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Biomedical subjects

D B Hope

Publications and source records attributed to D B Hope.

At least 19 recordsLinked to original sources
PubMed1985

Forskolin inhibits potassium-evoked release of vasopressin from rat neurohypophyses.

The effects of forskolin, an activator of adenylate cyclase, on the release of vasopressin from isolated rat neurointermediate lobes during a 10 min period of potassium stimulation were investigated. Forskolin was added 5 min after the onset of potassium stimulation. A concentration-dependent reduction in the amount of hormone released during the remaining 5 min period of stimulation was observed. The results suggest that an increase in cyclic AMP following depolarisation inhibits hormone release. This contrasts with several other reports showing that an increase in cyclic AMP prior to stimulation enhances release. We therefore propose that cyclic AMP plays a dual role in stimulus-secretion coupling in the neurohypophysis.

Animals↗
PubMed1985

Forskolin: its effects on potassium-evoked release of vasopressin from the rat neurohypophysis.

The effect of forskolin, added either before or 5 min after the onset of potassium-evoked release of vasopressin from isolated neurointermediate lobes of the rat has been investigated. A low concentration of forskolin (1 microM), added before stimulation, enhanced the potassium-evoked release of vasopressin throughout two successive 5 min periods of stimulation. Higher concentrations of forskolin (10-80 microM) produced no effect on the potassium-evoked release of hormone during the first 5 min of stimulation, but inhibited release during the second 5 min period. When added 5 min after the onset of potassium stimulation, forskolin (1-80 microM) reduced the amount of vasopressin released during the remaining 5 min of stimulation. Forskolin produced a concentration-dependent increase in cyclic AMP during both the control and potassium stimulation periods. The amount of cyclic AMP generated by forskolin during potassium stimulation was less than that produced during the corresponding control periods.

Animals↗
PubMed1984

Characterization of aspirin hydrolase of guinea-pig liver cytoplasm.

Previous subcellular fractionation studies of guinea-pig liver had shown aspirin hydrolysing activity to be located mainly in the microsomal fraction, and was due to a single carboxylesterase (EC 3.1.1.1) (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773). However, activity had been found in all cell fractions, and in this study they were analysed simultaneously by slab gel electrophoresis for aspirin hydrolysing activity. Two enzymes were identified, one of which was associated only with the particulate cell fractions and was the microsomal carboxylesterase described previously. The other activity was located exclusively in the cytoplasmic fraction, and could be inhibited by bis(4-nitrophenyl)phosphate, so identifying it as a carboxylesterase. It has an unusually low molecular weight of 35 000, compared with values of 60 000, or multiples thereof, normally found for liver carboxylesterases. It contributes about 14% of the total aspirin hydrolysing activity of liver homogenates, and could be distinguished from its particulate counterpart by differences in molecular weight and in sensitivity to inhibition by bis(4-nitrophenyl)phosphate (see White, K.N. and Hope, D.B. (1984) Biochim. Biophys. Acta 785, 138-147).

Animals↗
PubMed1984

Partial purification and characterization of a microsomal carboxylesterase specific for salicylate esters from guinea-pig liver.

Studies on liver carboxylesterases have predominantly involved the use of uncharged ester and amide substrates to monitor activity. A microsomal carboxylesterase (EC 3.1.1.1) from guinea-pig liver microsomes has been identified which specifically hydrolyses aspirin (White, K.N. and Hope, D.B. (1981) Biochem. J. 197, 771-773), a substrate which is negatively charged at physiological pH, and this work describes its partial purification and characterization. The enzyme is monomeric, it has a molecular weight of approx. 55 000 and is very sensitive to inhibition by the carboxylesterase inhibitor bis(4-nitrophenyl)phosphate. Although it could not be completely separated from contaminating carboxylesterases, substrate specificity was investigated using the negatively charged esters of salicylic acid. The enzyme is not specific for the acetyl ester of salicylic acid, aspirin, but hydrolyses the longer chain esters more rapidly, with the highest Vmax for the n-octanoyl ester. The enzyme was subject to substrate inhibition which increased with increasing chain length of the fatty acid on the ester, and approached 100% inhibition at concentrations of substrate below critical micellar concentrations.

Animals↗
PubMed1984

Phosphodiesterase activities for cyclic nucleotides in nerve endings from the bovine posterior pituitary gland.

The cyclic nucleotide phosphodiesterase (PDE) activities were studied in a nerve ending fraction from bovine neural lobes. Most of the activity was particulate and unaffected by calcium. Lineweaver-Burk plots for this fraction showed negative cooperativity with apparent Km values for cyclic AMP of 11 microM and for cyclic GMP of 4 microM. The soluble activities for both cyclic nucleotides were activated by calcium and inhibited by calmodulin-binding drugs (trifluoperazine and calmidazolium). The apparent Km values were 50 microM for cyclic AMP and 20 microM for cyclic GMP for the soluble activities. Sucrose density gradients resolved the soluble activities into two peaks. The activity with the higher sedimentation rate (MW 122,000 daltons) hydrolysed both cyclic nucleotides and was calcium-calmodulin-dependent. The other peak (MW 47,000 daltons) had a higher affinity for cyclic AMP than for cyclic GMP and was calcium-independent. Solubilized particulate activities gave two main peaks on the density gradient, both calcium-independent. One was mainly for cyclic AMP (MW 47,000 daltons) and the other mainly for cyclic GMP (MW 133,000 daltons). The function of PDEs in relation to secretion was discussed.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1982

Cyclic nucleotides and the release of vasopressin from the rat posterior pituitary gland.

When rat posterior pituitary glands were stimulated by a high concentration of potassium, a peak of cyclic AMP and a peak of cyclic GMP were detected after 0.5 min and 1 min, respectively, whereas the rate of release of vasopressin was maximal only after 2 min. When calcium was omitted from the medium, no significant changes in cyclic nucleotide levels were found and the vasopressin release remained at the basal rate. During cold-stimulated (10 degrees C) release of vasopressin, a peak of cyclic AMP was detected after 5 min simultaneously with the maximal rate of vasopressin release. The significance of the cyclic nucleotides in the release of vasopressin is discussed.

Animals↗
PubMed1982

Purification and characterization of posterior pituitary calmodulin and its activation of neurosecretosome Ca2+ + Mg2+-ATPase activities.

Calmodulin was isolated as an electrophoretically homogeneous protein from bovine posterior pituitary glands. The yield indicated that this gland is a particularly rich source. Purified bovine posterior pituitary calmodulin and bovine brain calmodulin had identical electrophoretic mobilities on 10% and 12% polyacrylamide gels. The protein was further identified by molecular weight determination and by amino acid analysis which showed that it contained trimethyllysine, one residue per molecule. Bovine posterior pituitary calmodulin was found to activate a preparation of calmodulin-deficient phosphodiesterase from bovine heart. In addition, pituitary calmodulin stimulated Ca+ + Mg2+-ATPase activity associated with a purified nerve ending plasma membrane fraction. This dependence could only be demonstrated after successive washing of the membranes with EGTA buffers, a procedure designed to remove endogenous calmodulin.

Amino Acids↗
PubMed1981

Identification of aspirinase with one of the carboxylesterases requiring a thiol group.

Aspirin-hydrolysing activity in guinea-pig liver is located mainly in the microsomal fraction. This activity was found by electrophoresis to be due to a single carboxylesterase band, out of 12 bands revealed with alpha-naphthyl acetate as substrate. The activity is inhibited completely and irreversibly by the carboxylesterase inhibitor bis-(-4-nitrophenyl) hydrogen phosphate, and also by thiol-blocking reagents.

Animals↗
PubMed1979

Hydroxy analogues of oxytocin and of lysine-vasopressin.

1 Synthetic analogues of oxytocin and of lysine-vasopressin with an hydroxyl group in either the L ro D configuration replacing the primary amino group have been tested for biological activity.2 [1-(L-2-Hydroxy-3-mercaptopropanoic acid)] oxytocin ([L-Hmp(1)]oxytocin) was 1.5 to 2 times more potent than oxytocin on the rat uterus in situ, the rat mammary strip and the rat mammary gland in situ and 3 times more potent on the rat isolated uterus.3 The pressor activity of [1-(L-2-hydroxy-3-mercaptopropanoic acid)-8-lysine]vasopressin ([L-Hmp(1), Lys(8)] vasopressin) was 2.2 and the antidiuretic activity 2.1 times that of lysine-vasopressin.4 The [D-Hmp(1)] analogues of oxytocin and vasopressin were much less potent than the [L-Hmp(1)] analogues.5 The responses to oxytocin and its hydroxy analogues in vivo were qualitatively indistinguishable but the pressor and antidiuretic responses to the hydroxy analogues of lysine-vasopressin were prolonged compared with those to the parent hormone.6 The hydroxy analogues of oxytocin and lysine-vasopressin were not inactivated by pregnancy plasma oxytocinase.7 The results are discussed in relation to the importance of the primary amino group for the biological activity and metabolism of the neurohypophysial hormones.

Animals↗
PubMed1977

Subcellular fractionation by centrifugation of homogenates of the neural lobe of the bovine pituitary gland: identification of different pools of hormone in the homogenate and isolation of neurosecretosomes.

1. Homogenates of bovine pituitary neural lobe tissue were subjected to differential centrifugation. Six subcellular fractions (I-VI) were obtained and the distribution among them of various cell organelles was studied by means of markers. Fraction II (800-3000 gav), the nerve-ending fraction (neurosecretosomes), contained sedimentable lactate dehydrogenase, hormone and a large proportion of the total Mg2 + +Na+ +K+-ATPase. 2. Lysis of the neurosecretosomes in hypotonic sucrose solutions led to loss of lactate dehydrogenase and vasopressin. 3. Centrifugation of the granule fraction (IV) on a sucrose gradient (1.3-2.0 M sucrose) gave a bimodal distribution of vasopressin. Purified neurosecretory granules were recovered from the denser band. Centrifugation on modified gradients (0.8-2.0 or 0.4-2.0 M sucrose) increased the yield of hormone in the denser band but the purity of the granules was decreased. 4. Considerable purification of the neurosecretosomes (fraction II) was achieved by "washing". Centrifugation of washed neurosecretosomes on sucrose density gradients led to the accumulation of all activities at a region between 1.4 and 1.5 M sucrose. 5. The distribution of Mg2+ +Na+ +K+-ATPase in centrifugal fractions indicated that the neurosecretosomes had been isolated in relatively high yield.

Adenosine Triphosphatases↗