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D A Powis

Publications and source records attributed to D A Powis.

At least 55 records · Page 3Linked to original sources
PubMed1987

Alpha 2-adrenoceptor blockade prevents cardiac glycoside-evoked neurotransmitter release from sympathetic nerves in dog saphenous vein.

1 The effect of alpha-adrenoceptor antagonists upon neurotransmitter release evoked by cardiac glycosides from sympathetic nerve terminals has been investigated in dog saphenous vein. 2 In rings of saphenous vein preloaded with [3H]-noradrenaline, acetylstrophanthidin (ACS) caused a concentration-dependent efflux of 3H (EC50 ca. 4.4 microM) that was attenuated by phentolamine and yohimbine but not by prazosin. 3 In helical strips of saphenous vein superfused with ACS at EC50 the efflux of 3H-compounds in general, and of [3H]-noradrenaline in particular, occurred after a short delay and increased with time to a maximum reached at 75 min. Phentolamine and phenoxybenzamine, but not prazosin reduced the efflux of [3H]-noradrenaline and of total 3H-compounds throughout the time-course of the ACS-evoked effect. 4 In helical strips of saphenous vein the glycoside ouabain also caused an increase in [3H]-noradrenaline and in total 3H-efflux that was attenuated by phentolamine. 5 By contrast with the above, in bovine adrenal medullary chromaffin cells, which appear to have no functional alpha-adrenoceptors, ACS caused a small, but significant increase in 3H-efflux which was not prevented by phentolamine. 6 Phentolamine, at concentrations that attenuate markedly the ouabain- or ACS-evoked increase in 3H-efflux from dog saphenous vein, did not cause significant inhibition of cocaine-sensitive [3H]-noradrenaline uptake nor did it reduce the extent of the 3H-efflux evoked either by tyramine or by reduced extracellular Na+. These findings imply that phentolamine does not affect ACS-evoked neurotransmitter release by an action on the catecholamine uptake mechanism. 7. It is concluded that the cardiac glycoside-evoked increase in neurotransmitter release from noradrenergic nerve terminals ofdog saphenous vein is modulated by a mechanism that involves an alpha2- adrenoceptor.

Adrenergic alpha-Agonists↗
PubMed1986

Unsuitability of the 86Rb+ uptake method for estimation of (Na+ + K+)-ATPase activity in innervated tissues.

(Na+ + K+)-ATPase activity was estimated by 86Rb+ uptake in dog saphenous vein to determine the validity of the technique in tissues that have a sympathetic innervation. When saphenous vein rings were incubated at 37 degrees C in Krebs' solution containing 86Rb+, the cardenolide acetylstrophanthidin caused a concentration-dependent inhibition of Rb+ uptake. The threshold for inhibition was approx. 10 nM acetylstrophanthidin and the maximum effect was obtained at 9 microM. In the upper part of this concentration range (greater than 1 microM) acetylstrophanthidin released noradrenaline from the sympathetic nerve terminals associated with the tissue. In this upper part of the acetylstrophanthidin concentration range the alpha-adrenoceptor antagonist phentolamine (8 microM) reduced, by up to 25%, the degree of 86Rb+ uptake inhibition caused by the cardenolide. In other experiments, saphenous vein strips were loaded with 86Rb+ and perifused with Krebs' solution containing acetylstrophanthidin. At concentrations which release noradrenaline, acetylstrophanthidin increased the efflux of 86Rb+. Phentolamine (8 microM) prevented the acetylstrophanthidin-evoked efflux of the isotope as did prior in vitro denervation of 86Rb+ loaded strips with 6-hydroxydopamine. Exogenous noradrenaline (1-100 microM) added to the perifusing fluid also caused an efflux of 86Rb+ that was attenuated by phentolamine. The data indicate for dog saphenous vein that with low concentrations of acetylstrophanthidin the extent of 86Rb+ accumulation might accurately reflect prevailing (Na+ + K+)-ATPase activity. At higher concentrations of acetylstrophanthidin, however, noradrenaline is released from the nerve endings and causes 86Rb+ efflux from the smooth muscle cells consequent upon alpha-adrenoceptor activation. Since this efflux reduces the extent of Rb+ accumulation, measurement of the latter does not adequately reflect uptake mediated by the activity of (Na+ + K+)-ATPase. This is significant because in most applications of the 86Rb+ uptake method it is the estimate of Rb+ accumulation made in the presence of a high concentration of cardenolide that forms the basis of all subsequent calculations with respect to (Na+ + K+)-ATPase activity.

Animals↗
PubMed1986

The subcellular distribution of a membrane-bound calmodulin-stimulated protein kinase.

Incubation of subcellular fractions isolated from rat cerebral cortex with [gamma-32P]ATP results in the phosphorylation of a number of proteins including two with apparent molecular weights of approximately 50,000 and 60,000 daltons. These phosphoproteins were shown to be the autophosphorylated subunits of a calmodulin-stimulated protein kinase by a number of physicochemical criteria, including their mobility on non-equilibrium pH gradient electrophoresis, their phosphopeptide profiles and phosphorylation characteristics. When a crude membrane fraction obtained following osmotic lysis of a P2 fraction was labeled and subsequently fractionated on sucrose density gradients, approximately 80% of the autophosphorylated kinase was associated with fractions enriched in synaptic plasma membranes. Other substrates of calmodulin kinase(s) were similarly distributed. Detergent extraction of synaptic plasma membranes to produce synaptic junctions and post-synaptic densities indicated that the majority of the autophosphorylated kinase was solubilized, apparently as a holoenzyme. The major post synaptic density protein (mPSDp) was not readily extracted by detergents and was largely unlabeled under the conditions used for phosphorylation, and yet this protein is structurally closely related to the kinase subunit. It is possible that this lack of labeling is due to the mPSDp being attached to the PSD in a different way or being present there in a different isoenzymic form from that of the readily autophosphorylated enzyme subunit. Thus, the data suggest that, in vitro at least, a number of pools of calmodulin kinase exist in neuronal membranes.

Adenosine Triphosphate↗
PubMed1986

Alpha 2-adrenoceptors do not regulate catecholamine secretion by bovine adrenal medullary cells: a study with clonidine.

Experiments have been performed with perfused bovine adrenal glands, with freshly isolated chromaffin cells, and with chromaffin cells maintained in tissue culture to investigate the suggestion that there are alpha-adrenoceptors present which regulate catecholamine secretion. Only one set of observations has lent support to this suggestion: the rather specific alpha 2-adrenoceptor agonist clonidine inhibits catecholamine secretion evoked by the physiological secretagogue, acetylcholine, and by the related nicotinic agonists, carbachol and nicotine. All other observations detract from the suggestion. Other alpha-adrenoceptor agonists (noradrenaline, adrenaline, tramazoline, phenylephrine, and alpha-methyl-noradrenaline) are virtually ineffective at inhibiting secretion evoked by carbachol. In addition, the alpha-adrenoceptor antagonists phentolamine, phenoxybenzamine, and yohimbine not only fail to enhance the secretion of catecholamines evoked by carbachol but also fail to offset the inhibitory action of clonidine. The data suggest that functional alpha 2-adrenoceptors of the classical type are not present upon bovine chromaffin cells and that, in this tissue, clonidine must act in some other way. In the bovine adrenal medullary chromaffin cell clonidine probably acts at the nicotinic receptor because it does not reduce catecholamine secretion evoked by depolarizing concentrations of potassium or veratridine but does reduce the carbachol-evoked influx of 22Na that can be measured in the presence of tetrodotoxin and ouabain and which probably reflects entry through the nicotinic channel. Furthermore, clonidine can abolish, in reversible fashion, the acetylcholine-activated inward current determined with patch-clamp.

Adrenal Medulla↗
PubMed1985

Failure of calcium to stimulate Na,K-ATPase in the presence of EDTA.

The effect of calcium on Na,K-ATPase activity of rat brain homogenates and its modification by the chelating agent EDTA has been investigated. In the absence of EDTA, free calcium (approximately 10(-6) mol/l) stimulates Na,K-ATPase activity; in the presence of EDTA the same concentration of free calcium is without effect on the enzyme. In the absence of EDTA the stimulation by calcium of Na,K-ATPase activity is enhanced by the additional presence of calmodulin but in the presence of EDTA, even when calmodulin is added to excess, calcium still fails to stimulate the enzyme. The possibility that EDTA interferes with an interaction between a calcium-calmodulin complex and Na,K-ATPase is discussed.

Adenosine Triphosphatases↗
PubMed1985

The effect of cooling on the responses of splenic capsular and vascular smooth muscle to nerve stimulation in the dog.

Field stimulation of strips of canine splenic capsule and splenic artery suspended in an organ bath causes in both a contraction that is enhanced by cooling from 37 degrees C to 27 degrees C. This finding contrasts with that from the isolated perfused canine spleen in which cooling to 27 degrees C enhanced the vascular response to splenic nerve stimulation but depressed the capsular response. The results of experiments to resolve the apparent discrepancy in the capsular smooth muscle response to stimulation indicate that the reduced response in the perfused spleen probably results from the capsular smooth muscle fibres being less optimally placed on their length: tension curve at 27 degrees C when spleen volume is less. There is thus no need to postulate any fundamental difference in properties between the smooth muscles from the two intrasplenic distributions.

Animals↗
PubMed1985

Cardiac glycosides, calcium and the release of neurotransmitter from peripheral noradrenergic nerves.

The requirement for external Ca (Cao) of neurotransmitter release evoked by cardenolides has been investigated in canine saphenous vein. Basal efflux of 3H-compounds from saphenous veins pre-loaded with 3H-noradrenaline was the same in the absence as in the presence of Cao; Cao is not required for basal efflux of neurotransmitter. Efflux of 3H-compounds was increased by cardenolides. Both ACS and ouabain caused a similar maximum net efflux of 3H suggesting that each evokes release from the same pool of 3H compounds. The similarity of the effects obtained with cardenolides to those obtained during exposure of saphenous vein preparations to potassium-free media suggests that 3H-efflux is the result of Na,K-ATPase inhibition. With ACS (ca. EC50) the net efflux of 3H-compounds early (less than 60 min) in the release period was greater in the absence of Cao than in its presence whereas at longer times the reverse was true; net efflux was less in the absence of Cao than in its presence. The difference in the 3H-efflux pattern was paralleled qualitatively throughout by efflux of 3H-noradrenaline. The ACS-evoked efflux of 3H-compounds in the presence and absence of Cao derives from sympathetic, noradrenergic nerves; 3H present in extraneuronal tissues was not released by the cardenolide. With ouabain (less than EC50) the total efflux of 3H over a 75 min period was greater in the absence of Cao than in its presence. The reverse was found with ouabain (greater than EC50): the total efflux of 3H was less in the absence of Cao than in its presence.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Stimulation of neuronal Na+, K+-ATPase by calcium.

The effect of calcium on ATP-phosphohydrolase activity of rat brain homogenates has been investigated. In both the presence and absence of the chelating agent EDTA, free calcium within the concentration range 1.2 x 10(-7) to 5.0 x 10(-4) moles/l consistently affected only the activity of Na+, K+-ATpase; the activities of Mg2+-ATPase and Na+-ATPase were essentially unchanged by Ca2+; Ca2+-ATPase could not be demonstrated. In either the presence or absence of EDTA, concentrations of free-Ca2+ above 3 x 10(-6) moles/l caused an inhibition of Na+,K+-ATPase activity. In the presence of EDTA, concentrations of free-Ca2+ below 3 x 10(-6) moles/l were ineffective at altering Na+, K+-ATPase activity but, in the absence of EDTA, free-Ca2+ in this concentration range caused a marked stimulation of the enzyme. Evidence is presented to show that the stimulation of Na+, K+-ATPase by calcium is modulated by the regulatory protein calmodulin. Since the stimulation occurs over the range of concentrations at which calcium would be expected to be encountered within the cell, it is suggested that this is the major physiological effect of calcium on Na+, K+-ATPase.

Animals↗
PubMed1981

Toward independent learning: curricular design for assisting students to learn how to learn.

It must be acknowledged that a university course in medicine cannot hope to present its students with everything they need to know in order to practice a lifetime's career in the profession. Even were the course years longer, increasing the content of factual knowledge would not equip the graduate for the practice of medicine. In part this is due to the fact that medical knowledge is continually changing or being reevaluated; in addition, facts alone cannot be correlated with practical application, and the process of how to use the facts is an essential part of the study and practice of medicine. It is essential, therefore, that an undergraduate medical course, among other things, teach the students to take responsibility for and to be able to structure their own learning. If the process of continual active learning is fostered during undergraduate days, it may readily be continued by the graduate alone in the absence of his teachers. The curriculum at the new medical school at Newcastle, New South Wales, Australia, is being developed with this priority in view. This paper describes some of the principles underpinning this philosophy and how such principles are being applied.

Curriculum↗
PubMed1980

The effects of acetylstrophanthidin and ouabain on the sympathetic adrenergic neuroeffector junction in canine vascular smooth muscle.

We performed experiments to determine the effects of acetylstrophanthidin (ACS) and ouabain on the adrenergic neuroeffector junction in dog saphenous veins. In quiescent strips incubated with 3H-norepinephrine (3H-NE), the drugs caused contraction and a progressive increase in overflow of 3H-NE and O-methylated metabolites; 3,4-dihydroxyphenylglycol (DOPEG) decreased. Tissue uptake of 3H-NE was partially inhibited. After surgical sympathectomy, both contraction and 3H-NE overflow were markedly attenuated. Following chemical sympathectomy with 6-hydroxydopamine, ouabain contractions were 11% of control, whereas the contractions due to exogenous norepinephrine were exaggerated. The initial overflow of 3H-NE was unaffected by tetrodotoxin, but the later and larger overflow with prolonged exposure was depressed. The former occurred in the absence of Ca2+, but the latter was Ca2+ dependent. Inhibition of the neuronal amine carrier by cocaine or desipramine and blockade of the neuronal alpha-adrenoceptors with phentolamine or phenoxybenzamine attenuated the release of 3H-NE evoked by ACS and ouabain. During electrical stimulation, ACS augmented the overflow of 3H-NE. This was attenuated by cocaine, desipramine, and the alpha-adrenolytic drugs. ACS, like pargyline, augmented the overflow of 3H-NE evoked by tyramine and depressed that of DOPEG. These experiments suggest that acetylstrophanthidin and ouabain (1) cause contraction of vascular smooth muscle by displacement of norepinephrine from neuronal stores, (2) reduce neuronal monoamine oxidase activity, (3) facilitate and may trigger Ca2+-dependent exocytotic release of norepinephrine, (4) partially inhibit the neuronal amine carrier mechanism but do not interfere with extraneuronal disposition of norepinephrine, and, finally (5) may have unexplained interactions with prejunctional alpha-adrenoceptors.

Animals↗
PubMed1979

The subdivision of beta-adrenoceptors in the cardiovascular system of the rat.

1 The antagonism by the beta-adrenoceptor blocking drugs, propranolol (non-selective) and practolol (beta-selective), of the cardiovascular actions of isoprenaline has been investigated in the rat. 2 All doses of practolol (0.1, 1 and 3 mg/kg) blocked the cardio-accelerator action of isoprenaline but only the largest dose blocked the vasodilator effect. 3 All doses of propranolol (0.01, 0.03 and 0.1 mg/kg) blocked the vasodilator effect of isoprenaline but only the largest dose diminished the tachycardia. 4 It is concluded that in the rat, as in other species, beta-adrenoceptors may be subdivided into beta 1 (cardiac) and beta 2 (peripheral vascular) types.

Animals↗
PubMed1979

Involvement of renal alpha- and beta-adrenoceptors in release of renin by carotid baroreflex.

In anesthetized vagotomized dogs with renal arterial pressure constant, carotid sinus hypotension (BCO) caused a reflex rise in systemic arterial pressure, a fall in renal blood flow, and a similar increase in renin release from both kidneys. Unilateral alpha-adrenoceptor blockade with phenoxybenzamine resulted in an increase in basal renal blood flow, a depression of basal renin release, and an abolition of the responses to BCO in the treated kidney. The untreated kidney responsed to BCO as before. Nonblocked and alpha-blocked kidneys released similar amounts of renin when renal blood flow was mechanically reduced by aortic constriction. Administration of propranolol to the nonblocked kidney prevented the release of renin but not the hemodynamic changes resulting from BCO. The experiments demonstrated that under certain conditions carotid sinus hypotension produced alpha-adrenoceptor-mediated changes in the kidney sufficient to cause increased renin release. A step in the renin release mechanism subsequent to the alpha-adrenoceptor-mediated changes in sensitive to propranolol.

Animals↗
PubMed1978

The differential effect of cooling on the responses of splenic capsular and vascular smooth muscle to nerve stimulation and noradrenaline.

The responses of the capsular and vascular smooth muscle of the dog's spleen to splenic nerve stimulation and to infused noradrenaline have been studied in the isolated, blood-perfused preparation at 37 degrees C, at 27 degrees C and again after rewarming to 37 degrees C. It was found that cooling per se had no effect on perfusion pressure but reduced splenic arterial blood flow, and caused no appreciable alteration in spleen volume. The increase in splenic flow resistance in response to nerve stimulation and noradrenaline was significantly greater at 27 degrees C than at 37 degrees C, but the concomitant reduction in spleen volume was significantly reduced. The enhanced effect of splenic nerve stimulation and noradrenaline on splenic flow resistance is discussed in terms of the relative contributions of an increased smooth muscle response and increased blood viscosity. The different effects of cooling on the responses of splenic vascular and capsular smooth muscle to nerve stimulation and noradrenaline are discussed in the context of the effect of cooling on other vascular and non-vascular smooth muscle.

Animals↗
PubMed1977

Responses of the cardiovascular system of the rat to alpha-and beta-adrenoceptor agonists.

1 The effects of intravenously infused phenylephrine and isoprenaline upon the cardiovascular system of the rat anaesthetized with pentobarbitone, have been investigated.2 Phenylephrine produces a dose-dependent rise in mean arterial blood pressure (MABP) that is due mainly to an increase in total peripheral vascular resistance (TPR), though at all doses tested cardiac output was invariably raised.3 The increase in cardiac output was due in each instance to an increase in stroke volume, heart rate being unchanged. This increase in cardiac output is probably brought about by effects of phenylephrine on the capacitance vessels rather than by an effect on the heart.4 Evidence is presented to show that the effects of phenylephrine are mediated largely by alpha-adrenoceptors, but that beta-adrenoceptors which affect TPR are also stimulated by the amine.5 Isoprenaline produces a dose-dependent fall in MABP that is due entirely to a fall in TPR since the cardiac output increases.6 Unlike phenylephrine, the increase in cardiac output obtained with isoprenaline was achieved by an increase in heart rate while stroke volume remained close to control values. It is contended that the augmented venous return required for the elevated cardiac output results in this case mainly from the isoprenaline-induced fall in TPR which enhances transfer of blood from arteries to the veins.7 Evidence is presented to show that the effects of isoprenaline are mediated mainly by beta-adrenoceptors.8 Under the present experimental conditions the adrenoceptor-mediated cardiovascular changes are little modified reflexly by the arterial baroreceptors.

Adrenergic alpha-Agonists↗