kinase C in intact cells. This possibility was investigated by reducing extracellular Ca2+ ((Ca2+]o) with EGTA or by inhibiting Ca2+ influx with inorganic Ca2+ blockers. These treatments reduced alpha 1-adrenoceptor-mediated translocation of protein kinase C. This suggested that elevation of Ca2+ influx alone triggers activation of protein kinase C. In support of this, it was found that treatments which elevate Ca2+ influx, including increased extracellular K+ and addition of the Ca2+ ionophore A23187, cause redistribution of protein kinase C. The effect of K+ was blocked by nifedipine and that of A23187 by EGTA, indicating that effects of these agents are Ca2+-dependent. The possible role of phospholipase C activation in these effects was examined by measuring the formation of [3H]diacylglycerol by cells labeled with [3H]arachidonic acid. Although [3H]diacylglycerol formation was easily detected in the presence or absence of an effective concentration of an inhibitor of diacylglycerol kinase, none of the agents which cause rapid translocation of protein kinase C were found to cause a rapid increase in the generation of [3H]diacylglycerol. These findings establish that an increase in Ca2+ influx is sufficient to trigger translocation of protein kinase C. In addition, we found that a very close correlation exists between translocation of protein kinase C by phenylephrine, K+, and A23187 and their ability to potentiate beta-adrenergic stimulation of cAMP and cGMP accumulation. This provides strong support to the proposal that translocation of protein kinase C is required for potentiation of beta-adrenergic stimulation of pinealocyte cAMP and cGMP accumulation.

Adrenergic beta-Antagonists↗
PubMed1988

Effects of protein kinase inhibitor (1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7) on protein kinase C activity and adrenergic stimulation of cAMP and cGMP in rat pinealocytes.

Protein kinase C is thought to be involved in the adrenergic regulation of pineal function. In this tissue, norepinephrine increases cAMP and cGMP accumulation through a synergistic dual receptor mechanism involving alpha 1- and beta-adrenergic receptors; the available evidence indicates that the alpha 1-adrenergic stimulation activates protein kinase C, and that this potentiates beta-adrenergic stimulation of pineal cAMP. The role of protein kinase C in the regulation of cGMP is unclear. In the present report, we determined whether an inhibitor of protein kinase C, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H7), inhibits pineal protein kinase C and the adrenergic stimulation of pineal cAMP and cGMP. H7 (10(-4) M) reduced soluble protein kinase C activity by 40%. Treatment of intact pinealocytes with H7 for 0-240 min reduced the effects of subsequent norepinephrine (NE) stimulation of cAMP and cGMP accumulation by at least 25%. H7 also inhibited 25-30% the maximum stimulation of both cAMP and cGMP produced by concurrent treatment with isoproterenol and two agents which elevate intracellular Ca2+, ouabain and A23187. However, H7 did not reduce the effects of selective beta-adrenergic activation, indicating that H7 was probably inhibiting the effects of NE by blocking alpha 1-adrenergic potentiation of beta-adrenergic stimulation, not beta-adrenergically activated mechanisms. H7 also reduced the stimulation of cAMP accumulation produced by the combined treatment of isoproterenol and an activator of protein kinase C, 4-beta-phorbol 12-myristate, 13-acetate, which is consistent with the view that H7 is acting by inhibiting protein kinase C activity. These observations are in agreement with the conclusion that potentiation of beta-adrenergic stimulation of cAMP by alpha 1-adrenergic agonists, protein kinase C activators, or [Ca2+]i elevating agents involves protein kinase C. In addition, these results are of special interest because they point to the possibility that protein kinase C is involved in the regulation of cGMP accumulation.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗
PubMed1988

Permissive role of calcium in alpha 1-adrenergic stimulation of pineal phosphatidylinositol phosphodiesterase (phospholipase C) activity.

Activation of alpha 1-adrenergic receptors increases [Ca+2]i and phosphatidylinositol phosphodiesterase (phospholipase C) activity in the pinealocyte. In this report the receptor involved in the stimulation of phospholipase C activity was further characterized, and the role of Ca2+ in this effect was investigated in some detail. Phospholipase C activity was estimated by measuring the production of [3H]inositol phosphates by [3H]inositol-labelled dispersed pinealocytes in suspension culture. Norepinephrine stimulated [3H]inositol monophosphate production severalfold; this was blocked by alpha 1-adrenergic antagonists, including prazosin, WB 4101, and phenoxybenzamine, but by neither an alpha 2- nor a beta-adrenergic antagonist, confirming that an alpha 1-adrenoceptor is involved in the regulation of phosphatidylinositol hydrolysis. Treatment with the Ca2+ chelator, EGTA, or with inorganic Ca2+ blockers, including Co2+, Mn2+, and La3+, reduced the norepinephrine-stimulated response, suggesting that the alpha 1-adrenergic stimulation of phospholipase C activity is Ca2+ dependent. However, phospholipase C activity was not increased by elevating intracellular Ca2+ with either the Ca2+ ionophore A23187 or with depolarizing concentrations of K+. These results indicate that although Ca2+ is necessary for alpha 1-adrenergic stimulation of phospholipase C activity, an increase in [Ca2+]i alone is not sufficient to stimulate the activity of this enzyme, and that effects which A23187 and depolarizing concentrations of K+ have on pineal function probably do not involve stimulation of phospholipase C activity.

Adrenergic alpha-Agonists↗
PubMed1988

Alpha 1-adrenergic potentiation of vasoactive intestinal peptide stimulation of rat pinealocyte adenosine 3',5'-monophosphate and guanosine 3',5'-monophosphate: evidence for a role of calcium and protein kinase-C.

alpha 1-Adrenergic agonists have recently been found to potentiate vasoactive intestinal peptide (VIP) stimulation of rat pinealocyte cAMP and cGMP. alpha 1-Adrenergic agonists also elevate pineal intracellular Ca2+ [( Ca2+]i) and activate protein kinase-C. In the present study, the possible involvement of Ca2+ and protein kinase-C in the alpha 1-adrenergic potentiation of VIP-stimulated cAMP and cGMP accumulation was examined with agents that alter [Ca2+]i or activate protein kinase-C. It was found that treatment with a Ca2+ chelator or with inorganic Ca2+ channel blockers inhibited alpha 1-adrenergic potentiation of VIP-stimulated cAMP and cGMP responses. Increasing [Ca2+]i by treatment with A23187, ouabain, or K+ potentiated VIP stimulation of cAMP and cGMP response. These observations indicate that Ca2+ mediates the alpha 1-adrenergic potentiation of VIP-stimulated cAMP and cGMP accumulation, as is true for the alpha 1-adrenergic potentiation of beta-adrenergic stimulated cAMP and cGMP accumulation. Activators of protein kinase-C mimicked the large effect alpha 1-adrenergic agonists have on cAMP accumulation in VIP-treated pinealocytes and had a small effect on cGMP accumulation in VIP-treated cells. These effects were not blocked by the Ca2+ chelator EGTA. However, the effects of a protein kinase-C activator on the cGMP response in VIP-stimulated cells were amplified by K+ (15 mM) or ouabain (1 microM), presumably through an action causing an increase in [Ca2+]i. These results suggest protein kinase-C is involved in the alpha 1-adrenergic potentiation of VIP-stimulated cAMP accumulation, as is the case for the alpha 1-adrenergic potentiation of beta-adrenergic stimulated cAMP. Protein kinase-C is also involved in cGMP accumulation, provided that there is a modest increase in [Ca2+]i.

Adrenergic alpha-Agonists↗