Role of acetylcholine in induction of repetitive activity in human atrial fibers

The actions of acetylcholine and its interactions with epinephrine were studied in human atrial tissues by recording transmembrane potentials and contractile force. Acetylcholine (0.55-5.5 microM) reduced force, shortened the duration and shifted to more negative values the plateau of action potentials, abolished phase 4 depolarization, and suppressed the activity of spontaneous fibers. During the recovery, often there was a rebound increase in some parameters of the action potential and in force. Epinephrine (0.3-2.8 microM) induced oscillatory potentials and aftercontractions and acetylcholine abolished them. However, during the washout of acetylcholine in the presence of epinephrine, the oscillatory potentials and aftercontractions were larger than before acetylcholine, and repetitive activity was often induced. The inhibitory and excitatory effects of acetylcholine were mimicked by methacholine (5.1 microM) and abolished by atropine (1.5 microM). The postacetylcholine rebound was also potentiated by theophylline (0.6-2 mM) but was not blocked by propranolol (1-3.4 microM), prazosin (1 microM), and diltiazem (0.1 microM). It is concluded that in human atrial fibers acetylcholine has inhibitory as well as excitatory effects that are exaggerated in the presence of epinephrine and are mediated by the activation of the muscarinic receptor. The interaction between acetylcholine and epinephrine involves an antagonism at an intracellular level.

Depressant effects of isoflurane and halothane on isolated human atrial fibers

The present experiments were designed to study the cellular mechanism responsible for the depressant effects of halothane and isoflurane on human atrial tissues obtained at cardiac surgery. The fibers were superfused in Tyrode's solution, and transmembrane potentials were recorded with a microelectrode technique. In atrial fibers showing fast response action potential (maximum velocity of depolarization [Vmax] greater than 100 V/s), halothane (0.75 vol%, 0.44 mM) and isoflurane (1.25 vol%, 0.53 mM) decreased slightly the upstroke velocity but depressed the plateau and twitch force significantly. In atrial fibers showing slow rate of phase-0 depolarization or when atrial fibers were depolarized in high [K]0, both halothane and isoflurane decreased the upstroke of slow response and the force. The depressant effects of anesthetics partially mimicked the actions of 1 microM tetrodotoxin and diltiazem and could be reversed by epinephrine or high [Ca]0. The delayed afterdepolarizations or aftercontractions and contracture induced by epinephrine or strophanthidin were also inhibited by both anesthetics. Halothane and isoflurane may depress normal electromechanical activity and arrhythmogenic triggered activity through a reduction of cation fluxes across the cell membrane.

Suppressive effects of somatostatin in dog Purkinje fibres

1. The effects of somatostatin (SS, 1 nM-3 microM) on the electrical and mechanical activities of isolated Purkinje fibres of the dog were studied. 2. In most Purkinje fibres driven electrically in normal [K]o Tyrode solution, SS decreased the force of contraction slightly and had very little effect on the fast response action potential. However, in sensitive fibres SS induced a moderate reduction of action potential duration and contractile force in normal [K]o and depressed the slow response action potentials in high [K]o. 3. In spontaneously beating Purkinje fibres, SS decreased the regular rhythms slightly but abolished bursts of fast rhythms at a concentration as low as 1 nM. 4. When the fibres were depolarized in the presence of 0.2 mM barium or in Na-free solution, SS suppressed the Ca-dependent slow response action potentials. 5. These findings suggest that SS may suppress abnormal automatic activity of dog Purkinje fibres through a reduction of transmembrane Ca influx or a modulation of intracellular calcium.

Arrhythmogenic effects of theophylline in human atrial tissue

We studied the effects of theophylline on the transmembrane action potential and the contractile force of human atrial fibers obtained from the hearts of 15 patients, undergoing corrective open-heart surgery. Atrial fibers were perfused with Tyrode solution and driven electrically at a constant rate of 60 beats per min. Theophylline (0.1-1 mM) steepened the diastolic depolarization, increased the amplitude of oscillatory potential during diastole and facilitated the development of spontaneous slow response action potentials. These arrhythmogenic effects of theophylline were suppressed after diltiazem (0.1-0.3 microM) pretreatment. The present findings provide the electrophysiologic evidence that abnormal atrial automaticity as a result of triggered activity may be the underlying cause for atrial ectopic activity and multifocal atrial tachycardia in patients taking theophylline.

Differential inotropic effects of halothane and isoflurane in dog ventricular tissues

The differential effects of halothane (0.25-0.75%) and isoflurane (0.5-1.25%) on the electromechanical activity of canine ventricular tissues were compared in vitro. In Purkinje fibres, halothane but not isoflurane could induce an initial increase of contractile force which was not blocked by diltiazem or propranolol. In ventricular muscles, halothane decreased the resting state contraction more markedly than isoflurane. The results suggest that halothane induces a greater negative inotropy than isoflurane through a differential alteration of intracellular Ca2+ stores.

Somatostatin effects in isolated human atrial fibres

Effects of hypothalamic hormone somatostatin on the action potential and contractile force of 54 human atrial preparations obtained at cardiac surgery were studied with standard microelectrode techniques. In the atrial fibres responding to electrical stimuli with fast response action potentials in 4 mM [K]0 Tyrode solution (maximum rate of phase 0 depolarization greater than 50 V/s), somatostatin (10(-7) to 10(-6) M) reduced slightly the duration of action potential and decreased twitch force dose-dependently. In spontaneously active atrial fibres, somatostatin (10(-7) to 10(-6) M) abolished the action potentials in the preparations with low maximal diastolic potential (MDP, -48 +/- 2.8 mV), but induced only mild suppressive effect in the preparations with high MDP (-70 +/- 2.4 mV). When the fibres were depolarized in 27 mM [K]0 Tyrode solution, somatostatin decreased the maximum rate of depolarization and amplitude of the slow response action potentials induced by electrical stimuli. The delayed afterdepolarizations or triggered action potentials induced by high-frequency electrical drive in the presence of epinephrine were also suppressed by somatostatin. The above findings suggest that somatostatin may suppress the abnormal automaticity and the triggered activity in human atrial fibres through a reduction of cellular calcium.

On the mechanism underlying the oscillatory current in cardiac Purkinje fibers

The mechanism underlying the oscillatory current (Ios) was investigated in sheep cardiac Purkinje fibers with a voltage-clamp technique. The Ios is initiated not only by repolarizing but also by a depolarizing clamp, provided that the preparation is preloaded with calcium by means of a conditioning clamp and that the depolarizing test clamp initiates the slow inward current. The Ios initiated by a depolarizing test clamp is usually smaller and has a longer time to peak than that initiated by a repolarizing clamp to the same potential. Brief depolarizing clamps can be followed by an Ios in fibers preloaded with calcium. The amplitude of Ios diminishes as the interval from the conditioning clamp increases. No Ios is initiated by repolarizing or depolarizing clamps to potentials positive to approximately -30 to -40 mV even when [Na]0 is lowered. If a test clamp is applied at a peak of Ios to potentials less negative than approximately -30 mV, the current disappears. The membrane conductance during the Ios is lower. It is concluded that Ios is initiated indirectly by repolarization and is due to a calcium-triggered release of calcium which may initiate an electrogenic Na-Ca exchange.

Electrophysiological effects of low pH in human atrial fibres

In the studies of the electrophysiological properties of human atrial fibres obtained at cardiac surgery, it has often been reported that the diastolic potential, the maximum upstroke velocity of the phase 0 depolarization (Vmax) and the amplitude of action potential are relatively low. The same findings were also obtained in our previous study when the tissue preparations were perfused with a Tyrode solution which, as usually described in literatures on cardiac cell studies had a NaHCO3 concentration of 12 mM and was aerated with a gas mixture of 95% O2-5% CO2 at 37 degrees C. Recently we found that the relatively poor electrical activities of the human atrial fibres were related to the low pH value (around 7.06) of the perfusate used. Raising the pH value of the perfusate either by increasing the NaHCO3 concentration or by reducing the CO2 in the bubbling gas mixture significantly improved the electrical activities of the fibres. There is evidence that the suppressive effect of low pH on the electrical activities is due to the hindering action of H+ on the transportation of other cations across the plasma membrane.

Electromechanical effects of caffeine in isolated human atrial fibres

Effects of caffeine on the action potential and contractile force of human atrial fibres obtained at cardiac surgery were studied with standard microelectrode technique. In 4 mmol . litre-1 [K]o, the only significant action produced by 0.3 to 3 mmol . litre-1 caffeine on the electro-mechanical activity of relatively normal atrial fibres was a slight shortening of the action potential duration at 50% repolarisation. When the fibres were depolarised in 27 mmol . litre-1 [K]o or in atrial fibres showing slow responses in 4 mmol . litre-1 [K]o, however, caffeine could increase the upstroke of slow response and the force. In 18% of atrial fibres showing slow responses in 4 mmol . litre-1 [K]o, caffeine induced spontaneous discharges and potentiated afterdepolarisations. The positive inotropic and the arrhythmogenic effects of caffeine could be diminished by pretreating the fibres with propranolol or Ca antagonists (diltiazem and verapamil). In fibres beating spontaneously in normal [K]o, caffeine accelerated spontaneous rhythms initially and then depressed them. Propranolol potentiated the later depression but did not block the initial acceleration. The results suggest that caffeine increases the transmembrane Ca influx and enhances the release of Ca from the intracellular stores in human atrial fibres. As a consequence, caffeine could induce arrhythmias in atria from certain individuals.

Positive inotropic effect of acetylcholine in isolated human atrial fibers

We studied the effects of acetylcholine on the transmembrane action potential and the contractile force of isolated human atrial fibers obtained at cardiac surgery. In 3 of 12 preparations, 10(-7)-10(-6) M acetylcholine induced a substantial increase in the contractile force following a brief initial decrease. Atropine diminished both the initial negative and the later positive inotropic effects induced by acetylcholine while propranolol abolished only the later effect.

The presence of the iron-sulfur protein (subunit V) of complex III in mitochondria of heme-deficient yeast cells lacking iron-sulfur clusters detectable by electron paramagnetic resonance

The presence of subunit V, the iron-sulfur protein, of complex III has been demonstrated in mitochondria from a mutant of Saccharomyces cerevisiae which lacks 5-aminolevulinic acid synthase and, hence, is devoid of heme. The mature form (24 K Da) of the iron-sulfur protein was observed in equal amounts in the heme-deficient and heme-sufficient cells with antiserum against subunit V and either the sensitive immuno-transfer technique or immunoprecipitation from dodecylsulfate-solubilized mitochondria. In addition, a slight shoulder with a molecular mass 1.5 kDa larger than the mature form was present in mitochondria from the heme-deficient cells. Electron paramagnetic resonance spectroscopy revealed the absence of iron-sulfur signals due to clusters S-1, S-2 and S-3 of succinate dehydrogenase or to Rieske's iron-sulfur cluster of complex III in mitochondria from the heme-deficient cells. The lack of iron-sulfur centers in these cells may be a consequence of the absence of sulfite reductase in the cells without heme.

Calcium overload and strophanthidin-induced mechanical toxicity in cardiac Purkinje fibers

In cardiac Purkinje fibers, strophanthidin increases and then decreases contractile force. The relationship between the decrease in force and calcium overload was studied by recording the electrical and mechanical activity under conditions known to increase calcium overload or its effects. Inhibitors of oxidative phosphorylation reduced the positive inotropy of strophanthidin and enhanced the decrease in force. These inhibitors also reduced the inotropic effect of high calcium. Increasing intracellular calcium by decreasing extracellular sodium concentration also resulted in a decrease in the strophanthidin inotropy. When arrhythmia was delayed, strophanthidin induced contracture and this was favored by blockers of glycolysis and by enhancing cellular calcium. Some of these effects were also observed in ventricular muscle fibers but at higher strophanthidin concentrations. The results suggest that the decline in contractile force during strophanthidin exposure is related to calcium overload, although it is made clear that in Purkinje fibers contractile force and resting force may be independently affected under suitable conditions.

Role of calcium in the inotropic effects of caffeine in cardiac Purkinje fibers

The inotropic effects of caffeine (1-3 mM) were studied in the presence and absence of strophanthidin in canine Purkinje fibers perfused in vitro. Caffeine (1 mM) induced a similar initial increase in contractile force in different calcium solutions (+22, +23 and +24% in 0.54, 2.7 and 8.1 mM calcium, respectively) and when propranolol (3.4 X 10(-6) M) was present. Also, caffeine increased contractile force in high potassium (16.2 mM) at a time when the slow action potentials were unaltered. After the increase, 1 mM caffeine decreased force by about 50%, and the decrease was larger when caffeine (3 mM) or [Ca]0 (8.1 mM) was higher. In the presence of caffeine, strophanthidin (3 X 10(-7)-1 X 10(-6) M) increased force (+302%) if caffeine (0.3 mM) and Ca concentrations (0.54 mM) were low. If either caffeine or calcium was increased, strophanthidin had no effect or decreased force. Strophanthidin alone increased force and then decreased it; caffeine increased force in the first stage and decreased it during the second stage. The positive inotropic effect (+224%) of low sodium (78.4 mM versus 149.4 mM in Tyrode solution) was also abolished by caffeine (-24%). In ventricular muscle fibers, caffeine increased force more (+59%) and reduced force less in the presence of strophanthidin. The results indicate that caffeine increases force initially by releasing calcium from intracellular stores. The caffeine-induced decline in force is modulated by calcium in that it is exaggerated by agents or procedures which increase cellular calcium (strophanthidin, high calcium, low sodium solutions) and is reversed in a low calcium solution.

Attenuation of alcohol drinking in tetrahydroisoquinoline-treated rats by morphine and naltrexone

In rats of either the Sprague-Dawley or Long-Evans strain, either tetrahydroisoquinoline (THP) was infused chronically ICV, or one of three protoberberine (PBN) compounds was administered subcutaneously at birth. When the animals were 120-180 days of age, a constant concentration of alcohol was offered simultaneously with water to those rats which demonstrated a clear-cut preference for alcohol. This concentration was selected on the basis of an alcohol preference screen. After alcohol intakes had stabilized, naltrexone was injected subcutaneously in a dose of either 1.0 or 5.0 mg/kg twice a day for three consecutive days. The higher dose (10.0 mg/kg total) of naltrexone suppressed the voluntary intake of alcohol by 26%, whereas the lower dose (2.0 mg/kg total) attenuated alcohol drinking by 14%. Both doses of naltrexone reduced food intake but did not appreciably affect water intake or body weight. When morphine was injected according to the same regimen in a dose of 10.0 or 2.5 mg/kg twice per day, a 49% reduction in alcohol intake was produced by the higher dose and a 32% decline followed the lower dose. Although morphine attenuated food intake, neither water intake nor body weight was affected. Saline control injections administered twice daily in the same way failed to alter any of the intake measures or body weight. These findings indicate that the long-lasting opiate antagonist naltrexone attenuates the voluntary consumption of alcohol in a manner similar to that produced by naloxone. The present results are discussed in terms of the evidence that an opiate agonist and antagonist may exert their actions by different mechanisms in the brain, possibly through separate subpopulations of opiate receptors.

Synthesis of the proteins of complex III of the mitochondrial respiratory chain in heme-deficient cells

The presence of several proteins of complex III of the respiratory chain has been demonstrated in mitochondria from a mutant of Saccharomyces cerevisiae lacking 5-aminolevulinic acid synthase and, hence, devoid of heme. The two 'core' proteins, apocytochrome b and the iron-sulfur protein, were observed in equal amounts in the heme-deficient and heme-sufficient cells with antiserum against complex III and the sensitive immuno-transfer technique. In addition, three other bands were detected with the complex III antiserum in the mitochondria from the cells lacking heme. One of these has a molecular weight similar to that reported for a precursor form of cytochrome c1. By contrast, when mitochondria from the heme-deficient cells were solubilized with mild detergents and treated with the complex III antiserum, almost no immunoprecipitation was obtained above that obtained with control serum. The presence of only one major labeled band with a molecular weight similar to subunit I was observed after gel electrophoresis. These results suggest that heme may be necessary for proper processing of the apoprotein of cytochrome c1 and for the assembly into the membrane of the subunits of complex III, rather than for the synthesis of the proteins.

Adrenergic receptors and increased reactivity of aortic smooth muscle in renal hypertensive rats

The contractile responses of aortic smooth muscle and the dissociation constant (Ka) of the norepinephrine (NE) acting on alpha-adrenergic receptors in isolated thoracic aorta from normotensive and renal hypertensive rats were studied. Male Wistar rats were made hypertensive by uninephrectomy with figure-of-eight ligatures around the contralateral kidney. Two to three weeks after operation, the arterial systolic pressure for sham-operated rats was 123 +/- 2, and that for hypertensive rats was 164 +/- 4 mm Hg. Spontaneous rhythmic contractions of aortic rings, which were potentiated by low [Na]o and abolished by nifedipine (10(-6) - 10(-5) M), were observed in 4 out of 18 hypertensive rats. Aortic rings from normotensive rats showed no spontaneous rhythmicity. Although rhythmic phasic contractions could be induced in normotensive tissues by NE or caffeine in the presence of NE, tissues of hypertensive rats responded more readily. Furthermore, the threshold and the ED50 for tonic contractile response to NE was lower in hypertensive aortic rings than in the rings obtained from normotensive aortas. The Ka of NE acting on alpha-adrenergic receptors was derived from concentration-response data for NE before and after irreversible blockade of a fraction of the receptors with dibenamine (1-2 X 10(-7) M) in the presence of cocaine and propranolol. Mean Kas obtained were 5.58 +/- 0.42 X 10(-7) and 2.12 +/- 0.28 X 10(-7) M for normotensive and hypertensive rats, respectively. The increased contractile responses of the hypertensive aorta may be explained, at least partially, by a higher resting [Ca]i and a greater affinity of NE acting on the alpha-adrenergic receptors.

Orientation of complex III in the yeast mitochondrial membrane: labeling with [125I] diazobenzenesulfonate and functional studies with the decyl analogue of coenzyme Q as substrate

Mitochondria (or mitoplasts) and submitochondrial particles from yeast were treated with [125I] diazobenzenesulfonate to label selectively proteins exposed on the outer or inner surface of the inner mitochondrial membrane. Polyacrylamide gel analysis of the immunoprecipitates formed with antibodies against Complex III or cytochrome b revealed that the two core proteins and cytochrome b were labeled in both mitochondria and submitochondrial particles, suggesting that these proteins span the membrane. Cytochrome c1 and the iron sulfur protein were labeled in mitochondria but not in submitochondrial particles, suggesting that these proteins are exposed on the cytosolic side of the inner membrane. The steady-state reduction of cytochromes b and c1 was determined with succinate and the decyl analogue of coenzyme Q as substrates. Addition of the coenzyme Q analogue to mitochondria caused reduction of 15-30% of ;the total dithionite-reducible b and 100% of the cytochrome c1: Addition of the coenzyme Q analogue to submitochondrial particles led to the reduction of 70% of the total dithionite-reducible cytochrome b but insignificant amounts of cytochrome c1. A model to explain the topography of Complex III in the inner membrane is proposed based on these results.

The antiarrhythmic effect of potassium and rubidium in strophanthidin toxicity

The effect of potassium and rubidium on the electrical and mechanical activity of canine Purkinje fibers were studied in vitro in the presence and absence of strophanthidin. High (5.4mM) K or 2.7 Rb decreased the force of contraction. In the presence of these ions, strophanthidin increased the force of contraction as usual but the onset of arrhythmias was delayed. During the toxic stage of strophanthidin, high K or Rb increased the force of contraction, abolished the arrhythias and improved markedly the action potential. In the presence of calcium overload induced by exposure to a K-poor or Na-free solution, K and Rb induced an increase in force of contraction. And in ventricular muscle these ions relaxed the contracture induced by strophanthidin. It is concluded that K and Rb (in addition to other mechanisms) exert an antiarrhythmic action by increasing potassium conductance and by reducing the calcium overload induced by strophanthidin.

Effect of calcium on strophanthidin-induced electrical and mechanical toxicity in cardiac Purkinje fibers

The role of calcium in the electrical and mechanical toxicity induced by strophanthidin (10(-6) M) was studied in canine Purkinje fibers perfused in vitro. Strophanthidin caused an increase in contractile force ("therapeutic effect") followed by a subsequent decrease and by the onset of arrhythmias ("toxic effects"). The onset of arrhythmias occurred sooner in low- and later in high-calcium solutions with respect to the normal calcium. The positive inotropic (therapeutic) effect of strophanthidin was reduced or prevented by caffeine (1 mM) or by high calcium. The late (toxic) decline in force during exposure to strophanthidin was temporarily reversed by decreasing [Ca]o to a low value. Similarly, the contractile force decreased when [Ca]o was increased from 8.1 to 16.2 mM and this decline was transiently reversed when [Ca]o was reduced to a low value. It is concluded that in Purkinje fibers 1) electrical toxicity may occur independently of an intracellular calcium accumulation, and 2) the mechanical toxicity may be due to an excessive accumulation of calcium in the fiber.

Sodium lack prevents strophanthidin toxicity in Purkinje fibers

The effect of strophanthidin on electrical and mechanical activity of canine cardiac Purkinje fibers perfused in vitro was tested in the presence and in the absence of sodium. The following results were obtained: (1) in Tyrode's solution, strophanthidin causes spontaneous fast rhythms; (2) perfusion of a tetraethylammonium (TEA) Na-free solution results in an initial slowing and quiescence followed by the development of spontaneous small potentials; (3) exposure to strophanthidin during the perfusion of the Na-free solution fails to induce an acceleration of spontaneous discharge; (4) on returning from Na-free to Tyrode's solution there is a temporary acceleration of the small potentials and this acceleration is larger and lasts longer when the Na-free solution contains strophanthidin, and (5) in fibers intoxicated with strophanthidin, exposure to Na-free solution stops the fast rhythms. It is concluded that sodium plays an important role in strophanthidin toxicity and no toxicity occurs when sodium is replaced by TEA.

A system for studying effects of microwaves on cells in culture

An improved design of the fluid-filled waveguide-exposure chamber is described for studying effects of microwave radiation on cells in vitro. The system with a micropipette sample holder may be used as a prototype to isolate the apparent nonthermal factor of microwave radiation on cells in culture from those effects resulting from cell temperature rise. This system also allows more precise calibration of incident and absorbed microwave energies. Compared with control, somatic cells of the Chinese hamster exhibited a lower rate of growth and difference in morphology after 2450 MHz microwave radiation for 20 min at a power density of 500 mW/cm2.

Role of sodium in strophanthidin toxicity of Purkinje fibers

The action of strophanthidin on the membrane potentials and contractile force of canine Purkinje fibers was studied in vitro. Purkinje fibers were exposed to strophanthidin (10(-6) M) until spontaneous fast discharge occurred. In Tyrode solution, strophanthidin increased and subsequently decreased ("mechanical toxicity") contractile force. The onset of spontaneous rhythms ("electrical toxicity") usually occurred when force was declining. In low-Na Tyrode (-71 mM NaCl), force increased: on exposure to strophanthidin, mechanical toxicity occurred sooner and electrical toxicity later. In low-Na low-Ca Tyrode, electrical toxicity developed sooner than in low-Na Tyrode. In high-Na Tyrode (+27 mM NaCl), force increased, and the time to electrical and mechanical toxicities was decreased. Increasing osmolarity (+54 mosM) with either sucrose or choline chloride increased force and shortened the time to the onset of strophanthidin toxicities. In the presence of arrhythmias, lowering [Na]o decreased transient oscillations and led to disappearance of arrhythmias. It is concluded that Na plays a role in strophanthidin-induced electrical toxicity, whereas Ca appears more important for mechanical toxicity.

Heparin neutralizing activity and coronary artery disease

It has been previously shown and confirmed in the present investigation that the disaggregation of adenosine diphosphate (ADP)-induced platelet aggregates occurs at a slow rate more frequently in the platelet-rich plasma (PRP) of men with coronary artery disease. ADP-induced platelet aggregation was studied in the citrated PRP of 32 men (21 with and 11 without coronary artery disease) to determine the relation between release of heparin neutralizing activity (HNA) from platelets and the rate of platelet diaggregation. Each of the five PRP with slow (less than 10 per cent) disaggregation were from men with coronary artery disease. Platelets from these five PRP released from 34 to 51 per cent of their content of HNA during ADP-induced aggregation in contrast to the 27 PRP with more rapid disaggregation, only three of which had a detectable release of HNA. Of the latter 27 PRP, 21 had a second phase of aggregation which usually reached a peak of light transmission less than that of the first phase. These data are consistent with (but do not prove) the hypothesis that HNA released during aggreation may be one of the factors tending to prevent disaggregation of ADP-induced platelet aggregates.

Absorption coefficient of atmospheric aerosol: a method for measurement

Recent models predict that the effect of atmospheric aerosol particles on global temperature depends in part on b(abs,)or on n(2), where b(abs) is the aerosol absorption coefficient, and n(2) is the imaginary part of the aerosol refractive index for radiation in visible wavelengths. Satisfactory techniques for measuring b(abs) have yet to be developed. A method for measurement of b(abs) is described and discussed. The apparatus uses a piece of opal glass to integrate light scattered by particles collected on Nuclepore filters. It is quite mobile, inexpensive, and can be calibrated. Analysis of the errors inherent in the measurement shows it is accurate to well within an order of magnitude for particles of radius larger than 0.1 microm.