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P F Baker

Publications and source records attributed to P F Baker.

At least 19 recordsLinked to original sources
PubMed1991

Comparison of the effects of a bufodienolide and ouabain on neuronal and smooth muscle preparations.

The effect of a bufodienolide (monohydroxy-14,15-epoxy-20,22-dienolide glycoside) purified from toad skin was compared with that of ouabain on 3H-noradrenaline release and on the tension of rabbit pulmonary arterial strips. This compound exerted an ouabain-like activity. The neuronal effects of this bufodienolide derivative on squid axon were also studied and compared with those of ouabain. Both compounds enhanced the resting and stimulation-evoked (2 Hz, 360 shocks) release of 3H-noradrenaline. Moreover, in the presence of either this bufodienolide or ouabain, the tension of the rabbit artery increased gradually, and the contraction evoked by electrical stimulation was potentiated. Both compounds enhanced, in a prazosin-sensitive way, smooth muscle responses to noradrenaline and to electrical stimulation. In higher concentrations, they contracted smooth muscle cells of pulmonary artery, an action which was insensitive to prazosin. The bufodienolide was about 8 times more active in inhibition of 22Na efflux than was ouabain, but did not affect Ca efflux, which is not sensitive to ouabain. It is therefore concluded that compounds with an inhibitory effect on Na+,K(+)-ATPase are able to affect chemical neurotransmission of blood vessels in such a way that in lower concentrations they potentiate the release of noradrenaline, and in higher concentrations they contract directly the smooth muscle. These findings indicate that such compounds if they are present in the circulation might be involved in the physiological regulation of blood pressure or in the genesis of hypertension.

Animals↗
PubMed1988

The effects of nitroprusside and putative agonists on guanylate cyclase activity in squid giant axons.

cGMP content of axoplasm from the giant axon of Loligo forbesi was investigated after subjecting the axon to various treatments. Repetitive electrical stimulation or depolarisation by high K+ caused no change in cGMP content. Glutamate and serotonin were also without effect. The nicotinic agonist carbachol (100 microM) increased cGMP levels by 90% (n = 5). A large transient elevation of cGMP content was evoked by external nitroprusside (10 nM-20 microM in intact axons. Nitroprusside injected into both extruded axoplasm and intact axons also increased cGMP content, the stimulation being considerably higher in intact axons where the axolemma was also present. Nitroprusside was also active in axons where the soluble cytoplasmic components were washed out by internal perfusion.

Animals↗
PubMed1988

Evidence implicating protein kinase C in exocytosis from electropermeabilized bovine chromaffin cells.

The calcium sensitivity of exocytosis from electro-permeabilized chromaffin cells is increased by activators of protein kinase C, such as TPA and certain phorbol esters, diacylglycerols, and mezerein. A range of putative inhibitors of protein kinase C block both the phorbol ester-sensitive component of secretion and also the underlying insensitive component. These inhibitors are also shown to inhibit medulla protein kinase C activity in vitro. The extent of secretion is reduced when electro-permeabilized cells are exposed to Ca2+ levels much in excess of 50 microM. The onset of inhibition is faster than the relatively slow rate of Ca-dependent exocytosis and is insensitive to inhibitors of proteolysis. Adrenal medulla protein kinase C activity is also irreversibly inhibited by high Ca2+ concentrations. Both the secretory response and the protein kinase C activity in vitro have similar nucleotide and cation specificities. Although these data do not definitely establish an involvement of protein kinase C in exocytosis, none argue against it.

Animals↗
PubMed1988

Control of intracellular pH in rat calvarial osteoblasts: coexistence of both chloride-bicarbonate and sodium-hydrogen exchange.

Intracellular pH was monitored continuously in cultured rat calvarial osteoblasts using the pH-sensitive fluorescent dye bis carboxyethyl carboxyfluorescein (BCECF), loaded into the cells as its membrane permeant ester. Recovery from an intracellular acid load generated by exposure to NH4Cl was unaffected by the anion exchange inhibitors 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disuphonic acid (SITS) and 4,4'-diisothiocyanato-stilbene 2,2'-disuphonic acid (DIDS) (100 microM), but blocked by the sodium-hydrogen exchange inhibitor amiloride (1 mM) and dependent on external sodium, suggesting that recovery is brought about by a sodium-hydrogen exchanger in the plasma membrane. The cells do, however, possess a SITS-sensitive chloride-bicarbonate exchanger, because iso-osmotic replacement of chloride by gluconate leads to intracellular alkalinization, that is inhibited by SITS, but independent of external sodium. Parathyroid hormone brings about an intracellular acidification, which may be due to an inhibiton of sodium-hydrogen exchange.

Acid-Base Equilibrium↗
PubMed1988

Glutamate transport in large muscle fibres of Balanus nubilus.

The uptake of glutamate and other acidic amino acids into barnacle single muscle fibres has been characterized. The uptake of glutamate consists of two components, one Na-independent and one Na-dependent. The Na-dependent uptake is saturable (half-maximal at 250 microM external glutamate) and is inhibited by a variety of analogues of which L-cysteate and D- and L-aspartate are the most potent. These amino acids are also transported into the muscle in a Na-dependent manner. The excitatory agonists kainate, quisqualate, and N-methyl-D-aspartate do not inhibit or affect uptake in any way. Progressive replacement of external Na by choline reduces uptake with very little effect on the apparent affinity for glutamate, suggesting that Na and glutamate bind to the transporter independently. The kinetics of activation are consistent with a requirement for at least two Na ions. Na activation of glutamate uptake can be inhibited by guanidinium with kinetics that are consistent with competitive inhibition at the Na binding site. Studies on the efflux of L-glutamate and other analogues have shown that efflux rates are only slightly increased by the removal of Na and do not seem to be affected in any clear manner by external levels of acidic amino acids.

Animals↗
PubMed1987

Calcium buffering in axons and axoplasm of Loligo.

1. Ca-selective micro-electrodes were used to measure free Ca concentration in axons and extruded axoplasm. 2. Free Ca in axons immersed in artificial sea water containing 3 mM-Ca averaged 77 nM in freshly dissected axons and 4.9 microM in cyanide- or azide-poisoned axons. 3. Extruded axoplasm maintained a free Ca only a little higher than that of the axons from which it was obtained. 4. Axoplasmic buffering was investigated by titrating isolated axoplasm with CaCl2 or K-EGTA and monitoring the change in free Ca. Energy-dependent and energy-independent components of Ca binding could be recognized in fresh axoplasm. The energy-dependent fraction could be further subdivided into Ruthenium Red-sensitive and Ruthenium Red-insensitive components and the energy-independent fraction into a component of high affinity and rather low capacity and another component of low affinity and large capacity. 5. The Ruthenium Red-sensitive process could accumulate many millimoles Ca per kilogram axoplasm while still maintaining a free Ca close to 100 nM. After injection of Ruthenium Red into fresh axoplasm, binding is dramatically altered so that it closely resembles that in a metabolically poisoned preparation. 6. The Ruthenium Red-insensitive process has a small capacity and appears to be capable of lowering free Ca to about 200 nM. It can, however, lower free Ca to 50-150 nM if oxalate is also present. 7. Simultaneous measurement of pH and free Ca showed that axoplasmic pH only begins to fall appreciably in response to added Ca when mitochondrial Ca buffering becomes impaired. 8. Raising axoplasmic levels of Na or Li, but not K, tends to bring about a concomitant rise in free Ca.

Adenosine Triphosphate↗
PubMed1986

The sodium-calcium exchange system.

Sodium-calcium counter-transport represents one of a number of processes for transporting calcium ions across cellular membranes. The physiological importance of the exchanger is outlined and its underlying mechanism discussed in terms of a comparison of the partial reactions of Na+-Ca2+ exchange in intact cells with those of plasma membrane vesicles.

Animals↗
PubMed1986

Observations on the muscarinic activation of catecholamine secretion in the chicken adrenal.

Cells were isolated by collagenase digestion of chicken adrenal glands. Catecholamine secretion could be stimulated by acetylcholine, carbamylcholine, potassium or veratridine. Methacholine, muscarine and oxotremorine were also effective secretagogues whereas nicotine was not. Secretion evoked by acetylcholine was blocked by low concentrations of atropine but was relatively insensitive to hexamethonium. Atropine-sensitive secretion required both external sodium and calcium, was unaffected by tetrodotoxin, blocked by methoxy verapamil and nifedipine, and potentiated by BAY-K-8644. These data suggest that muscarinic activation of these cells facilitates tetrodotoxin insensitive depolarization, thereby opening conventional voltage-sensitive calcium channels. The mechanism by which calcium activates catecholamine secretion was investigated in cells that had been made permeable by exposure to brief intense electric fields. Catecholamine release required Mg-adenosine 5' triphosphate, was half-maximally activated by 1 microM Ca2+ and could be inhibited by high concentrations of Mg2+. At low Ca2+ concentrations, release was potentiated by 12-O-tetradecanoylphorbol 13-acetate, dioctanoylglycerol, guanosine 5'-O-(3-thiotriphosphate) and 5'-guanylylimidodiphosphate, all of which increased the apparent affinity of exocytosis for Ca2+.

Acetylcholine↗
PubMed1986

Kinetic analysis of the triggered exocytosis/endocytosis secretory cycle in cultured bovine adrenal medullary cells.

Cultured bovine adrenal medullary cells are an excellent preparation for quantitative analysis of the secretory exocytosis/endocytosis cycle. In this paper we examine the kinetics of endocytosis after stimulation of secretion. Membrane retrieval was monitored by uptake of the fluid phase marker horseradish peroxidase. Horseradish peroxidase was found to be suitable because it can be washed off completely, assayed quantitatively, and its uptake increases linearly with concentration. If this marker is present during stimulation, the rate of uptake is initially slower than catecholamine secretion but faster at a later time, suggesting that the formation of endocytotic vesicles follows exocytosis. To monitor the time-dependent concentration of secretory vesicle-plasma membrane fusion product (omega-profiles), secretion was halted at various time intervals after stimulation and the excess membrane allowed to transform into endocytotic vesicles in the presence of horseradish peroxidase. By adding horseradish peroxidase at various times after inhibition of secretion, the time course of membrane retrieval could be measured directly. All our results are consistent with a two-step kinetic model in which exocytosis and membrane retrieval are consecutive events. The estimated volumes of the compartments involved are roughly equal. The rate of endocytosis is strongly temperature-dependent but unaffected by extracellular calcium in the range of 10(-8)-2.5 X 10(-3) M, suggesting that calcium is not required at the site of endocytotic membrane fusion. Membrane retrieval is also unaffected by Lanthanum (1 mM) but is slowed by hypertonic media.

Adrenal Medulla↗
PubMed1986

Comparison of the effects of potassium and membrane potential on the calcium-dependent sodium efflux in squid axons.

Experiments are described in which the [Ca]o-dependent component of 22Na efflux is monitored under conditions of membrane potential control by voltage clamp. The apparent affinity of the efflux system for external Ca is very low in choline sea water (apparent KD approximately 50 mM); but increases dramatically when choline is replaced isosmotically by Li or K (apparent KD approximately 1-2 mM). Ca influx changes in a parallel fashion. Tris behaves much like choline and guanidinium is about two-thirds as effective as Li. Replacement of Li by K has little effect on the apparent affinity for external Ca but brings about a small (30-40%) increase in the maximal flux. The increase in maximum flux can be removed by electrical hyperpolarization to the potential before application of K and, in the absence of K, can be mimicked by electrical depolarization. These experiments suggest that the stimulatory effect of K on the Ca-dependent Na efflux into Li sea water is electrical in origin. Partial replacement of choline by K stimulates the Ca-dependent Na efflux; but only part of this stimulation can be removed by electrical hyperpolarization and, in the absence of K, electrical depolarization only brings about a relatively small stimulation. This is because only part of the stimulation that follows addition of K to choline sea waters is electrical in origin: the rest reflects an increase in the apparent affinity for external Ca that is brought about by K acting chemically. The maximum efflux into K is about 40% higher than that into choline. That this may reflect an electrical effect is supported by the observation that electrical depolarization increases the flux into choline sea water containing 110 mM-Ca where the Ca-binding site is close to saturation. The voltage clamp was used to determine the voltage dependence of the Ca-dependent Na efflux into Li sea water, choline sea water and choline sea water containing 100 mM-Na. In all three cases the flux increased with depolarization and was still rising at +70 mV. The dependence on potential was not very steep, an e-fold increase occurred over approximately 50 mV.

Animals↗
PubMed1986

Influence of membrane potential on calcium efflux from giant axons of Loligo.

Experiments are described in which Ca efflux is monitored in axons under voltage clamp. As Ca efflux consists of more than one component, conditions were sought where one component predominates. Thus external Na-dependent Ca efflux can be studied in relative isolation either at pH 9.0 or in fully poisoned axons immersed in Ca-free media; external Ca-dependent Ca efflux can be studied in fully poisoned axons immersed in Na-free media and the Na-independent, energy requiring, pump is best examined in Na and Ca-free sea waters. Both in unpoisoned axons at pH 9.0 and fully poisoned axons at pH 7.8, the external Na-dependent Ca efflux is activated by hyperpolarization and inhibited by depolarization. Depolarizations achieved either electrically or by exposure to high K are roughly comparable and the inhibition brought about by high K can largely be removed by electrical hyperpolarization to the initial resting potential. In both Na sea waters and choline sea waters containing 100 mM-Na, Ca efflux is increased e-fold over approximately 50 mV. In choline sea water, external Ca-dependent Ca efflux from fully poisoned axons is unaffected by voltage over the range -80 to -30 mV. But addition of K or Li activates the flux and this activation is increased by hyperpolarization and decreased by depolarization, suggesting that the activating cation may also be transported into the axon. The Na-independent, energy-requiring, flux is inhibited by electrical hyperpolarization and stimulated by electrical depolarization. External K also stimulates the flux and part of this stimulation can be removed by electrical hyperpolarization. These data show that the energy-dependent pump is sensitive to membrane potential in the physiological range and suggest that it may be an electrogenic process. The finding that voltage affects the energy-dependent (uncoupled) pump and external Na-dependent fluxes in opposite directions may help explain why the total Ca efflux from intact axons responds to potential in a very variable manner.

Animals↗
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↗